Thanks Jared, that was a good explanation.
I have yet a little more that 50% of the book left to read, but if I do not
feel secure about STP (and other BCMSN related stuff) when I'm done, I will
surely consider buying the Cisco LAN Switching book.
Take care,
Ole
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Ole Drews Jensen
Systems Network Manager
CCNA, MCSE, MCP+I
RWR Enterprises, Inc.
[EMAIL PROTECTED]
http://www.insync.net/~drews/ccnp
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-----Original Message-----
From: Jared Carter [mailto:[EMAIL PROTECTED]]
Sent: Sunday, September 10, 2000 12:45 PM
To: [EMAIL PROTECTED]; [EMAIL PROTECTED]
Subject: BCMSN: STP topology changes
Ole-
When the root switch sends out the topology change BPDU, it does not specify
that Switch E is no longer available. It simply flips the Topology Change
bit in every Configuration BPDU saying that there has been a topology change
SOMEWHERE. This lets the switches age out entries in Forward Delay seconds,
15 by default, much faster than the default 300 seconds. It sends these TC
BPDUs out for Forward Delay + Max Age seconds (a default of 35 seconds).
Once this time is up, it stops flipping the TC bit in the configuration
BPDUs.
The Cisco Press book by Clark/Hamilton, Cisco LAN Switching has two chapters
on STP that are undeniably the best around. Its worth reading if you're
taking BCMSN.
/Jared
-----Original Message-----
From: Ole Drews Jensen [mailto:[EMAIL PROTECTED]]
Sent: Sunday, September 10, 2000 10:35 AM
To: '[EMAIL PROTECTED]'
Subject: BCMSN: STP topology changes
I am not sure if it's me running on 50% power this Sunday morning (most
likely), if it's the book that is written like a white house speech, or if
it's both. Anyway, the reason for this e-mail is that I am reading something
that to me doesn't make sense.
For those of you who would like to comment on this, if you have Cisco Press'
BCMSN book, I am refereing to "Handling Topology Changes in Spanning Tree"
on pages 142 and 143.
If you do not have that book, here's a quick description:
Switch A is the ROOT bridge.
Switch B and C are both connected to Switch A.
Switch D is connected to both Switch B and C.
Switch E is connected to Switch D.
Since A and D can see each other through both Switch B and C, the connection
between D and C is blocked.
The situation is that Switch D's link to E fails, and Switch D sends a
notice to Switch A (the root bridge) via Switch B.
The root bridge (Switch A) now sets the topology change in its configuration
for a period of time equal to the sum of the fwd delay and max age
parameters.
According to step 5 (in the book): "A bridge receiving the topology change
configuration message from the root bridge uses the fwd delay timer to age
out entries in its address table. This allows it to age out entries faster
than the normal five-minute default so that stations that are no longer
available due to the topology change will be aged out faster. It does this
until it no longer receives topology change configuration messages from the
root bridge."
The last line in that quote makes it sound like it keeps receiving messages
about the link E being down until it's up again - but what if the Switch E
never comes up again because it was sold to someone else - will the root
bridge keep sending messages about it forever??? - I don't think so, but
that is how I read it.
Can someone please clarify this for me?
Thanks in advance,
Ole
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Ole Drews Jensen
Systems Network Manager
CCNA, MCSE, MCP+I
RWR Enterprises, Inc.
[EMAIL PROTECTED]
http://www.insync.net/~drews/ccnp
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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