anyone else seeing very long AS paths?

Rodney Dunn rodunn at cisco.com
Thu Feb 19 14:15:02 CST 2009


We are working on a document for Cisco.com but in the interim
here is the bug that will fix the issue of a Cisco IOS device
sending an incorrectly formatted BGP update when as a result
of prepending it goes over 255 AS hops.

Note: The Title and Release-note on bug toolkit may be a
bit different as I just updated it to be more accurate.

Of all the scenarios I've looked at (thanks to those that responded
offline) there wasn't a condition found where this could happen
without AS path prepending being used.

Please respond offline or let's move the discussion over to
cisco-nsp at this point.

CSCsx73770
Invalid BGP formatted update causes peer reset with AS prepending

<B>Symptom:</B>
 
 A Cisco IOS device that receives a BGP update message and as a result of AS
prepending needs to send an update downstream that would have over 255 AS hops
will send an invalid formatted update. This update when received by a
downstream BGP speaker triggers a NOTIFICATION back to the sender which results
in the BGP session being reset.
 
 <B>Conditions:</B>
 
 This problem is seen when a Cisco IOS device receives a BGP update and
 due to a combination of either inbound, outbound, or both AS prepending it
needs to send an update downstream that has more than 255 AS hops.
 
 <B>Workaround:</B>
 
 The workaround is to implement <CmdBold> bgp maxas-limit X <NoCmdBold> on the
device that after prepending would need to send an update with over 255 AS
hops. Since IOS limits the inbound prepending value to 10 the most that
could be added iss 11 AS hops (10 on ingress, 10 on egress, and 1 for normal
eBGP AS hop addition). Therefore, a conservative value to configure would be
200 to prevent this condition.
 
 

Full support of Section 5.1.2 of RFC4271 is being tracked under
CSCsx75937
Add BGP support of AS paths longer than 255 per Section 5.1.2 of RFC4271

Thanks to those that worked offline with us to verify the field results
reported.

Rodney




On Tue, Feb 17, 2009 at 05:27:01PM -0500, Rodney Dunn wrote:
> If you want to take this offline send it unicast or we could
> move it to cisco-nsp.
> 
> What scenarios are you seeing that appear broken other than
> when a notification is sent when a > 255 hop update is received?
> That's the one I'm working on right now.
> 
> Rodney
> 
> On Tue, Feb 17, 2009 at 05:31:49PM -0500, German Martinez wrote:
> > On Tue Feb 17, 2009, Rodney Dunn wrote:
> > 
> > Hello Rodney,
> > It will be great if you can share with us your findings.  It seems
> > like we are hitting different bugs in different platforms.
> > 
> > Thanks
> > German
> > 
> > > Ivan,
> > > 
> > > It is confusing but from what I have tested you have it correct.
> > > 
> > > The confusing part comes from multiple issues.
> > > 
> > > a) The documentation about the default maxas limit being 75 appears to be
> > >    incorrect. I'll get that fixed.
> > > 
> > > b) Prior to CSCee30718 there was a hard limit of 255. After that fix
> > >    AS sets of more than 255 should work.
> > > 
> > > c) CSCeh13489 implemented the maxas command to mark it as invalid and
> > >    not send.
> > > 
> > > 
> > > There does appear to be an issue when you cross the 255 boundary
> > > and the next hop router sends a notification back.
> > > 
> > > I've got it recreated in the lab and we are working to clearly understand
> > > why that is. I'll post an update once we have more.
> > > 
> > > The way to prevent it is the upstream device that crosses the 255 boundary
> > > on sending needs to use the maxas limit command to keep it less than 255.
> > > 
> > > It doesn't work on the device that receives the update with the AS path
> > > larger than 255.
> > > 
> > > Rodney
> > > 
> > > On Tue, Feb 17, 2009 at 08:58:48PM +0100, Ivan Pepelnjak wrote:
> > > > > We were dropping ALL prefixes and the eBGP session was still 
> > > > > resetting. 
> > > > 
> > > > Upstream or downstream?
> > > > 
> > > > > 1) "bgp maxas-limit 75" had no effect mitigating this problem 
> > > > > on the IOS we were using. That is: it was previously verified 
> > > > > to be working just fine to drop paths longer than 75, but 
> > > > > once we started receiving paths >
> > > > > 255 then BGP started resetting.
> > > > 
> > > > I was able to receive BGP paths longer than 255 on IOS release 12.2SRC. The
> > > > paths were generated by Quagga BGP daemon.
> > > > 
> > > > 12.2SRC causes the downstream session to break when the installed AS-path
> > > > length is close to 255 and you use downstream AS-path prepending.
> > > > 
> > > > In your case, I'm assuming you were hit with an older bug (probably at the
> > > > 128 AS-path length boundary). It would be very hard to generate just the
> > > > right AS-path length to unintentionally break your upstream EBGP session (as
> > > > I said before, it's a nice targeted attack if you know your downstream
> > > > topology).
> > > > 
> > > > If your IOS is vulnerable to the older bugs that break inbound processing of
> > > > AS paths longer than 128, there's nothing you can do on your end. The
> > > > internal BGP checks reject the inbound update before the inbound filters (or
> > > > bgp maxas-limit) can touch it and reset the upstream BGP session.
> > > > 
> > > > Hope this helps
> > > > Ivan
> > > > 
> > > > Disclaimer: as I don't have internal access to Cisco, all of the above is a
> > > > result of lab tests.
> > > > 
> 




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