bgp update destroying transit on redback routers ?

Jeff Tantsura jeff.tantsura at
Fri Dec 2 22:14:35 UTC 2011

Hi Alexandre,

You are right, the behavior is exactly as per RFC4271 section 6:
"When any of the conditions described here are detected, a
NOTIFICATION message, with the indicated Error Code, Error Subcode, and Data fields, is sent, and the BGP connection is closed.
So because ASN 0 in AGGREGATOR is seen as a malformed UPDATE we send 3/9 and close the connection.

Ideally it should be treated as "treat-as-withdraw" as per draft-chen-ebgp-error-handling, however please note - this is still a draft, 
not a normative document and with all my support it takes time to implement.

Once again, we understand the implications for our customers and hence going to disable ASN 0 check.

P.S. We have strong evidence that the update in question was caused by a bug on a freshly updated router (I'm not going to disclose the vendor) 


-----Original Message-----
From: Alexandre Snarskii [mailto:snar at] 
Sent: Friday, December 02, 2011 6:36 AM
To: Jeff Tantsura
Cc: nanog at
Subject: Re: bgp update destroying transit on redback routers ?

On Thu, Dec 01, 2011 at 04:56:43PM -0500, Jeff Tantsura wrote:
> Hi,
> Let me take it over from now on, I'm the IP Routing/MPLS Product 
> Manager at Ericsson responsible for all routing protocols.
> There's nothing wrong in checking ASN in AGGREGATOR, we don't really 
> want see ASN 0 anywhere, that's how draft-wkumari-idr-as0 
> (draft-ietf-idr-as0-00) came into the worlds.

This draft says that

If a BGP speaker receives a route which has an AS number of zero in the AS_PATH (or AS4_PATH) attribute, it SHOULD be logged and treated as a WITHDRAW. This same behavior applies to routes containing zero as the Aggregator or AS4 Aggregator.

but observed behaviour was more like following: 

If a BGP speaker receives [bad route] it MUST close session immediately with NOTIFICATION Error Code 'Update Message Error' and subcode 'Error with optional attribute'.

In theory, there is no difference between theory and practice. 
But, in practice, there is. 

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