Symptom

When an L3out SVI interfaces has two ipv6 address configured, the inbound (passive setup) BGP TCP session can be closed immediately after three way handshake, as a result, ipv6 BGP session remains idle/down.

Conditions

There are three conditions to trigger this problem. 1. The version must be 15.2.3e train, 14.2-6h train does not have this issue. 2. ACI side must be BGP passive setup, meaning listen on TCP 179, achieve by prefix based neighbor or peer does not support BGP passive setup. 3. ACI side has two or more ipv6 address configured, usually configured for vPC pairs for HSRP like redundancy.

Workaround

Remove the secondary IPv6 address from ACI L3out interface, hence there is only one IPv6 address configured, to allow BGP session established, then add the secondary Ipv6 address back. However this workaround may need be re-applied if the leaf switch rebooted.

Further Problem Description