BGP troubleshooting basic issues

Active neighbor

It may happen that your neighborship is in state Active or Idle. It means that router has sent an open packet and is waiting for response. There could be several reasons for that:

  • neighbor is peering with a wrong IP address
  • neighbor doesnt have neighbor statement for this router
  • AS number is missconfigured
  • Interface you used in neighbor statement is down
  • There is no route to reach neighbor IP address or neighbor doesnt have route to reach you back!!

Because you can establish neighborship between hosts not directly connected you must verify if you got routing on the path how to reach your neighbor! Neighbors must have end to end IP reachability.

BGP issues with source IP address

  • when creating a BGP packet the neighbor statement defines the destination address and the outbound interface define the source address!
  • when a BGP packet is received for new BGP session the source address is compared to the list of neighbor statements:
    • if a match is found a relationship is established
    • if no match is found the packet is dropped
    • make sure that source address matches the IP address neighbor router is using in its neighbor statement
  • This happens when you are using loopback interfaces for neighbor establishment. But BGP packets use by default as a source IP address the outbound interface address. You can change this by command – neighbor {IP address} update-source {Loopback 0}