Consider the below topology: enter image description here

Imagine that both the vPC peer keepalive link and vPC peer link have gone down. I am trying to understand how will that lead to unpredictable traffic flows or loops.

From what i have read on Cisco docs:

If the vPC keepalive link fails first and then a peer 
link fails, the vPC secondary switch assumes the 
primary switch role and keeps its vPC member ports up.
If the peer link and keepalive link fails, there could 
be a chance that both vPC switches are healthy and the 
failure occurs because of a connectivity issue between 
the switches. In this situation, both vPC switches claim 
the primary switch role and keep the vPC member ports up. 
This situation is known as a split-brain scenario. Because 
the peer link is no longer available, the two vPC switches 
cannot synchronize the unicast MAC address and the IGMP 
group and therefore they cannot maintain the complete unicast 
and multicast forwarding table.

I understand the above, but i want to further dig deep into how loop might occur.How will unpredictable traffic scenario occur? I understand that there will be two vPC primary switch(both of them will act as STP root).Can you please clarify by using a simple topology?

On further searching, i see the below post here

enter image description here From what i understood, since we have 2 switches acting as vPC Primary(and hence both of them will be STP root as well), the links will be in forwarding state and hence there are chances of loop. Is that correct understanding? I am also trying to figure out, if a packet is sourced from S1, how is that likely to undergo loop?

  • Can you clarify if the pair of 7K's at the top is also running VPC (i.e. 7K/5K running back-to-back VPC) or if those 7K's are independent? In either case where have the STP roots been placed (nb - best-practice would usually have them on the 7K's)? – rnxrx Jan 29 '17 at 20:18

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