[c-nsp] BGP sanity check

[Adam Vitkovsky]

If there's a need for R1 and R2 to exchange equal number of prefixes than
just enable "bgp advertise-best-external" on both routers

adam
-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net
[mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Chuck Church
Sent: Sunday, December 09, 2012 11:28 PM
To: 'Gert Doering'
Cc: cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] BGP sanity check

Thanks.  I guess in my mind the numbers needed to add up more.  Both routers
are taking a full table, which is more or less the same prefixes with
different path information.    R2 claims it's sending about 412K to R1, yet
R1 only sends 100K to R2.  I would think the number should add up
approximately close to the full table size.  This is significantly over that
430K number.  Would the fact that each router may get the same path length
for a prefix explain why some obviously are sent by each towards the other?

Chuck


-----Original Message-----
From: Gert Doering [mailto:gert at greenie.muc.de]
Sent: Sunday, December 09, 2012 4:47 PM
To: Chuck Church
Cc: cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] BGP sanity check

Hi,

On Sun, Dec 09, 2012 at 10:37:37AM -0500, Chuck Church wrote:
> I'm thinking this might be because RTR 2's eBGP has the better path to 
> most destinations compared to RTR 1, thus RTR 1 sees this and doesn't 
> send those prefixes back towards RTR 2.  Does that sound right?

Exactly so.

> Here's a prefix in question:
> RTR 2:
> BGP routing table entry for 209.209.144.0/20, version 3845458
> Paths: (1 available, best #1, table default)
>   Advertised to update-groups:
>      9
>   20115 1299 4323 10397, (received & used)
>     68.115.217.2 from 68.115.217.2 (96.34.212.228)
>       Origin IGP, localpref 100, valid, external, best
> 
> RTR 1:
> BGP routing table entry for 209.209.144.0/20, version 23114388
> Paths: (2 available, best #1, table Default-IP-Routing-Table)
>   Not advertised to any peer
> <-------------------------------------  This tells me it's not sent, 
> trying to figure out why

Because the only peer he could send it to is RTR 2, and that's where the
best path is learned from -> paths are never sent back to origin (and only
best path is ever announced to peers).

gert
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