Because VLANs are what they are, virtual lans,
in other words many lan segments (self contained
broadcast domains). We're trying to accomplish
something in software, which was traditionally
implemented physically.
The Question 2 you is... What is the traditional
way of moving 1 packet from a lan segment to
another that doesnt share the same broadcast
domain? (i.e. Not just connected by a bridge or
layer 2 switch)
Answer: Routing.
Clients don't find IP address of other clients in
different broadcast domains. To them, they simply
don't exist. Only the common Router between them exists.
(Layer 2 is completely Ignorant of Layer 3). They only
ARP the IP address of the Router. Or should I say RARP.
They're usually configured with the gw IP already.
Wayne
-----Original Message-----
From: Bob Vance [mailto:[EMAIL PROTECTED]]
Sent: Wednesday, January 17, 2001 2:50 AM
To: CISCO_GroupStudy List (E-mail)
Subject: RE: why is routing needed with VLANs - ARP?
What I'm saying is that, before we implement VLANs, we have a flat
address space, with obviously, no routing.
Now, suppose that I arbitrarily decide not to forward broadcasts out
ports 6-10 through some IOS command.
Everything will still work quite happily (except anything relying on
those broadcasts, of course).
...
Ooops. I think that I just saw the answer.
One of those broadcast thingys is lil' ole ARP.
So, how does a client find the IP address of a destination if the
destination is outside the VLAN?
It's funny that this wasn't pointed out in any of my VLAN reading
(admittedly limited to ICND coursebook and Caslow).
It just arbitrarily says unicasts are blocked or routing is
required without giving a reason.
Oh, well.
-------------------------------------------------
Tks | <mailto:[EMAIL PROTECTED]>
BV | <mailto:[EMAIL PROTECTED]>
Sr. Technical Consultant, SBM, A Gates/Arrow Co.
Vox 770-623-3430 11455 Lakefield Dr.
Fax 770-623-3429 Duluth, GA 30097-1511
=================================================
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of
Bob Vance
Sent: Tuesday, January 16, 2001 11:35 AM
To: CISCO_GroupStudy List (E-mail)
Subject: why is routing needed with VLANs
OK.
I must be brain dead, today.
(and, yes, Chuck, I *have* had my morning dose of Diet Coke :)
and, yes, I know, "What's so special about 'today' "?
)
As far I can understand it so far, about the only benefit that I see
from VLANs is reducing the size of broadcast domains.
Suppose that I have a switch in the closet with one big flat address
space (well, it couldn't be that big with only one switch, now, could
it ?>). Then someone says,
"You know, we're getting a lot of blah-blah broadcast traffic.
Let's VLAN.
"
OK, fine. We VLAN and put whatever services in each VLAN that are
required to handle the broadcasts (e.g., DHCP service). So, now the
switch doesn't send broadcasts outside a particular VLAN.
But, what's so magic about a VLAN that the switch also decides not to
send unicasts outside a VLAN. Before the VLANs, the switch maintained
a MAC table and knew which port to go out to get to any unicast address
in the entire space. So, why can't it continue to do that after we
arbitrarily implement some constraint on broadcast addresses?
It seems to me that the same, exact MAC table, with an additional VLAN
field would not require that restriction. If it's a broadcast, send the
packet only out ports with a VLAN-id that matches the source port's
VLAN-id. If it's a unicast, handle it just like we used to.
Similarly, even if we have 5 switches, I just don't see the requirement
that we (as switch-code designers) must block unicasts and resort to a
routing requirement.
Even with 500 switches ... well, let's not get ridiculous :)
I feel that there is a simple point that I've overlooked, so I will
continue to RTFM while I await your responses.>)
-------------------------------------------------
Tks | <mailto:[EMAIL PROTECTED]>
BV | <mailto:[EMAIL PROTECTED]>
Sr. Technical Consultant, SBM, A Gates/Arrow Co.
Vox 770-623-3430 11455 Lakefield Dr.
Fax 770-623-3429 Duluth, GA 30097-1511
=================================================
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