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I am trying to understand the inner workings of HSRP.

During a failover, on Wireshark I see "Interface State TLV". I get what is happening in the "Group State TLV", but during the interface state TLV, it goes:

Interface State TLV (Act=1 Pass=0)

every so often, then during a failover it goes through the sequence:

Interface State TLV (Act=1 Pass=1) Interface State TLV (Act=0 Pass=1)

How does it function during the "Interface State TLV (Act=1 Pass=1)" stage, or does it all happen so fast it doesn't matter?

  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer. – Ron Maupin Dec 15 '19 at 5:29
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I believe, It is not a problem when you have "Act=1 Pass=1" for some milliseconds. I believe, you can see this from both routers simultaneously during failover transition. In this very short moment, both routers are active (packet goes to both of them for virtual ip will be passed).

It prevents the situation, when (during some milliseconds) both routers declared themselves as passive, so some packets can be dropped.

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