BGP Notes

• BGP connection is a TCP connection on port 179
• Only the TCP server needs to approve the source of the packet it receives. Neighbor update-source is actually required only on one side, as long as correct IP addresses are being used
• When two routers attempt a BGP connection simultaneously, only one is preserved, the one from the router with the highest BGP router ID
• You can use the command "debug ip packet detail XXX" where XXX is an ACL matching the src/dst we are interested in
• iBGP learned routes are not advertised to other iBGP peers
• iBGP packets have TTL=255 while eBGP packets have TTL=1 by default
• If you see a message "... Active open failed - no route to peer..." it can be an indication of eBGP multihop missing
• Cisco eBGP performs the following independently
  1. Neighbor is directly connected on a known subnet
  2. Sends packets with TTL=1
• In order to form a neighborship between directly connected router using Loopbacks, you can use eBGP multihop, which actually disables #1 from the above. Alternatively you can use "neighbor disable-connected-check" to do the same.
• eBGP multihop command defaults to TTL=255
• BGP authentication is implemented through TCP option 19
• Unique BGP router ID within the BGP domain is required, in order to avoid traffic blackholes. That is due to loop prevention algorithm.