A, B, C, D belong to the same AS. IBGP neighbor relation is established between A, B, C, D and two RR. When the link beween B and D interrupts, the service traffic flows the link between A and C. After a while BGP neighbor relation between C, D and two RR is down.
After enabling simple classifier at the interface where there is block, BGP neighbor recovers.
There is link block because the traffic flows the link between A and C. BGP packet between two RR and C, D is discarded. Keepalive timeout neighbor of BGP is down.
Under default the router does not enable classifier. It regards forwarding traffic as BE traffic and forwards it. It is suggested to deploy simple classifier on core layer of similar topology. The router can recognize forwarding packet to ensure that protocol packet is first handled. It is suggested to DSCP clear operation at network edge based on network safety to avoid the attack of th packet with high priority.