This article explains about as-path loop & how to troubleshoot loop avoidance mechanism for asymmetric routing issues.

When we see routes going into hidden state & causing asymmetric routing issues. Redundancy test failing for particular prefixes only need to check that route extensive output.

Troubleshooting: Need to check if the below criteria points are matching.

1. Whenever customer is performing the redundancy test & traffic for one particular operator.
2. They have 2 BGP peers from operator where both the router are learning the routes & a vrrp in between them.
3. Initially suspected the issue to be with vrf which might not be taking traffic but that is not the case as we have multiple vendors connecting to the operator using same physical interfaces.
4. Check BGP route advertised & received from operator & the IBGP peer of 2 routers primary & secondary.
5. We saw that route is not being learnt at primary & gets struck in hidden state, even though secondary router is advertising which could be the possible issue for traffic not shifting during the failover as there is no route entry.
6. After checking the hidden route detail for one operator we saw the hidden reason to be unusable AS path as it was getting looped at 6479 which was its own AS number.
7. Now that we know the reason for hidden route being as-looped Need to configure as loops allow for family unicast inside IBGP group on R1 & later as a result route was active & dint go to hidden state.
8. The route receive counts also increased on R1 post the changes , end customer tried to ssh but still facing same issue & then when check same issue found on R2 router as well no routes present & that prefix was in hidden state.
9. Followed the same above step on R2 as well post configuration changes routes were active on both the routers now.
10. Customer now again disabled primary link & traffic was now successfully re-routed via the secondary link.
11. Reversed above step 10 & disabled 2nd link that time also traffic worked as expected.

Resolution: To Conclude it was asymmetry routing issues due to which traffic was not re routed whenever link went down. Now the same issue is resolved post making the hidden routes active.