Re: STP and 7 hops [7:44408]
convergence
of
the
spanning
tree.
I'd think that if a bridge were to be the third bridge away
from
the
root,
and
another switch was the third bridge on the far side of the
root,
I
wouldn't
expect to see any problems with MaxAge because I can't see
the
root
being
too
far from some of the bridges in the network. Now if a
bridge
were
to
be
the
seventh, I could see how that would impose a greater delay
and
possibly
negatively impact the MaxAge parameter. Now my question
would
be...
does
this
really apply in today's networks or is this more of a
limitation
of
yesteryear's software-based bridges?
And essentially, that's the conclusion Priscilla and I came
to
-
that
the
7
hop radius doesn't really seem to apply to today's switched
environments...
You might want to check with her again though - Priscilla
just
authored
a
new
book on troubleshooting campus networks and may updated her
thinking.
-- Leigh Anne Chisholm (CCNP, CCDP) -Original
Message-
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED]]On
Behalf
Of
Steven A. Ridder
Sent: Saturday, May 18, 2002 5:16 AM
To: [EMAIL PROTECTED]
Subject: Re: STP and 7 hops [7:44408]
I believe the 5-4-3 rule is for repeaters, not switches.
Brian Hill wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
Steven,
The 7 hop limit is from the root bridge, as STP
calculates
the
tree
from
the
root. Historically, I am not sure why it's 7, but
Ethernet
has
a
base
hop
limit of 4 switches (5-4-3 rule), so it doesn't really
matter
so
much.
The
reason for the 4 hop limit in Ethernet is simple: For 10
Mb
or
full
duplex
100 Mb connections, the limit is mostly to reduce noise
from
the
amplification of the signal as it passes through the
switches/hubs,
where
as
in 100Mb half-duplex connections, it is mostly to keep
the
propogation
delay
within specs.
Hope this helps,
Brian Hill
CCNP, CCDP, MCSE 2000 (Charter Member),MCSE+I (NT4.0),
MCSA (Charter Member), MCP+I, MCP(21), Inet+, Net+, A+
Lead Technology Architect, TechTrain
Author: Cisco, The Complete Reference
http://www.alfageek.com
Priscilla Oppenheimer
http://www.priscilla.com
Priscilla Oppenheimer
http://www.priscilla.com
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7i=44689t=44408
--
FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html
Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: STP and 7 hops [7:44408]
s
gigastacked, then how does STP come into play. In a
configuration
such
as
this, again if I understand the documentation correctly, the
electronics
and
the switch OS behave differently.
Leigh Anne Chisholm wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
Actually, the 5-4-3 rule has everything to do with
detecting
collisions.
It's
a limiting factor of distance so that a collision will be
detected
within
the
first 64 bytes of a frame's transmission (also known as
Ethernet's
minimum
frame size). It's preferable to detect a collision before
the
frame
leaves
the buffer of the transmitting interface - so that
retransmission
can
be
accomplished at the data link layer rather than left to
upper
layers.
Several months ago, Priscilla and I debated the 7 switch
rule.
If
you
wanted
to search the archives for the entire thread, it was titled
What's
the
diameter of your switched network? [7:17489] and was
discussed
at
the
end
of
August, 2001. Here's an excerpt from one of my posts
regarding
the 7
hop
limit:
From other statements I've read (Cisco published material)
and
from
the
original excerpt I published, I'd imagine that the
placement
of
the
root
does
matter.
Part of this restriction is coming from the age field BPDU
carry:
when a BPDU is propagated from the root bridge towards the
leaves
of
the
tree, the age field is incremented each time it goes though
a
bridge.
Eventually, when the age field of a BPDU goes beyond max
age,
it
is
discarded. Typically, this will occur if the root is too
far
away
from
some
bridges of the network. This issue will impact convergence
of
the
spanning
tree.
I'd think that if a bridge were to be the third bridge away
from
the
root,
and
another switch was the third bridge on the far side of the
root,
I
wouldn't
expect to see any problems with MaxAge because I can't see
the
root
being
too
far from some of the bridges in the network. Now if a
bridge
were
to
be
the
seventh, I could see how that would impose a greater delay
and
possibly
negatively impact the MaxAge parameter. Now my question
would
be...
does
this
really apply in today's networks or is this more of a
limitation
of
yesteryear's software-based bridges?
And essentially, that's the conclusion Priscilla and I came
to
-
that
the
7
hop radius doesn't really seem to apply to today's switched
environments...
You might want to check with her again though - Priscilla
just
authored
a
new
book on troubleshooting campus networks and may updated her
thinking.
-- Leigh Anne Chisholm (CCNP, CCDP) -Original
Message-
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED]]On
Behalf
Of
Steven A. Ridder
Sent: Saturday, May 18, 2002 5:16 AM
To: [EMAIL PROTECTED]
Subject: Re: STP and 7 hops [7:44408]
I believe the 5-4-3 rule is for repeaters, not switches.
Brian Hill wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
Steven,
The 7 hop limit is from the root bridge, as STP
calculates
the
tree
from
the
root. Historically, I am not sure why it's 7, but
Ethernet
has
a
base
hop
limit of 4 switches (5-4-3 rule), so it doesn't
really
matter
so
much.
The
reason for the 4 hop limit in Ethernet is simple: For
10
Mb
or
full
duplex
100 Mb connections, the limit is mostly to reduce noise
from
the
amplification of the signal as it passes through the
switches/hubs,
where
as
in 100Mb half-duplex connections, it is mostly to keep
the
propogation
delay
within specs.
Hope this helps,
Brian Hill
CCNP, CCDP, MCSE 2000 (Charter Member),MCSE+I (NT4.0),
MCSA (Charter Member), MCP+I, MCP(21), Inet+, Net+, A+
Lead Technology Architect, TechTrain
Author: Cisco, The Complete Reference
http://www.alfageek.com
Re: STP and 7 hops [7:44408]
Here's the address for the 802.x documents from IEEE: pad pad pad http://standards.ieee.org/getieee802/ -- RFC 1149 Compliant. Get in my head: http://sar.dynu.com Brian Hill wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... Priscilla Oppenheimer wrote: The IEEE annex that covered full-duplex (802.3x) probably didn't mention slot time. That annex was rolled into the 802.3 2000 edition, however, which of course does cover slot time since it still covers CSMA/CD, repeaters, etc. (in addition to full-duplex operation.) Thanks, I was beginning to think I had bad info on slot time as well. It turns out you were saying that all devices that use CSMA/CD (i.e. multiaccess), not all Ethernet devices, use the slot time, which is how I understood it in the first place, I just misunderstood what you were saying :( Well, now we are getting into EE talk. ;-) Everything is analog at some level, isn't it? But an Ethernet repeater works on a Manchester encoded digital signal. (MLT-3 encoding for 100 Mbps). I think your second statement is closest to the truth (that the repeater converts the analog signal into a digital representation and creates a new analog signal). But I don't know the exact details. Great, that makes perfect sense :) There is that Manchester encoding again. Do you know where I can find good documentation on it? I'm sorry I was so punchy in the previous message. No problem Priscilla :) You guys (and gals) actually cleared up a lot of old misconceptions in this whole long list of stuff, which is great considering that documents on the dirty inner workings of Ethernet are hard to find unless you happen to be an IEEE member. :P Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=44514t=44408 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: STP and 7 hops [7:44408]
But they won't let you get IEEE 802.3 for free right now! It's been updated to the 2002 edition. I can understand them not letting you get that edition for free for the first six months, but I'm mad that you can't get the 2000 edition any more either. I think they must be worried that everyone would go for the free one, and that could be a problem if vendors, especially, were still implementing to the old version. But hopefully this situation will be remedied soon and we will be able to get the 2002 eidtion for free. Regarding the question about learning Manchester encoding, it is quite simple, and it's also not very relevant to anything real, but I did cover it in my Ethernet Troubleshooting paper at: http://www.certificationzone.com Priscilla At 05:51 AM 5/20/02, Steven A. Ridder wrote: Here's the address for the 802.x documents from IEEE: pad pad pad http://standards.ieee.org/getieee802/ -- RFC 1149 Compliant. Get in my head: http://sar.dynu.com Brian Hill wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... Priscilla Oppenheimer wrote: The IEEE annex that covered full-duplex (802.3x) probably didn't mention slot time. That annex was rolled into the 802.3 2000 edition, however, which of course does cover slot time since it still covers CSMA/CD, repeaters, etc. (in addition to full-duplex operation.) Thanks, I was beginning to think I had bad info on slot time as well. It turns out you were saying that all devices that use CSMA/CD (i.e. multiaccess), not all Ethernet devices, use the slot time, which is how I understood it in the first place, I just misunderstood what you were saying :( Well, now we are getting into EE talk. ;-) Everything is analog at some level, isn't it? But an Ethernet repeater works on a Manchester encoded digital signal. (MLT-3 encoding for 100 Mbps). I think your second statement is closest to the truth (that the repeater converts the analog signal into a digital representation and creates a new analog signal). But I don't know the exact details. Great, that makes perfect sense :) There is that Manchester encoding again. Do you know where I can find good documentation on it? I'm sorry I was so punchy in the previous message. No problem Priscilla :) You guys (and gals) actually cleared up a lot of old misconceptions in this whole long list of stuff, which is great considering that documents on the dirty inner workings of Ethernet are hard to find unless you happen to be an IEEE member. :P Priscilla Oppenheimer http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=44545t=44408 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: STP and 7 hops [7:44408]
o 20 seconds, so that
doesn't limit you to 7 hops.
Also, with gigaswitch, do all those 9 switches remain in the spanning
tree?
I don't know anything about gigaswitch, as you can probably tell, but I
would think the 7 limit would only apply to the logical topology, if it
applies at all.
Priscilla
Steven A. Ridder wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
I asked Cisco, and STP treats the stack as separate switches.
Chuck wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
in your travels have you ever run into the gigastack, and its
relation
to
STP?
the question came up a short time ago. The Cisco gigastack
documentation
does not cover STP - only how gigastack works.
essentially, using daisy chained gigastack connections, one can
connect
up
to 9 switches. If I understand correctly, the Cisco electronics
and
switch
OS consider this stack a single entity for management purposes -
i.e.
can
be
managed from a single IP address. However, Cisco does not say one
way
or
another if the electronics and the switch OS treat this stack as
a
single
device for STP purposes.
I'm guessing, based on this and other discussions that it would
have
to
be
a
single device for STP purposes. It just would be nice if Cisco
would
provide
the specific information.
That brings up the corollary question - if you have on two
switches
gigastacked, then how does STP come into play. In a configuration
such
as
this, again if I understand the documentation correctly, the
electronics
and
the switch OS behave differently.
Leigh Anne Chisholm wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
Actually, the 5-4-3 rule has everything to do with detecting
collisions.
It's
a limiting factor of distance so that a collision will be
detected
within
the
first 64 bytes of a frame's transmission (also known as
Ethernet's
minimum
frame size). It's preferable to detect a collision before the
frame
leaves
the buffer of the transmitting interface - so that
retransmission
can
be
accomplished at the data link layer rather than left to upper
layers.
Several months ago, Priscilla and I debated the 7 switch rule.
If
you
wanted
to search the archives for the entire thread, it was titled
What's
the
diameter of your switched network? [7:17489] and was discussed
at
the
end
of
August, 2001. Here's an excerpt from one of my posts regarding
the 7
hop
limit:
From other statements I've read (Cisco published material) and
from
the
original excerpt I published, I'd imagine that the placement of
the
root
does
matter.
Part of this restriction is coming from the age field BPDU
carry:
when a BPDU is propagated from the root bridge towards the
leaves
of
the
tree, the age field is incremented each time it goes though a
bridge.
Eventually, when the age field of a BPDU goes beyond max age,
it
is
discarded. Typically, this will occur if the root is too far
away
from
some
bridges of the network. This issue will impact convergence of
the
spanning
tree.
I'd think that if a bridge were to be the third bridge away
from
the
root,
and
another switch was the third bridge on the far side of the
root,
I
wouldn't
expect to see any problems with MaxAge because I can't see the
root
being
too
far from some of the bridges in the network. Now if a bridge
were
to
be
the
seventh, I could see how that would impose a greater delay and
possibly
negatively impact the MaxAge parameter. Now my question would
be...
does
this
really apply in today's networks or is this more of a
limitation
of
yesteryear's software-based bridges?
And essentially, that's the conclusion Priscilla and I came to
-
that
the
7
hop radius doesn't really seem to apply to today's switched
environments...
You might want to check with her again though - Priscilla just
authored
a
new
book on troubleshooting campus networks and may updated her
thinking.
-- Leigh Anne Chisholm (CCNP, CCDP) -Original
Message-
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On
Behalf
Of
Steven A. Ridder
Sent: Saturday, May 18, 2002 5:16 AM
To: [EMAIL PROTECTED]
Subject: Re: STP and 7 hops [7:44408]
I believe the 5-4-3 rule is for repeaters, not
RE: STP and 7 hops [7:44408]
I just saw your message!? You are right-on with the STP comments, which I snipped, but this other thing caught my eye. At 11:08 AM 5/18/02, Leigh Anne Chisholm wrote: PS. Priscilla - if a network is supposed to be designed at the access layer first, then distribution layer, then the core, why is your book called Top-Down Network Design? Shouldn't it be Bottom's Up? I know I personally prefer the latter... (-: I guess you must be looking at a typical picture of Cisco's hierarchical model that has the core at the top. Just turn it upside down. There's nothing that says the core has to be displayed at the top. (My favorite pictures show the model as concentric circles actually.) Top-Down Network Design doesn't actually say that access should be designed first, but I guess if I were to think about it, I would say that's usually true. How can you know what you'll need in the core if you haven't figured out the edges first? But it's definitely an iterative process too, as I say in the first few pages of the book. It probably sounds like I don't have much of a sense of humor about this. I do really! ;-) Priscilla Priscilla Oppenheimer http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=44550t=44408 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: STP and 7 hops [7:44408]
Heh... it's only available to us IEEE members =) Mike Priscilla Oppenheimer wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... But they won't let you get IEEE 802.3 for free right now! It's been updated to the 2002 edition. I can understand them not letting you get that edition for free for the first six months, but I'm mad that you can't get the 2000 edition any more either. I think they must be worried that everyone would go for the free one, and that could be a problem if vendors, especially, were still implementing to the old version. But hopefully this situation will be remedied soon and we will be able to get the 2002 eidtion for free. Regarding the question about learning Manchester encoding, it is quite simple, and it's also not very relevant to anything real, but I did cover it in my Ethernet Troubleshooting paper at: http://www.certificationzone.com Priscilla At 05:51 AM 5/20/02, Steven A. Ridder wrote: Here's the address for the 802.x documents from IEEE: pad pad pad http://standards.ieee.org/getieee802/ -- RFC 1149 Compliant. Get in my head: http://sar.dynu.com Brian Hill wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... Priscilla Oppenheimer wrote: The IEEE annex that covered full-duplex (802.3x) probably didn't mention slot time. That annex was rolled into the 802.3 2000 edition, however, which of course does cover slot time since it still covers CSMA/CD, repeaters, etc. (in addition to full-duplex operation.) Thanks, I was beginning to think I had bad info on slot time as well. It turns out you were saying that all devices that use CSMA/CD (i.e. multiaccess), not all Ethernet devices, use the slot time, which is how I understood it in the first place, I just misunderstood what you were saying :( Well, now we are getting into EE talk. ;-) Everything is analog at some level, isn't it? But an Ethernet repeater works on a Manchester encoded digital signal. (MLT-3 encoding for 100 Mbps). I think your second statement is closest to the truth (that the repeater converts the analog signal into a digital representation and creates a new analog signal). But I don't know the exact details. Great, that makes perfect sense :) There is that Manchester encoding again. Do you know where I can find good documentation on it? I'm sorry I was so punchy in the previous message. No problem Priscilla :) You guys (and gals) actually cleared up a lot of old misconceptions in this whole long list of stuff, which is great considering that documents on the dirty inner workings of Ethernet are hard to find unless you happen to be an IEEE member. :P Priscilla Oppenheimer http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=44564t=44408 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: STP and 7 hops [7:44408]
in your travels have you ever run into the gigastack, and its relation to STP? the question came up a short time ago. The Cisco gigastack documentation does not cover STP - only how gigastack works. essentially, using daisy chained gigastack connections, one can connect up to 9 switches. If I understand correctly, the Cisco electronics and switch OS consider this stack a single entity for management purposes - i.e. can be managed from a single IP address. However, Cisco does not say one way or another if the electronics and the switch OS treat this stack as a single device for STP purposes. I'm guessing, based on this and other discussions that it would have to be a single device for STP purposes. It just would be nice if Cisco would provide the specific information. That brings up the corollary question - if you have on two switches gigastacked, then how does STP come into play. In a configuration such as this, again if I understand the documentation correctly, the electronics and the switch OS behave differently. Leigh Anne Chisholm wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... Actually, the 5-4-3 rule has everything to do with detecting collisions. It's a limiting factor of distance so that a collision will be detected within the first 64 bytes of a frame's transmission (also known as Ethernet's minimum frame size). It's preferable to detect a collision before the frame leaves the buffer of the transmitting interface - so that retransmission can be accomplished at the data link layer rather than left to upper layers. Several months ago, Priscilla and I debated the 7 switch rule. If you wanted to search the archives for the entire thread, it was titled What's the diameter of your switched network? [7:17489] and was discussed at the end of August, 2001. Here's an excerpt from one of my posts regarding the 7 hop limit: From other statements I've read (Cisco published material) and from the original excerpt I published, I'd imagine that the placement of the root does matter. Part of this restriction is coming from the age field BPDU carry: when a BPDU is propagated from the root bridge towards the leaves of the tree, the age field is incremented each time it goes though a bridge. Eventually, when the age field of a BPDU goes beyond max age, it is discarded. Typically, this will occur if the root is too far away from some bridges of the network. This issue will impact convergence of the spanning tree. I'd think that if a bridge were to be the third bridge away from the root, and another switch was the third bridge on the far side of the root, I wouldn't expect to see any problems with MaxAge because I can't see the root being too far from some of the bridges in the network. Now if a bridge were to be the seventh, I could see how that would impose a greater delay and possibly negatively impact the MaxAge parameter. Now my question would be... does this really apply in today's networks or is this more of a limitation of yesteryear's software-based bridges? And essentially, that's the conclusion Priscilla and I came to - that the 7 hop radius doesn't really seem to apply to today's switched environments... You might want to check with her again though - Priscilla just authored a new book on troubleshooting campus networks and may updated her thinking. -- Leigh Anne Chisholm (CCNP, CCDP) -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Steven A. Ridder Sent: Saturday, May 18, 2002 5:16 AM To: [EMAIL PROTECTED] Subject: Re: STP and 7 hops [7:44408] I believe the 5-4-3 rule is for repeaters, not switches. Brian Hill wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... Steven, The 7 hop limit is from the root bridge, as STP calculates the tree from the root. Historically, I am not sure why it's 7, but Ethernet has a base hop limit of 4 switches (5-4-3 rule), so it doesn't really matter so much. The reason for the 4 hop limit in Ethernet is simple: For 10 Mb or full duplex 100 Mb connections, the limit is mostly to reduce noise from the amplification of the signal as it passes through the switches/hubs, where as in 100Mb half-duplex connections, it is mostly to keep the propogation delay within specs. Hope this helps, Brian Hill CCNP, CCDP, MCSE 2000 (Charter Member),MCSE+I (NT4.0), MCSA (Charter Member), MCP+I, MCP(21), Inet+, Net+, A+ Lead Technology Architect, TechTrain Author: Cisco, The Complete Reference http://www.alfageek.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=44478t=44408 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: STP and 7 hops [7:44408]
I asked Cisco, and STP treats the stack as separate switches. Chuck wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... in your travels have you ever run into the gigastack, and its relation to STP? the question came up a short time ago. The Cisco gigastack documentation does not cover STP - only how gigastack works. essentially, using daisy chained gigastack connections, one can connect up to 9 switches. If I understand correctly, the Cisco electronics and switch OS consider this stack a single entity for management purposes - i.e. can be managed from a single IP address. However, Cisco does not say one way or another if the electronics and the switch OS treat this stack as a single device for STP purposes. I'm guessing, based on this and other discussions that it would have to be a single device for STP purposes. It just would be nice if Cisco would provide the specific information. That brings up the corollary question - if you have on two switches gigastacked, then how does STP come into play. In a configuration such as this, again if I understand the documentation correctly, the electronics and the switch OS behave differently. Leigh Anne Chisholm wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... Actually, the 5-4-3 rule has everything to do with detecting collisions. It's a limiting factor of distance so that a collision will be detected within the first 64 bytes of a frame's transmission (also known as Ethernet's minimum frame size). It's preferable to detect a collision before the frame leaves the buffer of the transmitting interface - so that retransmission can be accomplished at the data link layer rather than left to upper layers. Several months ago, Priscilla and I debated the 7 switch rule. If you wanted to search the archives for the entire thread, it was titled What's the diameter of your switched network? [7:17489] and was discussed at the end of August, 2001. Here's an excerpt from one of my posts regarding the 7 hop limit: From other statements I've read (Cisco published material) and from the original excerpt I published, I'd imagine that the placement of the root does matter. Part of this restriction is coming from the age field BPDU carry: when a BPDU is propagated from the root bridge towards the leaves of the tree, the age field is incremented each time it goes though a bridge. Eventually, when the age field of a BPDU goes beyond max age, it is discarded. Typically, this will occur if the root is too far away from some bridges of the network. This issue will impact convergence of the spanning tree. I'd think that if a bridge were to be the third bridge away from the root, and another switch was the third bridge on the far side of the root, I wouldn't expect to see any problems with MaxAge because I can't see the root being too far from some of the bridges in the network. Now if a bridge were to be the seventh, I could see how that would impose a greater delay and possibly negatively impact the MaxAge parameter. Now my question would be... does this really apply in today's networks or is this more of a limitation of yesteryear's software-based bridges? And essentially, that's the conclusion Priscilla and I came to - that the 7 hop radius doesn't really seem to apply to today's switched environments... You might want to check with her again though - Priscilla just authored a new book on troubleshooting campus networks and may updated her thinking. -- Leigh Anne Chisholm (CCNP, CCDP) -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Steven A. Ridder Sent: Saturday, May 18, 2002 5:16 AM To: [EMAIL PROTECTED] Subject: Re: STP and 7 hops [7:44408] I believe the 5-4-3 rule is for repeaters, not switches. Brian Hill wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... Steven, The 7 hop limit is from the root bridge, as STP calculates the tree from the root. Historically, I am not sure why it's 7, but Ethernet has a base hop limit of 4 switches (5-4-3 rule), so it doesn't really matter so much. The reason for the 4 hop limit in Ethernet is simple: For 10 Mb or full duplex 100 Mb connections, the limit is mostly to reduce noise from the amplification of the signal as it passes through the switches/hubs, where as in 100Mb half-duplex connections, it is mostly to keep the propogation delay within specs. Hope this helps, Brian Hill CCNP, CCDP, MCSE 2000 (Charter Member),MCSE+I (NT4.0), MCSA (Charter Member), MCP+I, MCP(21), Inet+, Net+, A+ Lead Technology Architect, TechTrain Author: Cisco, The Complete Reference http://www.alfageek.com Message Posted at: http://www.groupstudy.com/form/re
Re: STP and 7 hops [7:44408]
were the people you asked able to address the max STP diameter of 7 issue
then?
Steven A. Ridder wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
I asked Cisco, and STP treats the stack as separate switches.
Chuck wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
in your travels have you ever run into the gigastack, and its relation
to
STP?
the question came up a short time ago. The Cisco gigastack documentation
does not cover STP - only how gigastack works.
essentially, using daisy chained gigastack connections, one can connect
up
to 9 switches. If I understand correctly, the Cisco electronics and
switch
OS consider this stack a single entity for management purposes - i.e.
can
be
managed from a single IP address. However, Cisco does not say one way or
another if the electronics and the switch OS treat this stack as a
single
device for STP purposes.
I'm guessing, based on this and other discussions that it would have to
be
a
single device for STP purposes. It just would be nice if Cisco would
provide
the specific information.
That brings up the corollary question - if you have on two switches
gigastacked, then how does STP come into play. In a configuration such
as
this, again if I understand the documentation correctly, the electronics
and
the switch OS behave differently.
Leigh Anne Chisholm wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
Actually, the 5-4-3 rule has everything to do with detecting
collisions.
It's
a limiting factor of distance so that a collision will be detected
within
the
first 64 bytes of a frame's transmission (also known as Ethernet's
minimum
frame size). It's preferable to detect a collision before the frame
leaves
the buffer of the transmitting interface - so that retransmission can
be
accomplished at the data link layer rather than left to upper layers.
Several months ago, Priscilla and I debated the 7 switch rule. If you
wanted
to search the archives for the entire thread, it was titled What's
the
diameter of your switched network? [7:17489] and was discussed at the
end
of
August, 2001. Here's an excerpt from one of my posts regarding the 7
hop
limit:
From other statements I've read (Cisco published material) and from
the
original excerpt I published, I'd imagine that the placement of the
root
does
matter.
Part of this restriction is coming from the age field BPDU carry:
when a BPDU is propagated from the root bridge towards the leaves of
the
tree, the age field is incremented each time it goes though a bridge.
Eventually, when the age field of a BPDU goes beyond max age, it is
discarded. Typically, this will occur if the root is too far away from
some
bridges of the network. This issue will impact convergence of the
spanning
tree.
I'd think that if a bridge were to be the third bridge away from the
root,
and
another switch was the third bridge on the far side of the root, I
wouldn't
expect to see any problems with MaxAge because I can't see the root
being
too
far from some of the bridges in the network. Now if a bridge were to
be
the
seventh, I could see how that would impose a greater delay and
possibly
negatively impact the MaxAge parameter. Now my question would be...
does
this
really apply in today's networks or is this more of a limitation of
yesteryear's software-based bridges?
And essentially, that's the conclusion Priscilla and I came to - that
the
7
hop radius doesn't really seem to apply to today's switched
environments...
You might want to check with her again though - Priscilla just
authored
a
new
book on troubleshooting campus networks and may updated her thinking.
-- Leigh Anne Chisholm (CCNP, CCDP) -Original Message-
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf
Of
Steven A. Ridder
Sent: Saturday, May 18, 2002 5:16 AM
To: [EMAIL PROTECTED]
Subject: Re: STP and 7 hops [7:44408]
I believe the 5-4-3 rule is for repeaters, not switches.
Brian Hill wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
Steven,
The 7 hop limit is from the root bridge, as STP calculates the
tree
from
the
root. Historically, I am not sure why it's 7, but Ethernet has a
base
hop
limit of 4 switches (5-4-3 rule), so it doesn't really matter so
much.
The
reason for the 4 hop limit in Ethernet is simple: For 10 Mb or
full
duplex
100 Mb connections, the limit is mostly to reduce noise from the
amplification of the signal as it passes through the
switches/hubs,
where
as
in 100Mb half-duplex connections, it is mostly to keep the
propogation
delay
within specs.
Hope this helps,
Brian Hill
CCNP, CCDP, M
RE: STP and 7 hops [7:44408]
At 10:49 PM 5/18/02, Brian Hill wrote: Slot time is an issue for all CSMA/CD networks, regardless of transmission speed. It is certainly discussed as a fundamental issue in all versions of IEEE 802.3 from the first in January 1985. This would be the first time I have heard this statement. I was under the impression that the slot time's primary purpose was to facilitate collision detection. In other words, that the slot time represented the length of time an Ethernet host listened to its own packet to detect a collision. Is this not true? If it is true, how does the slot time have anything to do with full duplex Ethernet? Full duplex is used on a point-to-point link where each side has a dedicated transmit circuit. It's not multiple access (MA). Since it's not MA, the sender doesn't need to sense the carrier first (CS). Collision detection (CD) isn't necessary because the two stations sending at the same time is legal. So full duplex wasn't what I had in mind when I said that slot time is an issue for all CSMA/CD networks. I don't know why you are mentioning it here. The IEEE annex that covered full-duplex (802.3x) probably didn't mention slot time. That annex was rolled into the 802.3 2000 edition, however, which of course does cover slot time since it still covers CSMA/CD, repeaters, etc. (in addition to full-duplex operation.) However, thinking about it, based solely on the switching mode, it seems that all switches (and even a lot of hubs now) buffer the packet in RAM and then forward it, which means, as someone stated, that the packet is rebuilt. A hub that did that wouldn't really be a hub. The extra delay would cause a problem, for one thing. Priscilla, I can't find the logic in this. If the hub doesn't buffer the frame, I don't see any way it could possibly rebuild it. A hub doesn't understand frames, doesn't buffer them, and doesn't rebuild them. It rebuilds bits. It regenerates the signal one bit at a time. It syncs up on the signal by looking at the preamble. It regenerates the preamble (to avoid the problem of the preamble shrinking from repeaters taking time to sync up on it) and forwards the rest of the bits. It also extends fragments that are less than 96 bits (including the preamble). If it has any RAM, it's only a few bits big. We used to teach the gory details of repeater behavior but that's about all I remember at this time. I mean, from what I can tell, either the hub amplifies the original signal (which you and documentation state is untrue), or it has to somehow record the incoming signal (into RAM?) and then send the regenerated signal back out, doesn't it? I am not talking about buffering the entire packet, therby increasing the delay , my thought process was simply that either it sent the signal on, amplifying it, or it stored and analyzed the signal, then forwarded it back out. This is based on my understanding that the signal itself is analog, even if it is represented digitally. In other words, either you can increase the analog signal by running it through a circuit, or you have to convert the original analog signal into a digital representation (+3.12 volts might be determined to be a binary 1 or simply 3 volts, for instance), and then create a new analog signal. I am no EE either, however, so perhaps my thinking is flawed. Is this how digital repeating is done? Well, now we are getting into EE talk. ;-) Everything is analog at some level, isn't it? But an Ethernet repeater works on a Manchester encoded digital signal. (MLT-3 encoding for 100 Mbps). I think your second statement is closest to the truth (that the repeater converts the analog signal into a digital representation and creates a new analog signal). But I don't know the exact details. I'm sorry I was so punchy in the previous message. Priscilla Priscilla Oppenheimer http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=44485t=44408 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: STP and 7 hops [7:44408]
At 03:51 PM 5/19/02, Chuck wrote:
were the people you asked able to address the max STP diameter of 7 issue
then?
Maybe they blew off the max STP diameter of 7 issue. ;-) It is just a
recommendation. Even IEEE 802.1D just uses that word (recommendation).
There's nothing in the protocol that would stop you from having a larger
network, is there? The Max Age timer defaults to 20 seconds, so that
doesn't limit you to 7 hops.
Also, with gigaswitch, do all those 9 switches remain in the spanning tree?
I don't know anything about gigaswitch, as you can probably tell, but I
would think the 7 limit would only apply to the logical topology, if it
applies at all.
Priscilla
Steven A. Ridder wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
I asked Cisco, and STP treats the stack as separate switches.
Chuck wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
in your travels have you ever run into the gigastack, and its relation
to
STP?
the question came up a short time ago. The Cisco gigastack
documentation
does not cover STP - only how gigastack works.
essentially, using daisy chained gigastack connections, one can connect
up
to 9 switches. If I understand correctly, the Cisco electronics and
switch
OS consider this stack a single entity for management purposes - i.e.
can
be
managed from a single IP address. However, Cisco does not say one way
or
another if the electronics and the switch OS treat this stack as a
single
device for STP purposes.
I'm guessing, based on this and other discussions that it would have to
be
a
single device for STP purposes. It just would be nice if Cisco would
provide
the specific information.
That brings up the corollary question - if you have on two switches
gigastacked, then how does STP come into play. In a configuration such
as
this, again if I understand the documentation correctly, the
electronics
and
the switch OS behave differently.
Leigh Anne Chisholm wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
Actually, the 5-4-3 rule has everything to do with detecting
collisions.
It's
a limiting factor of distance so that a collision will be detected
within
the
first 64 bytes of a frame's transmission (also known as Ethernet's
minimum
frame size). It's preferable to detect a collision before the frame
leaves
the buffer of the transmitting interface - so that retransmission can
be
accomplished at the data link layer rather than left to upper layers.
Several months ago, Priscilla and I debated the 7 switch rule. If
you
wanted
to search the archives for the entire thread, it was titled What's
the
diameter of your switched network? [7:17489] and was discussed at
the
end
of
August, 2001. Here's an excerpt from one of my posts regarding the 7
hop
limit:
From other statements I've read (Cisco published material) and from
the
original excerpt I published, I'd imagine that the placement of the
root
does
matter.
Part of this restriction is coming from the age field BPDU carry:
when a BPDU is propagated from the root bridge towards the leaves of
the
tree, the age field is incremented each time it goes though a bridge.
Eventually, when the age field of a BPDU goes beyond max age, it is
discarded. Typically, this will occur if the root is too far away
from
some
bridges of the network. This issue will impact convergence of the
spanning
tree.
I'd think that if a bridge were to be the third bridge away from the
root,
and
another switch was the third bridge on the far side of the root, I
wouldn't
expect to see any problems with MaxAge because I can't see the root
being
too
far from some of the bridges in the network. Now if a bridge were to
be
the
seventh, I could see how that would impose a greater delay and
possibly
negatively impact the MaxAge parameter. Now my question would be...
does
this
really apply in today's networks or is this more of a limitation of
yesteryear's software-based bridges?
And essentially, that's the conclusion Priscilla and I came to - that
the
7
hop radius doesn't really seem to apply to today's switched
environments...
You might want to check with her again though - Priscilla just
authored
a
new
book on troubleshooting campus networks and may updated her thinking.
-- Leigh Anne Chisholm (CCNP, CCDP) -Original Message-
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf
Of
Steven A. Ridder
Sent: Saturday, May 18, 2002 5:16 AM
To: [EMAIL PROTECTED]
Subject: Re: STP and 7 hops [7:44408]
I believe the 5-4-3 rule is for repeaters, not switches.
Brian Hill wrote in message
[EMAIL PROTECTED
Re: STP and 7 hops [7:44408]
ial) and
from
the
original excerpt I published, I'd imagine that the placement of
the
root
does
matter.
Part of this restriction is coming from the age field BPDU carry:
when a BPDU is propagated from the root bridge towards the leaves
of
the
tree, the age field is incremented each time it goes though a
bridge.
Eventually, when the age field of a BPDU goes beyond max age, it
is
discarded. Typically, this will occur if the root is too far away
from
some
bridges of the network. This issue will impact convergence of the
spanning
tree.
I'd think that if a bridge were to be the third bridge away from
the
root,
and
another switch was the third bridge on the far side of the root, I
wouldn't
expect to see any problems with MaxAge because I can't see the
root
being
too
far from some of the bridges in the network. Now if a bridge were
to
be
the
seventh, I could see how that would impose a greater delay and
possibly
negatively impact the MaxAge parameter. Now my question would
be...
does
this
really apply in today's networks or is this more of a limitation
of
yesteryear's software-based bridges?
And essentially, that's the conclusion Priscilla and I came to -
that
the
7
hop radius doesn't really seem to apply to today's switched
environments...
You might want to check with her again though - Priscilla just
authored
a
new
book on troubleshooting campus networks and may updated her
thinking.
-- Leigh Anne Chisholm (CCNP, CCDP) -Original Message-
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On
Behalf
Of
Steven A. Ridder
Sent: Saturday, May 18, 2002 5:16 AM
To: [EMAIL PROTECTED]
Subject: Re: STP and 7 hops [7:44408]
I believe the 5-4-3 rule is for repeaters, not switches.
Brian Hill wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
Steven,
The 7 hop limit is from the root bridge, as STP calculates the
tree
from
the
root. Historically, I am not sure why it's 7, but Ethernet has
a
base
hop
limit of 4 switches (5-4-3 rule), so it doesn't really
matter
so
much.
The
reason for the 4 hop limit in Ethernet is simple: For 10 Mb or
full
duplex
100 Mb connections, the limit is mostly to reduce noise from
the
amplification of the signal as it passes through the
switches/hubs,
where
as
in 100Mb half-duplex connections, it is mostly to keep the
propogation
delay
within specs.
Hope this helps,
Brian Hill
CCNP, CCDP, MCSE 2000 (Charter Member),MCSE+I (NT4.0),
MCSA (Charter Member), MCP+I, MCP(21), Inet+, Net+, A+
Lead Technology Architect, TechTrain
Author: Cisco, The Complete Reference
http://www.alfageek.com
Priscilla Oppenheimer
http://www.priscilla.com
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7i=44492t=44408
--
FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html
Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: STP and 7 hops [7:44408]
I didn't ask about the max number of switches, just that if the stack=1 hop
or number of switches in stack=hop.
As for what Pricilla stated, I guess the 7 hop is a recommendation, not a
set-in-stone rule. Which is good, since I just proposed a network that has
an 8 hop diameter.
Chuck wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
were the people you asked able to address the max STP diameter of 7 issue
then?
Steven A. Ridder wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
I asked Cisco, and STP treats the stack as separate switches.
Chuck wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
in your travels have you ever run into the gigastack, and its relation
to
STP?
the question came up a short time ago. The Cisco gigastack
documentation
does not cover STP - only how gigastack works.
essentially, using daisy chained gigastack connections, one can
connect
up
to 9 switches. If I understand correctly, the Cisco electronics and
switch
OS consider this stack a single entity for management purposes - i.e.
can
be
managed from a single IP address. However, Cisco does not say one way
or
another if the electronics and the switch OS treat this stack as a
single
device for STP purposes.
I'm guessing, based on this and other discussions that it would have
to
be
a
single device for STP purposes. It just would be nice if Cisco would
provide
the specific information.
That brings up the corollary question - if you have on two switches
gigastacked, then how does STP come into play. In a configuration such
as
this, again if I understand the documentation correctly, the
electronics
and
the switch OS behave differently.
Leigh Anne Chisholm wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
Actually, the 5-4-3 rule has everything to do with detecting
collisions.
It's
a limiting factor of distance so that a collision will be detected
within
the
first 64 bytes of a frame's transmission (also known as Ethernet's
minimum
frame size). It's preferable to detect a collision before the frame
leaves
the buffer of the transmitting interface - so that retransmission
can
be
accomplished at the data link layer rather than left to upper
layers.
Several months ago, Priscilla and I debated the 7 switch rule. If
you
wanted
to search the archives for the entire thread, it was titled What's
the
diameter of your switched network? [7:17489] and was discussed at
the
end
of
August, 2001. Here's an excerpt from one of my posts regarding the
7
hop
limit:
From other statements I've read (Cisco published material) and from
the
original excerpt I published, I'd imagine that the placement of the
root
does
matter.
Part of this restriction is coming from the age field BPDU carry:
when a BPDU is propagated from the root bridge towards the leaves of
the
tree, the age field is incremented each time it goes though a
bridge.
Eventually, when the age field of a BPDU goes beyond max age, it is
discarded. Typically, this will occur if the root is too far away
from
some
bridges of the network. This issue will impact convergence of the
spanning
tree.
I'd think that if a bridge were to be the third bridge away from the
root,
and
another switch was the third bridge on the far side of the root, I
wouldn't
expect to see any problems with MaxAge because I can't see the root
being
too
far from some of the bridges in the network. Now if a bridge were
to
be
the
seventh, I could see how that would impose a greater delay and
possibly
negatively impact the MaxAge parameter. Now my question would be...
does
this
really apply in today's networks or is this more of a limitation of
yesteryear's software-based bridges?
And essentially, that's the conclusion Priscilla and I came to -
that
the
7
hop radius doesn't really seem to apply to today's switched
environments...
You might want to check with her again though - Priscilla just
authored
a
new
book on troubleshooting campus networks and may updated her
thinking.
-- Leigh Anne Chisholm (CCNP, CCDP) -Original Message-
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On
Behalf
Of
Steven A. Ridder
Sent: Saturday, May 18, 2002 5:16 AM
To: [EMAIL PROTECTED]
Subject: Re: STP and 7 hops [7:44408]
I believe the 5-4-3 rule is for repeaters, not switches.
Brian Hill wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
Steven,
The 7 hop limit is from the root bridge, as STP calculates the
tree
from
the
root. Historically, I am not sure why it's 7, but Ethernet has a
base
h
Re: STP and 7 hops [7:44408]
lly, the 5-4-3 rule has everything to do with detecting collisions. It's a limiting factor of distance so that a collision will be detected within the first 64 bytes of a frame's transmission (also known as Ethernet's minimum frame size). It's preferable to detect a collision before the frame leaves the buffer of the transmitting interface - so that retransmission can be accomplished at the data link layer rather than left to upper layers. Several months ago, Priscilla and I debated the 7 switch rule. If you wanted to search the archives for the entire thread, it was titled What's the diameter of your switched network? [7:17489] and was discussed at the end of August, 2001. Here's an excerpt from one of my posts regarding the 7 hop limit: From other statements I've read (Cisco published material) and from the original excerpt I published, I'd imagine that the placement of the root does matter. Part of this restriction is coming from the age field BPDU carry: when a BPDU is propagated from the root bridge towards the leaves of the tree, the age field is incremented each time it goes though a bridge. Eventually, when the age field of a BPDU goes beyond max age, it is discarded. Typically, this will occur if the root is too far away from some bridges of the network. This issue will impact convergence of the spanning tree. I'd think that if a bridge were to be the third bridge away from the root, and another switch was the third bridge on the far side of the root, I wouldn't expect to see any problems with MaxAge because I can't see the root being too far from some of the bridges in the network. Now if a bridge were to be the seventh, I could see how that would impose a greater delay and possibly negatively impact the MaxAge parameter. Now my question would be... does this really apply in today's networks or is this more of a limitation of yesteryear's software-based bridges? And essentially, that's the conclusion Priscilla and I came to - that the 7 hop radius doesn't really seem to apply to today's switched environments... You might want to check with her again though - Priscilla just authored a new book on troubleshooting campus networks and may updated her thinking. -- Leigh Anne Chisholm (CCNP, CCDP) -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Steven A. Ridder Sent: Saturday, May 18, 2002 5:16 AM To: [EMAIL PROTECTED] Subject: Re: STP and 7 hops [7:44408] I believe the 5-4-3 rule is for repeaters, not switches. Brian Hill wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... Steven, The 7 hop limit is from the root bridge, as STP calculates the tree from the root. Historically, I am not sure why it's 7, but Ethernet has a base hop limit of 4 switches (5-4-3 rule), so it doesn't really matter so much. The reason for the 4 hop limit in Ethernet is simple: For 10 Mb or full duplex 100 Mb connections, the limit is mostly to reduce noise from the amplification of the signal as it passes through the switches/hubs, where as in 100Mb half-duplex connections, it is mostly to keep the propogation delay within specs. Hope this helps, Brian Hill CCNP, CCDP, MCSE 2000 (Charter Member),MCSE+I (NT4.0), MCSA (Charter Member), MCP+I, MCP(21), Inet+, Net+, A+ Lead Technology Architect, TechTrain Author: Cisco, The Complete Reference http://www.alfageek.com Priscilla Oppenheimer http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=44499t=44408 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: STP and 7 hops [7:44408]
Priscilla Oppenheimer wrote: The IEEE annex that covered full-duplex (802.3x) probably didn't mention slot time. That annex was rolled into the 802.3 2000 edition, however, which of course does cover slot time since it still covers CSMA/CD, repeaters, etc. (in addition to full-duplex operation.) Thanks, I was beginning to think I had bad info on slot time as well. It turns out you were saying that all devices that use CSMA/CD (i.e. multiaccess), not all Ethernet devices, use the slot time, which is how I understood it in the first place, I just misunderstood what you were saying :( Well, now we are getting into EE talk. ;-) Everything is analog at some level, isn't it? But an Ethernet repeater works on a Manchester encoded digital signal. (MLT-3 encoding for 100 Mbps). I think your second statement is closest to the truth (that the repeater converts the analog signal into a digital representation and creates a new analog signal). But I don't know the exact details. Great, that makes perfect sense :) There is that Manchester encoding again. Do you know where I can find good documentation on it? I'm sorry I was so punchy in the previous message. No problem Priscilla :) You guys (and gals) actually cleared up a lot of old misconceptions in this whole long list of stuff, which is great considering that documents on the dirty inner workings of Ethernet are hard to find unless you happen to be an IEEE member. :P Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=44508t=44408 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: STP and 7 hops [7:44408]
Steven, The 7 hop limit is from the root bridge, as STP calculates the tree from the root. Historically, I am not sure why it's 7, but Ethernet has a base hop limit of 4 switches (5-4-3 rule), so it doesn't really matter so much. The reason for the 4 hop limit in Ethernet is simple: For 10 Mb or full duplex 100 Mb connections, the limit is mostly to reduce noise from the amplification of the signal as it passes through the switches/hubs, where as in 100Mb half-duplex connections, it is mostly to keep the propogation delay within specs. Hope this helps, Brian Hill CCNP, CCDP, MCSE 2000 (Charter Member),MCSE+I (NT4.0), MCSA (Charter Member), MCP+I, MCP(21), Inet+, Net+, A+ Lead Technology Architect, TechTrain Author: Cisco, The Complete Reference http://www.alfageek.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=44427t=44408 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: STP and 7 hops [7:44408]
I believe the 5-4-3 rule is for repeaters, not switches. Brian Hill wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... Steven, The 7 hop limit is from the root bridge, as STP calculates the tree from the root. Historically, I am not sure why it's 7, but Ethernet has a base hop limit of 4 switches (5-4-3 rule), so it doesn't really matter so much. The reason for the 4 hop limit in Ethernet is simple: For 10 Mb or full duplex 100 Mb connections, the limit is mostly to reduce noise from the amplification of the signal as it passes through the switches/hubs, where as in 100Mb half-duplex connections, it is mostly to keep the propogation delay within specs. Hope this helps, Brian Hill CCNP, CCDP, MCSE 2000 (Charter Member),MCSE+I (NT4.0), MCSA (Charter Member), MCP+I, MCP(21), Inet+, Net+, A+ Lead Technology Architect, TechTrain Author: Cisco, The Complete Reference http://www.alfageek.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=44431t=44408 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: STP and 7 hops [7:44408]
root. Historically, I am not sure why it's 7, but Ethernet has a base hop limit of 4 switches (5-4-3 rule), so it doesn't really As far as I remember, this is not true. 5-4-3 applies only to repeaters/hubs - not bridges/switches. R/H amplify received signal, while B/S regerate it entirely. Also, 5-4-3 rule has a lot to do with detecting collisions, which is non-issue in bridged networks. Marko. Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=44435t=44408 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: STP and 7 hops [7:44408]
Actually, the 5-4-3 rule has everything to do with detecting collisions. It's a limiting factor of distance so that a collision will be detected within the first 64 bytes of a frame's transmission (also known as Ethernet's minimum frame size). It's preferable to detect a collision before the frame leaves the buffer of the transmitting interface - so that retransmission can be accomplished at the data link layer rather than left to upper layers. Several months ago, Priscilla and I debated the 7 switch rule. If you wanted to search the archives for the entire thread, it was titled What's the diameter of your switched network? [7:17489] and was discussed at the end of August, 2001. Here's an excerpt from one of my posts regarding the 7 hop limit: From other statements I've read (Cisco published material) and from the original excerpt I published, I'd imagine that the placement of the root does matter. Part of this restriction is coming from the age field BPDU carry: when a BPDU is propagated from the root bridge towards the leaves of the tree, the age field is incremented each time it goes though a bridge. Eventually, when the age field of a BPDU goes beyond max age, it is discarded. Typically, this will occur if the root is too far away from some bridges of the network. This issue will impact convergence of the spanning tree. I'd think that if a bridge were to be the third bridge away from the root, and another switch was the third bridge on the far side of the root, I wouldn't expect to see any problems with MaxAge because I can't see the root being too far from some of the bridges in the network. Now if a bridge were to be the seventh, I could see how that would impose a greater delay and possibly negatively impact the MaxAge parameter. Now my question would be... does this really apply in today's networks or is this more of a limitation of yesteryear's software-based bridges? And essentially, that's the conclusion Priscilla and I came to - that the 7 hop radius doesn't really seem to apply to today's switched environments... You might want to check with her again though - Priscilla just authored a new book on troubleshooting campus networks and may updated her thinking. -- Leigh Anne Chisholm (CCNP, CCDP) -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Steven A. Ridder Sent: Saturday, May 18, 2002 5:16 AM To: [EMAIL PROTECTED] Subject: Re: STP and 7 hops [7:44408] I believe the 5-4-3 rule is for repeaters, not switches. Brian Hill wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... Steven, The 7 hop limit is from the root bridge, as STP calculates the tree from the root. Historically, I am not sure why it's 7, but Ethernet has a base hop limit of 4 switches (5-4-3 rule), so it doesn't really matter so much. The reason for the 4 hop limit in Ethernet is simple: For 10 Mb or full duplex 100 Mb connections, the limit is mostly to reduce noise from the amplification of the signal as it passes through the switches/hubs, where as in 100Mb half-duplex connections, it is mostly to keep the propogation delay within specs. Hope this helps, Brian Hill CCNP, CCDP, MCSE 2000 (Charter Member),MCSE+I (NT4.0), MCSA (Charter Member), MCP+I, MCP(21), Inet+, Net+, A+ Lead Technology Architect, TechTrain Author: Cisco, The Complete Reference http://www.alfageek.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=6t=44408 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: STP and 7 hops [7:44408]
At 06:21 AM 5/18/02, Brian Hill wrote: Steven, The 7 hop limit is from the root bridge No. It's from edge to edge. The root is at the top of a hierarchy of switches. Think of the president's spot in an org chart, for example. The 7 hops is from one worker at the bottom of one branch talking to another worker at the bottom of another branch, with data travelling across the entire hierarchy, up to the top and then back down. IEEE says that 7 is recommended. They don't explain it very well though. It comes from the olden days when DEC invented STP. According to one of my mentors Marty Adkins (CCIE low number), DEC engineers said it allowed for conservative BPDU propagation, but also for serialization delay and WAN latency. Their spec allowed one (or two?) bridge hops to have a slow serial link, as low as 56 Kbps. With more than seven bridge hops, a frame might take longer than one second to travel end-to-end. That would cause DEC's LAT protocol (which can only be bridged) to time out and retransmit. If the max retry count was exceeded, the session was dropped. , as STP calculates the tree from the root. Historically, I am not sure why it's 7, but Ethernet has a base hop limit of 4 switches (5-4-3 rule), The 5-4-3 rule has nothing to do with switches. It's for repeaters. It restricts the max size of a collision domain. so it doesn't really matter so much. The reason for the 4 hop limit in Ethernet is simple: For 10 Mb or full duplex 100 Mb connections, the limit is mostly to reduce noise from the amplification of the signal as it passes through the switches/hubs, The rule doesn't have to do with amplification and switches don't come into this discussion at all. where as in 100Mb half-duplex connections, it is mostly to keep the propogation delay within specs. Yes. The rule keeps propagation delay within specs. Specifically, the 5-4-3 rule exists to make sure that a sending station is still sending when a collision reflects back from the max size of network when the sender is sending a minimum-size frame (64 bytes). Hope this helps, Brian Hill CCNP, CCDP, Were at the pool drinking beers with that other guy when you wrote your reply? ;-) Priscilla MCSE 2000 (Charter Member),MCSE+I (NT4.0), MCSA (Charter Member), MCP+I, MCP(21), Inet+, Net+, A+ Lead Technology Architect, TechTrain Author: Cisco, The Complete Reference http://www.alfageek.com Priscilla Oppenheimer http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=44451t=44408 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: STP and 7 hops [7:44408]
Priscilla Oppenheimer wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... The 7 hop limit is from the root bridge No. It's from edge to edge. That's why it's called 'diameter'. not 'radius'.. hehe =) Mike W. Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=44456t=44408 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: STP and 7 hops [7:44408]
HmmmFor the 7 hop from edge thing, I stand corrected. Now that you mention radius vs. diameter, I see what you mean. My thinking was that the diameter was calculated with the root at the center (3 switches to either side for 7 switches total). The problem with that is that it doesn't necessarily have to be three per side, it could be 6 on one and none on the other :( Just poor analysis on my part. As for the 5-4-3 rule, I am going to disagree on a few counts. Here's how I remember it (from an admittedly long time ago): First, the slot time (64 byte time) isn't much of an issue unless running at 100Mbps or faster, and isn't an issue at all running full duplex. As I remember it, the problem with the slot time is that at 100Mbps, the slot time drops to something like 5.12 ms, which necessarily reduces the repeated network's diameter, as if a packet collides after the slot time it becomes a late collision, and the original host may not properly detect the collision. I seem to remember that there always being a recommendation, however, not to repeat the packet more than 5 times due to S/N Ratio problems creeping in after amplification. As for the switch vs. hub thing, I seem to remember based on the S/N thing that anything that amplified the original signal caused this problem. I also remember the distinction that under normal circumstances, the switch does not modify the packet in any way (L3/4 switches and trunking excluded). However, thinking about it, based solely on the switching mode, it seems that all switches (and even a lot of hubs now) buffer the packet in RAM and then forward it, which means, as someone stated, that the packet is rebuilt. So, I agree with most of what you have said after all, with the exception of the S/N ratio having nothing to do with it. I do remember reading that the S/N ratio degradation was an issue after many amplifications of the original signal. As for me drinking by the pool, no, I am out of town at present, and just rattled off the reply. As I hope I have shown, I did have reasons for what I said, just perhaps the weren't thought out well enough. Brian Hill CCNP, CCDP, MCSE 2000 (Charter Member),MCSE+I (NT4.0), MCSA (Charter Member), MCP+I, MCP(21), Inet+, Net+, A+ Lead Technology Architect, TechTrain Author: Cisco, The Complete Reference http://www.alfageek.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=44460t=44408 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: STP and 7 hops [7:44408]
Here's a pretty good explanation of the STP timers, diameter, etc. Understanding and Tuning Spanning-Tree Protocol Timers http://www.cisco.com/warp/public/473/122.html Much easier to follow than the IEEE standard. Marty Adkins Email: [EMAIL PROTECTED] Chesapeake NetCraftsmen o:410.757.3050, p:[EMAIL PROTECTED] 1290 Bay Dale Drive, Suite 312 http://www.netcraftsmen.net Arnold, MD 21012-2325 Cisco CCIE #1289 Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=44463t=44408 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: STP and 7 hops [7:44408]
At 07:29 PM 5/18/02, Brian Hill wrote: First, the slot time (64 byte time) isn't much of an issue unless running at 100Mbps or faster, and isn't an issue at all running full duplex. Slot time is an issue for all CSMA/CD networks, regardless of transmission speed. It is certainly discussed as a fundamental issue in all versions of IEEE 802.3 from the first in January 1985. It is also discussed in Ethernet Version 2.0 from 1982. In fact, hold it here.. I also have a copy of the Ethernet memo written by Bob Metcalfe in 1973. (This is the memo where the term Ethernet was first used. It's more than a memo. It's the actual specs.) Yes, it talks about slot time too. Oh, and here's the original patent from 1977. Yup, in there too. The 5-4-3 rule came about when people were trying to design 10Base5 networks, i.e. 10 Mbps with 500-meter thick coax segments in the 1970s and 1980s. The rule was an oversimplification of the actual requirements to help people design networks with Ethernet repeaters while maintaining the 512 bit slot time. Nobody used the term full duplex or half duplex in a conversation having to do with Ethernet. Ethernet was multiaccess. Stations sensed the carrier before sending, and there was a goal that only one station should send at a time, so that's sort of like half duplex, which was a phrase mostly used to characterize a two-wire point-to-point connection between a terminal and a host. The term half duplex wasn't used to describe Ethernet until the full-duplex interfaces started popping up. Half duplex was applied retroactively to the older types of interfaces that did normal CSMA/CD. (I just checked the Ethernet Version 2 specification and the original IEEE 802.3 specification and couldn't find any mention of the phrase half duplex. I don't have them in soft copy though, so I'm limited to the scanning my eyes can do.) As I remember it, the problem with the slot time is that at 100Mbps, the slot time drops to something like 5.12 ms The slot time is 5.12 microseconds for 100 Mbps Ethernet. That is indeed an issue. That's why it's rare to implement 100 Mbps Ethernet with repeaters. , which necessarily reduces the repeated network's diameter, as if a packet collides after the slot time it becomes a late collision, and the original host may not properly detect the collision. I seem to remember that there always being a recommendation, however, not to repeat the packet more than 5 times due to S/N Ratio problems creeping in after amplification. I can't find any mention of what you're saying, and I also question the premise. Ethernet repeaters do digital regeneration of the signal. I don't think they amplify noise. They clean up the digital signal. As for the switch vs. hub thing, I seem to remember based on the S/N thing that anything that amplified the original signal caused this problem. Regardless, it's completely out of the realm of a discussion on how switches behave. I also remember the distinction that under normal circumstances, the switch does not modify the packet in any way (L3/4 switches and trunking excluded). True, but think about the meaning of the sentence. Layer 2 switches don't modify the packet (frame). We aren't talking about signals here. We're miles above that. However, thinking about it, based solely on the switching mode, it seems that all switches (and even a lot of hubs now) buffer the packet in RAM and then forward it, which means, as someone stated, that the packet is rebuilt. A hub that did that wouldn't really be a hub. The extra delay would cause a problem, for one thing. Of course a switch rebuilds the frame. Even a cut-through switch obeys CSMA/CD rules. Even though it starts forwarding as soon as it recognizes the destination address, it does sense carrier first and then monitor for collisions and retransmit if necessary. (It does store the frame, just in case). We discussed this in great detail a few weeks ago. So, I agree with most of what you have said after all, with the exception of the S/N ratio having nothing to do with it. I do remember reading that the S/N ratio degradation was an issue after many amplifications of the original signal. As for me drinking by the pool, no, I am out of town at present At the pool, though? A lot of hotels have pools. ;-) Seriously, there's no point in a having a philosophical walk down memory lane on this topic. It's still in my locally-used cache, plus I have mountains of documentation to back up what I say (not being at a hotel). Unless we get into the bowels of the physical sublayers, in which case I will relinquish my nerd hat, not being an electrical engineer, I respectfully suggest that discussing the other issues is a waste of bandwidth. Priscilla , and just rattled off the reply. As I hope I have shown, I did have reasons for what I said, just perhaps the weren't thought out well enough. Brian Hill CCNP, CCDP, MCSE 2000 (Charter Member),MCSE+I
RE: STP and 7 hops [7:44408]
Look Priscilla, I am not trying to get in a pissing contest here, and I am not trying to waste bandwidth (though I don't know if I would worry about that, considering all of the things that flow through the Internet). All I was attempting to do was clarify what I was thinking, and I ADMITTED that you were correct on most of the issues. However, I also know what I have read and been told through the years, and while that doesn't include the original IEEE documents, it DOES include quite a few books, including a number of Cisco books. Perhaps they are wrong and you are rightI am fine with that, but I would like to clarify my understanding if that is the case. Therefore, I will (try) to make some succinct comments on your statements below: Priscilla Oppenheimer wrote: At 07:29 PM 5/18/02, Brian Hill wrote: First, the slot time (64 byte time) isn't much of an issue unless running at 100Mbps or faster, and isn't an issue at all running full duplex. Slot time is an issue for all CSMA/CD networks, regardless of transmission speed. It is certainly discussed as a fundamental issue in all versions of IEEE 802.3 from the first in January 1985. This would be the first time I have heard this statement. I was under the impression that the slot time's primary purpose was to facilitate collision detection. In other words, that the slot time represented the length of time an Ethernet host listened to it's own packet to detect a collision. Is this not true? If it is true, how does the slot time have anything to do with full duplex Ethernet? , which necessarily reduces the repeated network's diameter, as if a packet collides after the slot time it becomes a late collision, and the original host may not properly detect the collision. I seem to remember that there always being a recommendation, however, not to repeat the packet more than 5 times due to S/N Ratio problems creeping in after amplification. I can't find any mention of what you're saying, and I also question the premise. Ethernet repeaters do digital regeneration of the signal. I don't think they amplify noise. They clean up the digital signal. Ahah, then I stand corrected yet again. I did a search on this, and found out that an analog repeater simply amplifies the signal, thus retaining any noise caused by attenuation, while a digital repeater actually regenerates the signal, essentially recreating the packet in the process. In most of the books I have read regarding basic Ethernet functionality, they simplify it by claiming that all repeaters simply amplify the original signal, which would retain any noise already present in the original. However, this new information regarding repeating is according to BICSI, which I would assume is correct, and matches up with your statements. As for the switch vs. hub thing, I seem to remember based on the S/N thing that anything that amplified the original signal caused this problem. Regardless, it's completely out of the realm of a discussion on how switches behave. Agreed, I was simply trying to explain my original thinking. The thought process was that a switch simply operated on the same principle as a hub, with the exception of the fact that it recognizes and forwards based on MAC. In other words, my thinking was that a switch amplified the signal like a hub, when in truth, neither do...They both rebuild the signal. I also remember the distinction that under normal circumstances, the switch does not modify the packet in any way (L3/4 switches and trunking excluded). True, but think about the meaning of the sentence. Layer 2 switches don't modify the packet (frame). We aren't talking about signals here. We're miles above that. Yes, but I was thinking of the easiest way to do this electrically, again, by amplifying. However, thinking about it, based solely on the switching mode, it seems that all switches (and even a lot of hubs now) buffer the packet in RAM and then forward it, which means, as someone stated, that the packet is rebuilt. A hub that did that wouldn't really be a hub. The extra delay would cause a problem, for one thing. Priscilla, I can't find the logic in this. If the hub doesn't buffer the frame, I don't see any way it could possibly rebuild it. I mean, from what I can tell, either the hub amplifies the original signal (which you and documentation state is untrue), or it has to somehow record the incoming signal (into RAM?) and then send the regenerated signal back out, doesn't it? I am not talking about buffering the entire packet, therby increasing the delay, my thought process was simply that either it sent the signal on, amplifying it, or it stored and analyzed the signal, then forwarded it back out. This is based on my understanding that the signal itself is analog, even if it is represented digitally. In other words, either you can increase the analog signal by running it through a circuit, or you have to
