From: Guyler, Rik (rguyler@shp-dayton.org)
Date: Thu Oct 27 2005 - 12:36:37 GMT-3
Don't forget the dedicated link between DSW's. This prevents the DSW's from
bringing both sides of the HSRP pair to active. Without this link (I prefer
an Etherchannel for some redundancy) you are right...both sides could go
active and then you would cause problems for the clients on the ASW's.
Also, I didn't see in your description but all ASW stacks should have at
least one link back to each DSW and of course each DSW has at least two
links into the core on different devices. The number of VLAN's per each ASW
stack really doesn't matter although there are practical limitations that
should be observed. Each of my stacks have at least an end-device VLAN and
a management VLAN. In some cases there may be an additional end-device VLAN
or two as well but usually no more.
When you approach the design of this area, break things down into
distribution blocks and consider each block a small network unto itself.
Beyond that block into the core you should be Layer 3 only so in essence you
are taking several L2/L3 networks (distribution blocks) and joing them
through the core. There are also other Layer 2 considerations within the
distribution block, such as STP optimization that should be addressed in
coordination with HSRP.
I just replaced all of the Sups in my DSW's (4000/4500) during daytime hours
without anybody even noticing so the redundant nature of the design is
solid.
-----Original Message-----
From: Carlos G Mendioroz [mailto:tron@huapi.ba.ar]
Sent: Thursday, October 27, 2005 11:07 AM
To: ccielab@groupstudy.com
Subject: OT: HSRP in multilayer switching architecture
Hi,
I'm back again with this topic, "HSRP backtraffic howto" so to say.
While reading (again) the campus network design paper, I see that you can
use HSRP to have default GW for ASWs (Access switches) to reach the world in
case of DSW (Distribution switch) failure.
So basically ASWs do have dedicated IP subnets (usually two for load
balancing) and then is a DSW goes down, the remaining DSW (they go in
pairs) takes the whole traffic.
Great.
But what happens if a ASW - DSW link goes down ?
HSRP still works because DSWs are linked by the failing link, so both
pretend to be active (isolated I guess) and the one talking to the ASW does
the job. But what about the traffic going to the ASW ? Both DSW are layer 3
active on that IP subnet, and unless there is hardware indication of the
link down at the switch, traffic will be half dropped.
Am I missing something ?
Regards,
-- Carlos G Mendioroz <tron@huapi.ba.ar> LW7 EQI Argentina
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