Confuse about HSRP and STP

Unanswered Question
Oct 7th, 2007

Say i have two Distribution Switches "A" and "B". Switch A is using PVST+ for all Odd Vlan. Switch B is using PVST+ for Even Vlan.


Now I have one Edge Switch "C" connected to both Switches A and B. The current STP priority configuration for Switch A odd vlan is 60 and 61 for Switch B. For HSRP Switch A priority is 90 and 80 for Switch B.


Say we are using odd vlan 299. I know that it will choose Switch A to route all the packets.


1) However, if i swap the HSRP priority configuration of Switch A and B, will Switch C still use Switch A to route the packets?


2) What will be the effect of changing HSRP priority? Will it causes a loop?

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lgijssel Sun, 10/07/2007 - 21:56

HSRP priority configuration determines which port or vlan will become the HSRP active interface. Swapping the priority will cause the other switch to become HSRP active. In that case, the other switch (B) will forward your traffic on the IP layer. It will not cause a loop.

However, there is one catch: STP topology is unaffected by this change so the same uplink (i.e. from C to A) will remain in use because the path between C and B is STP-blocked at switch C. Subsequently, the packets must be switched over the link between A and B before switch B will be able to process them. This implies an extra Layer2-hop and more load on your backbone link. This is exactly why Cisco advises to keep STP and HSRP topologies consistent.


regards,

Leo

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