Symptom
When the best path is from a router reflector client neighbor, bgp may block the update. if debug bgp update is turned you, you will see following in the debug: "Deny 1 UPDATE to filter-group 0.3 (regular, pelem regular) for ... ... reason cluster id not set for RR client: 1.1.1.1"
Conditions
This will only happen on a bgp route reflector when a bgp cluster id is not configured under router bgp.
Workaround
restart of process bgp may help. Explicitly configure the cluster-id will help.
Example:
route bgp 65000
bgp cluster-id 65000 <--configure a cluster-id
Further Problem Description
You may also see the BGP process restart and dump core, and BGP neighbor flapping after an ISSU SMU install
