HSRP and STP

Unanswered Question
Jun 30th, 2007

OK, here's a question regarding theory of operation.

Imagine you have 2 access switches with dual-homed L2 uplinks to 2 distribution/aggregation switches. You know, a typical campus vlan-type setup.

Now, imagine distribution switch 2 acts as the STP root bridge for, say, vlan 10. And also assume that the Active router for that same vlan is on switch 1.

STP will place the L2 trunk to switch 2 in a forwarding state because switch 2 is the root bridge for that vlan.

Now, if switch 2 is the active HSRP router for this vlan, how does the traffic make it to switch 1? I assume switch 2 will forward the vlan traffic to switch 1 via an L3 crosslink between them. Is that correct?

If so, it seems a little clumsy, no? Shouldn't the design engineer have configured the STP root bridge to also contain the HSRP active router for any particular vlan?

I have this problem too.
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tlizzerri Sat, 06/30/2007 - 04:39

CORRECTION:

Sorry, I had a typo in my initial post. The question should have read:

Now, if switch 2 contains the STANDBY HSRP router for this vlan, how does the traffic make it to switch 1, where the ACTIVE router for this vlan resides? I assume switch 2 will see itself as the standby and then forward the vlan traffic to switch 1 via an L3 crosslink between them. Is that correct?

chrihussey Sat, 06/30/2007 - 04:44

Yes that is correct.

As a general practice I have the root bridge be the active HSRP address in those configurations.

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