Re: revised draft of BGP-4

["Michael F. Nittmann" <[e-mail]>]

On Wed, 14 Jun 1995, Yakov Rekhter wrote:

> 
> Folks,
> 
> Today I sent to CNRI a revised version of BGP-4 spec.
> 
> The only difference between this version and the current protocol spec
> (other than changing dates and my affiliation) are the following two
> paragraphs:
> 
> 
> First paragraph:
> 
> 	If the OPEN message carries any other Optional Parameter (other than Authentication
> 	Information), and the local system doesn't recognize the Parameter, the
> 	Parameter shall be ignored.
> 
> Second paragraph:
> 
> 	The information carried by the AS_PATH attribute is checked for
> 	AS loops. AS loop detection is done by scanning the full AS path
> 	(as specified in the AS_PATH attribute), and checking that the
> 	autonomous system number of the local system does not appear
> 	in the AS path. If the autonomous system number appears in the AS path
> 	the route may be stored in the Adj-RIB-In, but unless the router is
> 	configured to accept routes with its own autonomous system in the AS path, 
> 	the route shall not be passed to the BGP Decision Process. 


Following case: assume a multihomed AS splits up due to internal 
connectivity loss. Both parts announce routes to systems with transit 
agreement. 
By accepting routes with the own AS path, the other part of the AS can be 
reached via the transit system.
this shouls be the default.

The limitation in this paragraph is only important for single homed AS 
systems, which are not the majority case.

I would suggest to invert this, that the default behaviour will be to 
accept routes with the same AS path, but not to duplicate the same route 
at all, e.g.

	1.0.0.0 is announced by two routers of AS 0 to AS 1 and AS 2 .
	
	AS 0 splits due to internal connectivity loss (split means, the 
	fallback link is too slow to take all traffic)
	
	Router A announces 1.0.0.0, router B (of AS 0) lost the route due to 
	the split and picks it up from AS 2, which has a transit agreement with 
	AS 0.


	Router B can announce to the severed part of AS 0 that 1.0.0.0 is 
	now not reachable best via the internal 'natural' link, but via AS 2.	



Mike


> 	Operations of a router that is configured to accept routes with its own autonomous
> 	system number in the AS path are outside the scope of this document.
> 
> 
> Please comment on this document as soon as possible, since we are planning to advance
> the document to a Full Standard at the Stockholm IETF.
> 
> Thanks.
> 
> Yakov Rekhter ([e-mail])
> 
> 
> 

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Michael F. Nittmann                                             [e-mail]
Network Architect						[e-mail] 
B3 Corporation, Marshfield, WI (CIX Member)               [phone] xt. 158
US Cyber (SM), Washington DC				  [phone]
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