2013年7月30日星期二

BGP ebgp-multihop

Question:

I have configured a test Catalyst Switches Price lab with BGP. I have two different AS setup with dual connections between the systems. I configured the loopbacks and update-source between the AS but BGP did not become active until I changed the ebgp-multihop count from default and set hop count to 2. My question is, is there a troubleshooting command I can use that will show me that the ebgp-multihop is not set correctly.

Answer:

If the ebgp-multihop is missing for an indirectly-connected eBGP neighbor, at the bottom of the show ip bgp neighbor X.X.X.X you will find these lines:

[ ... cut ... ]
  Connections established 0; dropped 0
  Last reset never
  External BGP neighbor not directly connected.
  No active TCP connection

There is the indication of the eBGP peer not being directly connected.

You may also be interested in reading the following thread about the ebgp-multihop and disable-connected-check that can be used specifically for loopback peerings between directly connected eBGP neighbors like yours.



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