topic BGP AS-PATH preferred route in ExtremeSwitching (EXOS/Switch Engine)

BGP AS-PATH preferred route

Alexandr_P — Fri, 15 Dec 2017 21:39:00 GMT

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!

RE: BGP AS-PATH preferred route

Necheporenko__N — Sat, 16 Dec 2017 14:58:00 GMT

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

RE: BGP AS-PATH preferred route

Stephane_Grosj1 — Sun, 17 Dec 2017 06:18:00 GMT

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.

RE: BGP AS-PATH preferred route

David_Choi — Mon, 18 Dec 2017 08:56:00 GMT

Hi Alexandr,

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

RE: BGP AS-PATH preferred route

Alexandr_P — Mon, 18 Dec 2017 08:56:00 GMT

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!

RE: BGP AS-PATH preferred route

Alexandr_P — Mon, 18 Dec 2017 14:07:00 GMT

Thank you all for help!!!