BGP dampening

[m.scano]

We have an eBGP peering in which we advertise a prefix via the network command, with a configuration similar to this one:

---

router bgp XXX

network 10.10.10.0 mask 255.255.255.0

!

ip route 10.10.10.0 255.255.255.0 Null0 254

---

The prefix 10.10.10.0/24 is also learned from our IGP.

Even though we have the static route, when the dynamic route goes down, BGP notice and delete the route, installing immediately after again (this time pointing to Null0 static).

This situation lead us to dampened routes.

Any experience/suggestion?

Thanks,

Maurizio

[m.lammerse]

Hi Maurizio,

AS-instabilities are often masked by route summarization. Is 10.10.10.0/24 the only subnet that your AS originates? Or is it possible for you to aggregate prefixes to, for example, 10.10.0.0/16? That way, your BGP peer would never notice the disappearing of 10.10.10.0/24.

HTH,

Marcel

[ruwhite]

Another option--and one that's possibly easier in this situation--is to get the IGP to advertise two /25's covering the address space, rather than the /24 itself. This will add one route to your IGP routing tables, but that's not much cost, I don't think, to solve this.

Then the BGP speaker will always be advertising off the /24 static to null0, and the route won't ever flap. It's the same idea, but rather than shortening the length of the static route, lengthen the prefix of the IGP routes that would normally match this route, if possible.

I find it pretty interesting that you're dampened after one flap--that's a pretty tight set of dampening parameters.

Russ.W

[m.scano]

That is a good idea and I will try it. Anyway, do you think that this is the correct behaviour for BGP?

Thanks,

Maurizio

[ruwhite]

"Anyway, do you think that this is the correct behaviour for BGP?"

The problem really isn't BGP, but rather a combination of two things:

-- How long it takes IOS to pick the static route up, install it, and get it advertised through BGP once the IGP route has fallen out of the routing table.

-- How your upstream's dampening parameters are configured.

I think there have been some recent improvements in the time it takes to install a floating static. You can adjust this some:

http://www.cisco.com/en/US/products/sw/iosswrel/ps1829/products_feature_guide09186a00802801fe.html

Which might help you in this situation.

:-)

Russ.W

[m.scano]

You're right, that's the workaround I use in other cases, but I also have particular prefixes that I can't summarize in any way.

Thanks for your help,

Maurizio

[jtantsura]

I think the first thing you should do is to talk to your upstream about their dampening policy.

It's not really common to configure dampening that aggressive towards customers.

Jeff