Re: Undocumented iBGP Behavior (Confirmed by Cisco)

From: Przemyslaw Karwasiecki (karwas@xxxxxxxxxxx)
Date: Tue Feb 05 2002 - 22:45:27 GMT-3

   
Looking closer at routers behoviour in my lab i still belive
that what you see is perfectly normal:

1) In phase 1 both routers suck all routes from AS701 and AS1.
2) In phase 2 they send to each other via iBGP connection
   all routes received from eBGP peers.
3) In phase 3 scanner (or whatever internal name is) is running
   over both adj-rib-in and selecting best routes
4) In phase 4 best routes are moved to loc-rib
5) In phase 5 some of eBGP routes which has lost
   in BGP selection in phase 3 and has been advertised
   over iBGP in phase 2 needs to be withdrawn 

---

In case when your left router receives from right router
better route via Genuity it will keep it, but it will
not advertise it back to right router because it was
learned via iBGP and it is not acting as route reflector.

This is the reason why you see that right routers is
receiving only 700 routes from left router.....

Browse through routing table on left router.
My guess is that all 103300 rotes there is via AS1.

Left router will not advertise routes which are not
selected (UUNET) and is not advertising back routes
learned from right rouer via iBGP.

Am I right, or completly mistaken?

Przemek

On Tue, 2002-02-05 at 20:18, W. Alan Robertson wrote:
> ----- Original Message -----
> From: "Ouellette, Tim" <tim.ouellette@eds.com>
>
>
> > The 2nd router that only has 700 routes in it's routing
> > table that it learned from it's IBGP still has the other
> > 103k routes in it's adj-rib-in from it's ebgp peer right,
> > they are just sitting dormant?  So if the other router
> > somehow lost it's ebgp peer, it'll send withdraws to the
> > ibgp peer and the other guy will take over with 104k
> > routes correct?
>
> Exactly...
>
> > Could you define what you meant buy "if an iBGP peer
> > learns that another iBGP peer already has a better route
> > to a specific prefix,  it will issue a withdrawl to that
> > peer for the prefix(es)."
>
> Let me see if I can articulate that a little better...
>
>
>   [ eBGP ]     [ eBGP ]
>   [AS 701]     [ AS 1 ]
>       |           |
>   104k|           |104K
>       |           |
>       |           |
>   [ BGP  ]     [  BGP ]
>   [AS "X"]     [AS "X"]
>       |           |
>       |  <-104K   |

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