anyone else seeing very long AS paths?

German Martinez gmartine at
Tue Feb 17 18:54:34 UTC 2009

On Tue Feb 17, 2009, Michael Ulitskiy wrote:

CSCee30718 – it removes the default value of bgp max-as from the router.

The solution is introduced in CSCeh13489 

BGP shouldn't propogate an update w excessive AS Path > 255
Symptoms: A router may reset its Border Gateway Protocol (BGP) session.

Conditions: This symptom is observed when a Cisco router that peers with
other routers receives an Autonomous System (AS) path with a length that is
equal to or greater than 255.

Workaround: Configure the bgp maxas limit command in such
as way that the maximum length of the AS path is a value below 255. When the
router receives an update with an excessive AS path value, the prefix is
rejected and recorded the event in the log.

This workaround has been suggested previously by Hank.

Anyone knows about any possible CPU impacts in case that you implement 
bgp maxas?


> My bgp speaking devices are a couple of 7200s running 12.2(40). 
> Not the newest IOS out there, but it's been doing the job just fine up until yesterday.
> Yesterday, when that malformed announcement hit my routers they didn't crash, 
> but they did reset bgp sessions (even though I didn't accept the route) and they kept doing so 
> until I got my upstream to filter it out.
> According to cisco bug toolkit CSCdr54230 should be fixed in 12.2, so obviously it's not enough.
> Does anybody know what IOS version has fix this problem, 'cause I couldn't find this info at CCO?
> Thanks,
> Michael
> On Tuesday 17 February 2009 10:21:07 am Etaoin Shrdlu wrote:
> > Jared Mauch wrote:
> > 
> > > On Tue, Feb 17, 2009 at 08:07:36AM +0200, Hank Nussbacher wrote:
> > 
> > >>"They" will keep trying and until a vast majority of ISPs implement 
> > >>maxas, this will keep happening.
> > 
> > > 	Or until people who are still running multi-year old cisco code
> > > actually upgrade?  This seems to primarily impact:
> > > 
> > > 	1) Old cisco code
> > > 	2) PC based bgp daemons
> > 
> > > Both of which likely just need to be upgraded.  I actually suspect 
> > > that a lot of people who dropped their bgp sessions did not notice
> > > something happened, and still will not upgrade their code....I
> > > suspect these people don't even know they have a bgp speaking device
> > > anymore.
> > 
> > On the other hand, the fact that various entities have gone out of their 
> > way to advertise that they're running old hardware/out-of-date software 
> > has been noted elsewhere. I'd strongly suggest, if you're reading NANOG, 
> >   that you update, before someone less pleasant and friendly than myself 
> > finds you. Please.
> > 
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