Extreme congestion (was Re: inter-domain link recovery)

Thu Aug 16 10:24:34 UTC 2007

On Wed, Aug 15, 2007 at 12:58:48PM -0700, Tony Li wrote:
> 
> On Aug 15, 2007, at 9:12 AM, Stephen Wilcox wrote:
> 
> >>Remember the end-to-end principle.  IP backbones don't fail with  
> >>extreme
> >>congestion, IP applications fail with extreme congestion.
> >
> >Hmm I'm not sure about that... a 100% full link dropping packets  
> >causes many problems:
> >[...]
> >L3: BGP sessions drop, OSPF hellos are lost.. routing fails
> >L2: STP packets dropped.. switching fails
> 
> 
> It should be noted that well designed equipment will prioritize  
> control data both on xmit and receive so that under extreme  
> congestion situations, these symptoms do not occur.

Hi Tony,
 s/will/should/

The various bits of kit I've played with have tended not to cope under a massively maxed out circuit (I dont mean just full, I mean trying to get double the capacity into a link). This includes Cisco and Foundry kit.. not sure with other vendors such as Extreme or Juniper.

Often the congestion/flaps causes high CPU, which also can cause failure of protocols on the control plane.

Also, if you have something like router-switch-router it may be that the intermediate device looks after its control plane (ie STP) but for example sees BGP as just another TCP stream which it cannot differentiate.

Whilst it may be that control plane priority, cpu protection are features now available.. they have not always been and I'm fairly sure are not available across all platforms and software now. And as we know, the majority of the Internet does not run the latest high end kit and software..

Steve