From: Mike Taylor (mike.taylor@mail.com)
Date: Wed Sep 10 2003 - 13:04:00 GMT-3
Hi all. I'm hoping you'll take a look at this interesting campus network
design scenario, and apologize in advance for the length of this message.
We're working with a customer who has implemented a campus network design
quite unlike that which we've seen elsewhere. Their network design is very
close to that of a standard collapsed-core model (see figure 4 of the
following link if you need to know just what that is - beware of wrap):
http://www.cisco.com/en/US/netsol/ns110/ns146/ns147/ns17/networking_solutions
_white_paper09186a00800a3e16.shtml
Normally with this design, you'd configure the core devices to be the
spanning-tree roots for all VLANs. You would configure uplink-fast on the
access-layer switches to improve spanning-tree convergence times. You would
tune the HSRP active router to be the same as the spanning-tree root per VLAN,
etc, etc.
The network in question, however, has been configured such that the
spanning-tree roots for each VLAN are found on the access-layer devices.
Uplink-fast configuration has been moved to the core switches. Routing and
HSRP still happen on the core devices. Additionally, access-layer devices
(for the most part) support only a single unique VLAN (i.e. all client ports
on access-layer device X would be configured as access-links to VLAN25, and
device X would be the spanning-tree root for VLAN25).
Why was the network designed this way? Since the access-layer devices support
a single VLAN, there would be no opportunity to load-balance multiple VLANs
across separate trunks (as you would in a "normal" design). They moved the
spanning-tree roots for their VLANs to the edge so that both uplinks to the
core would be forwarding (and in their minds, load balancing). What really
happens is not true load balancing - all traffic sourced from devices
connected to the access-layer switch (and destined for some other broadcast
domain) follows the uplink towards the current HSRP active router (their
default gateway). Return traffic might come back to the access-layer switch
via either (forwarding) trunk. While this creates asymmetric traffic
patterns, we haven't found this to cause any issues and it does create a
pseudo load-balancing situation. In our lab, we tested convergence times for
various failover situations and found them to be in line with those of a
"normal" collapsed-core design.
Can anyone find gotchas with this design? We're having a hard time proving
that this design is worse or less efficient than a "normal" collapsed-core
model, even though the theory of the two designs are nearly opposite. What do
you big-brained people think?
Thanks!
Mike Taylor - CCIE #9658, MCSE, CNE
Network Engineer - Network Solutions, Inc.
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