Header Only - DO NOT REMOVE - Extreme Networks

BGP AS-PATH preferred route


Userlevel 6
Hello, all!

Does anybody faced with issue, BGP routes with the same Weight and LPref and Summit is choose preferred route with larger AS-PATH?
EXOS 15.6.2

For example:
X670G2 # sh bgp
routes detail network
123.123.123.0/24

Routes:
Route: 123.123.123.0/24, Peer 1.2.3.4
Origin IGP, Next-Hop 1.2.3.4, LPref 500, MED 0
Weight 3000,
As-PATH: ( 45002 )
Community: 32808:1 32808:2 32808:3


Route: 123.123.123.0/24, Peer 5.6.7.8, BEST, Active
Origin IGP, Next-Hop 5.6.7.8, LPref 500, MED 0
Weight 3000,
As-PATH: ( 45001 45002 )

Community: 32808:1 32808:2 32808:3

Thank you!

5 replies

Userlevel 4
Hello Alexandr,

Inside the BGP onfederation, the best path selection cannot be calculated with AS PATH length attribute. This can be taken care only by adding addition configuration like Local preference or Weight.
In your output shortest patch is selected based on Lowest Next hop IP address.

Best Regards,
Nikolay
Userlevel 7
To be more precise:

https://tools.ietf.org/html/rfc5065#section-5.3

"When comparing routes using AS_PATH length, CONFED_SEQUENCE and CONFED_SETs SHOULD NOT be counted."

If you don't remember what is CONFED_SEQ and CONFED_SET: https://tools.ietf.org/html/rfc5065#section-3

So, AS-Path is still used, but not with Member-AS. And you were looking only at member-AS, as the "()" stresses it in your output.
Userlevel 4
Hi Alexandr,

Just wondering if the next hop (1.2.3.4) is reachable (active) or not.
Userlevel 6
David Choi wrote:

Hi Alexandr,

Just wondering if the next hop (1.2.3.4) is reachable (active) or not.

Hello, David!

Both peers - 1.2.3.4. and 5.6.7.8 is reachable.
If one of them was unavailable, there would be no questions 🙂

Thank you!
Userlevel 6
Thank you all for help!!!

Reply