Re: something strange about bgp community

[Max Tulyev <maxtul () netassist ua>]

Hi Song,

that's normal. This is a signat to AS702 to do some special with this
route when it will see this route.

Community is transistive by its design, so you don't need to be directly
connected to AS702 to send some signal to it.

For example:

you->AS101-AS102-AS103-target
  \->AS104-/\/\/\-AS105-/

where /\/\/\ is disrupted path with packet loss. AS104-AS105 is better
as AS-PATH lengh. If "target" accepts community, I can add
"target:notaccept" to AS104 and avoid that path.

Some brain-dead ;) transit providers clear all community when import
announces, but for me it is at least unfair.

On 07.01.15 15:35, Song Li wrote:
> Hi everyone,
>
> Today when I check one route in Routeviews I find something strange as
> follows:
>
> route-views>sh ip bgp 176.108.0.0
> BGP routing table entry for 176.108.0.0/19, version 23405621
> Paths: (33 available, best #28, table default)
>   Not advertised to any peer
>   Refresh Epoch 1
>   202018 35320 35320 57800
>     5.101.110.2 from 5.101.110.2 (5.101.110.2)
>       Origin IGP, localpref 100, valid, external
>       Community: 702:120 2914:429 20485:52990 20485:53990 20485:54040
> 20485:54050 47541:10001
>       rx pathid: 0, tx pathid: 0
>
> the AS-Path is "202018 35320 35320 57800" but the community is 702:120
> 2914:429 20485:52990 ....
>
> According to RFC 1997, the community format is AA:NN and AA means the
> AS#. Here, AS702, AS2914 and AS20485 do not appear in the AS-Path and as
> a result they should not appear in the community. Could anybody tell me
> what's the reason they do appear in the community of this route?
>
> Thanks!