From: Howard C. Berkowitz (hcb@gettcomm.com)
Date: Tue Mar 30 2004 - 14:57:40 GMT-3
At 7:32 PM +0200 3/30/04, David Hurtado wrote:
>Hello everybody,
>
>I'm trying to understand the BGP new feature: BGP Prefix-Based
>Outbound Route Filtering, but the Doc Cd is not clear.
>
>Could somebody tell me some link or archive where this feature is
>better explained?
>
>Thanks for the help
Let me make some general comments about what it is trying to do, from
the NETF perspective, and steer you to several Internet Drafts.
Outbound Route Filtering, in general, is a method for making eBGP
networks more scalable. Several flavors have been proposed, and the
eventual goal is to have a single specification that covers:
prefix-based
AS-path based
community-base
ORF is really an extension to the functionality of BGP, ironically in
a way that finally gives some truth to misinformation commonly taught
about BGP. You've probably heard the phrase "BGP transmits policies."
Prior to ORF, it never did. What BGP transmitted was the information
on which policy-deciding features inside the router (i.e., part of
the protocol processing but not the protocol) could make policy
decisions. For example, MED from a single ASP suggests that a policy
decision to prefer certain peerings for certain routes is preferable,
but it isn't the policy of the receiving AS, which would actually
make the decision whether or not to honor MED.
ORF actually sends a set of filtering policies to another router.
The idea is that once a router gets the ORF rules from its peer, it
will save bandwidth and update processing by applying outbound deny
filters on its own interface, because the ORF sender has just said it
will deny those routes if it receives them.
ORF is one of those things, like egress filtering, that doesn't
necessarily benefit the router/AS that does it, but if the majority
of AS do it, everybody wins.
Some relevant Internet Drafts:
http://www.ietf.org/internet-drafts/draft-ietf-idr-route-filter-10.txt
http://www.ietf.org/internet-drafts/draft-ietf-idr-aspath-orf-06.txt
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