[c-nsp] DDOS, router acted "oddly".

[Drew Weaver]

        I believe I know why I had the issue I had last evening when a 500Mbps DDOS hit our network. I believe it is due to queuing issues, but I am not sure, I wanted to ask you folks what you thought.

The topology of the 'attack ' is as such:

Attacker - Internet - 3Gbps aggregate(4 connections) - 2x Cisco GSR 12000 - 4x Gig-E - Catalyst 6509 - 100Mbps -- target host

So last evening we were hit with a 500Mbps DDOS attack, this shouldn't have been a big deal as we have over 3Gbps in aggregate bandwidth and this 500Mbps pushed our total utilization up to around 1300Mbps. However, we noticed that the DDOS was degrading connectivity for all hosts on the network.

* The (multiple) gig-e connections between the GSRs and the Catalyst 6509 were nowhere near their maximum capacity
* I see no errors in the log files of either of the two GSRs which were involved
* The 100Mbps port which the target host was connected to was obviously pegged.
* There were no errors logged on that particular catalyst (although I believe the problem is obviously with the GSRs)

I don't really see any "good?" reason why all of the traffic flowing through both of the GSR 12ks would have been reduced to a crawl unless there was some kind of queue backlash between the Catalyst and the GSR 12ks.

Does anyone have any advice or insight?

Thanks,
-Drew