BGP looking glass showing AS twice ??

[taimoor4engg]

Hey Guys, 

Need assistance in BGP looking glass, i check my transit routes in BGP looking glass and its showing AS twice.

BGP routing table entry for 88.196.6.0/19
Versions:
  Process           bRIB/RIB  SendTblVer
  Speaker          158007868   158007868
Last Modified: Apr  9 04:53:06.616 for 12:32:04
Paths: (18 available, best #6)  Advertised IPv4 Unicast paths to update-groups (with more than one peer):
    0.2 0.4 0.6 0.13 
  Path #1: Received by speaker 0
  Not advertised to any peer
  1299 13761 3603 3603
    62.115.156.248 from 62.115.156.248 (2.255.254.40)

My AS path showing twice, 1299 13761 3603 3603.


What is that mean if I see my AS twice ?

[gachilleas]

If you have not configure it in your route-map out (as-path prepend etc) probably your ISP is doing this manipulation (to distinguish between backup paths etc).

No need to worry about this.

[taimoor4engg]

Actually we do prepend, it is still showing.

match ip address prefix-list 19_ONLY
set as-path prepend 3603

[Richard Burts]

Perhaps there is something here that I do not understand and if so please provide clarification. If you are doing prepend on your route advertisements then why are you surprised to see your AS in the path twice? That is what prepend does.

[gachilleas]

My guess was right... this is the meaining of prepend.

Prepend means you add your as path as many times as you define.

In your configuration you add your as path one more time... so you will have it two times (one is be deafult in bgp advertisements).

Kind regards

[Giuseppe Larosa]

Hello @taimoor4engg ,

>> set as-path prepend 3603

this means adds one instance of ASN  3603 to to the AS path before sending it out , this is why you see two instances of  ASN 3603 in looking glass in the AS path attribute.

This is exactly how AS path prepending works as noted by @Richard Burts  and @gachilleas .

Hope to help

Giuseppe