NetworkTalk & BGP

C.2.b) “Show ip bgp neighbor ip-address”

1. This command is useful in order to get:

a) Information about the TCP counter, timers and sessions and also the BGP parameters of the BGP session.
Information on any of the optional qualifiers referring to routes or paths is given. This feature is useful when you are troubleshooting path selection.

2. In the case of RT-B, we have three outputs to analyze.

a) The 1st give us paths information on the Ibgp session with RT-A.
We could say that as expected the best path is the local one, point to point connection with RT-A. Though, this is not the case because all prefixes received from RT-A are set with a LocalPref of 90.
Best path is the one coming from the static route, to Customer-G’s router (“11.11.224.3”), on which LocalPref has been left to its default value (100).
Remember that network 11.11.226.0/24 is not advertised to Ebgp speakers, if it was the case, the best path should be ISP-B Ebgp neighbor.
This command shows us its usefulness on how to check path selection of your traffic and your BGP peers.

Log C-2b-1

b) The 2nd one is about Ebgp session with ISP-B. Obviously the best path is through ISP-B’s Ebgp session. Remember that BGP table of ISP-B is set to LocalPref of 100, instead of 90 like for the ther BGP tables,

Log C-2b-2

c) The 3rd one is about Ebgp session with ISP-C. For the same reason; best path is ISP-B’s Ebgp neighbor since LocalPref is 100. To conclude, the best path by default for outgoing traffic in router RT-B is the one from ISP-B’s link.

Log C-2b-3

3. For additional information, in the case of RT-A, we have the following output.

a) The 1st one is about Ibgp session with RT-B.

Log C-2b-4

b) The 2nd one is about Ebgp session with ISP-D-Link1.

Log C-2b-5

c) The 3rd one is about Ebgp session with ISP-D-Link2.

Log C-2b-6

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