Re: BGP unsupported capability code

[Danny McPherson <danny () tcb net>]

On Aug 17, 2006, at 10:57 PM, Joe Maimon wrote:
> If A tries to peer with B, and B sends a BGP capability 64 to A, if  
> A does not support that capability what would proper and/or  
> reasonable behavior for A be?

Speaker A MAY send a NOTIFICATION message with Open
Message Error/Error Subcode "Unsupported Capability" and
a data field listing capability 64 as the trigger for the
NOTIFICATION to speaker B (thereby terminating the session).
However, RFC 3392 does NOT require that the local BGP
speaker terminate the connection, which has particular utility
when considering the implications such a hard requirement
may otherwise impose on functions such as dynamic BGP
capabilities.

> (a "published" source for it, if you could possibly do so.)
>
> a) send
>
> unsupported capability code 64 lengh 6
> ## 2006-08-17 19:17:05 : [bgp/stack]:     send NOTIFICATION msg  
> code (Open-Error)
> subcode(Open Message Error: Unsupported Capability) to peer  
> w9.x4.y7.z9 via socket 415
>
> b) ignore it
>
> c) admin defined
>
> rfc3392.txt only appears to address the case where if A tries to  
> peer with B, and B sends a BGP capability 64 to A, if A does not  
> support that capability, B MAY terminate the connection with the  
> above notification.
>
> (section 3 paragraph 5)

If the peer doesn't support the capability and this is conveyed
from A to B via a NOTIFICATION message (as illustrated in the
log message above), then given the scenario you provide B
SHOULD NOT include the capability (64) in any subsequent
OPEN message sent to A when attempting to reestablish a
BGP connection -- IF B so chooses to attempt to automatically
reestablish a connection.  Note that the above says SHOULD
NOT, not MUST NOT.

I'm not quite sure what your point above is, as I think the
document sufficiently outlines the required behavior.

Although BGP is perhaps (?) still of interest to the NANOG
community, I suspect such protocol intricacies are not and
would submit that queries of nature are best directed to
idr () ietf org.

-danny