BGP Dampening question

Fri Jul 16 05:41:10 UTC 2004

On Jul 15, 2004, at 11:46 PM, D Train wrote:

> I was needing to know if anyone could assist in helping me find a 
> solution to a problem I am experiencing. Here is the scenario:
>
> I have an AS 20, that has 2 circuits one to city A, and one to City B. 
> City A and City B are in another AS, lets say AS 1. In my AS 20, I am 
> learning the default route via EBGP, from City A,through my primary 
> link, and also have a static route configured to traverse my secondary 
> link, to City B. If I keep seeing the physical connection to City A 
> flapping, of course bgp will flap, but will I be able to use route 
> dampening to control the instability in AS 20? Will I be able to tweak 
> route dampening to where I will be able to just use the secondary for 
> say a set time, before it will try to use the primary link, even if 
> this connection is continuously flapping? I am hoping that I will be 
> able to tweak dampening to where it will just use my secondary link, 
> until I can fix my primary link, w/o having to manually shut the 
> interface, or shut bgp?
> I apologize if this is a bit off topic...

First, this is about the most on-topic post I've seen in a while.

Second, yes, you can do what you want with flap dampening.  Your router 
will penalize the announcements from A for every time it flaps, and 
will wait until it has stopped flapping for a user definable time 
before sending packets to A again.

That said, I would not use flap dampening for this.  If A is flapping 
THAT much, time to get another provider, or another local loop.  If it 
only flaps occasionally, not a big deal, the routers will handle it.

Besides, since you only have connectivity to one AS, there is really no 
need for you to announce to the global table at all.  Just use a 
private AS to get the routes and have the other AS originate your CIDR 
from their AS.

You can still get the default route, still static to the backup link.  
If the circuit to A flaps, routing will converge quickly.  You can 
tweak the timers to do it very quickly, since no one else is listening.

In fact, you do not even need BGP.  This set up is simple enough to use 
anything else - even RIP.  (I'm serious - this is a trivial routing 
exercise, so one could even argue the most brain-dead protocol is best 
suited for it.)

-- 
TTFN,
patrick