Cisco uRPF failures

Jo Rhett jrhett at
Thu Sep 4 11:35:34 CDT 2008

(changing subject line)

On Sep 3, 2008, at 7:06 PM, Rubens Kuhl Jr. wrote:
>> This statement is patently false.  The uRPF failures I dealt with  
>> were based
>> entirely on the recommended settings, and were confirmed by Cisco.   
>> Last I
>> heard (2 months ago) the problems remain.  Cisco just isn't being  
>> honest
>> with you about them.
> Would you mind telling us what is the scenario so we can avoid it ?

That's the surprising thing -- no scenario.  Very basic  
configuration.  Enabling uRPF and then hitting it with a few gig of  
non-routable packets consistently caused the sup module to stop  
talking on the console, and various other problems to persist  
throughout the unit, ie no arp response.  We were able to simulate  
this with two 2 pc's direction connected to a 6500 in a lab.  If I  
remember right, we had to enable CEF to see the problem, but since CEF  
is a kitchen sink that dozens of other features require you simply  
couldn't disable it.

We also discovered problems related to uRPF and load balanced links,  
but those were difficult to reproduce in the lab and we couldn't  
affect their peering, so we had to disable uRPF and ignore so I don't  
have much details.

I kept thinking that this was a serious problem that Cisco would  
address quickly, but that turns out not to be the case.  To this day  
I've never found a network operator using uRPF on Cisco gear.
   (note: network operator. it's probably fine for several-hundred-meg  
enterprise sites)

Jo Rhett
Net Consonance : consonant endings by net philanthropy, open source  
and other randomness

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