Question on BGP Best Path Selection (LOCAL )

[limtohsoon]

Hi Sir,

Attached is output of BGP table that I came across in a book. There's one prefix in the table with two paths.

I notice that the LOCAL_PREF attribute is not shown for routes learned via eBGP. My understanding of LOCAL_PREF is that it is configured local on the router and only exchanged within the same AS with other iBGP speakers.

Can anyone tell, Which criteria does BGP use to select the best path for the prefix 172.18.0.0? It chooses the iBGP route (next hop 172.30.255.150). In my opinion, it uses the LOCAL_PREF attribute.

I read the document "BGP Best Path Selection Algorithm" that path without LOCAL_PREF is considered as having the value set with the "bgp default local-preference" command, or 100 by default. So in my case, BGP is comparing LOCAL_PREF 300 (via 172.30.255.150) against LOCAL_PREF 100 (via 10.100.83.1).

Please correct me if I got the wrong concept.

Thank you.

B.Rgds,

Lim TS

[Harold Ritter · Accepted solution]

You are correct. The internal path (local-pref 300) wins over the external path (local-pref 100) because of its higher local-preference.

Hope this helps,

[limtohsoon]

Hi Harold,

Thanks for the helpful info.

Just one more question. I'd like to know the effects of configuring the command "bgp default local-preference 200". Below are my current understanding:

(1) All routes advertised to iBGP peers carry a LOCAL_PREF of 200.

(2) Routes learned via eBGP will assume a LOCAL_PREF of 200.

Again, kindly correct me if I got the wrong concept.

Thank you.

B.Rgds,

Lim TS

[Harold Ritter]

This is correct. It changes the default value from 100 to 200. eBGP learnt prefixes will automatically be assigned a local-pref of 200, which will be carried as these prefixes are advertised to iBGP peers.

Hope this helps,