is it possible to know why would BGP prefer path over the other !?

mohammed hashim · ‎04-27-2017

Hi,

if there are multiple paths learnt over BGP, BGP then will do best path selection process, is there any way to see which of the sequence of the process was selected (preferred) ??

R2#show ip bgp 8.8.8.8
BGP routing table entry for 8.8.8.8/32, version 124
Paths: (2 available, best #2, table default)

Advertised to update-groups:
2 3

Refresh Epoch 1
200, (Received from a RR-client)
10.0.37.7 (metric 3072) from 10.0.37.7 (100.0.0.7)
Origin IGP, metric 0, localpref 100, valid, internal
rx pathid: 0, tx pathid: 0

Refresh Epoch 1
200, (Received from a RR-client)
10.0.12.1 from 10.0.12.1 (100.0.0.1)
Origin IGP, metric 0, localpref 100, valid, internal, best
rx pathid: 0, tx pathid: 0x0

“We Love Oranges AS Oranges Mean Pure Refreshment”
W	Weight (Highest)
L	LOCAL_PREF (Highest)
O	Originate (local)
AS	AS_PATH (shortest)
O	ORIGIN Code (IGP > EGP > Incomplete)
M	MED (lowest)
P	Paths (External > Internal)
R	RID (lowest)

Georg Pauwen · ‎04-27-2017

Hello,

the 'show ip bgp network' command should show you the entries in the BGP routing table for a given network, and the logic as to why a specific path was selected. Below is an example:

Example: BGP Best Path Selection

http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html#anc3

Masoud Pourshabanian · ‎04-29-2017

Hello,

The output of BGP command in Nexus tells you why the route is the best but in IOS, you need to investigate yourself. Weight, Local Preference, Originate, As-path, Med , Paths are easy to identify in output which is line by line per route..

For IGP metric,RID, you need to see the detail for each route in BGP table.

Masoud

Masoud Pourshabanian · ‎04-29-2017

I totally ignored your example in my previous comment.

As Julio mentioned,

First check the metric for IGP next hop in your routing table. The lowest is preferred

show IP route 10.0.37.7

show IP route 10.0.12.1

If they are equal which I think they are not, go the RID. The lowest RID is prefered

Masoud

Julio E. Moisa · ‎04-29-2017

Hi

It could be by lowest router-id (100.0.0.1) also check why it is showed: (metric 3072)

Also a show ip bgp could be useful to see the as-path

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