multilayer switches allow for HSRP group reuse on different Vlans/SVIs
This is seen not as a problem but as a scalability workaround in order to overcome some limitations of low end multilayer switches that didn't support more then 16 groups in some cases.
About the MAC address reuse: this is not a problem in multilayer switches as they are able with TCAM tables to track the Vlan where the frame has been received so that the packet is processed by the correct SVI.
Said this, a potential for problem is still present in case two different broadcast domains are joined by accident.
In this case the Vlan multiplexing is not enough to distinguish in context frames and out of context frames.
This can cause high cpu usage and I have seen this also on C6500 with Sup 720 running 12.2(18)SXF code.
A possible workaround for this is the following:
reuse the same HSRP group number on different Vlans but use a different MD5 authentication on each, so that out of context frames ( in the case described above) are early discarded.
The HSRP Group number defines the last byte of the virtual mac-address of the HSRP. This mac-address should be unique withing a broadcast domain. and is used for the local communication between HSRP members. should be there any conflict within this address or different Mac address between HSRP members (virtual Mac), then there will be no HSRP for that Group.
How would multiple SVI's in a single HSRP Group effect failover? You would think an event within a VLAN could cause everything to failover in the HSRP Group including spanning tree loop, or even preempt on an SVI.
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