Subject: RE: Active Route -- OT

From: "Natale, Jonathan" <[email-masked]>
Date: Wed, 2 Oct 2002 13:29:34 -0400

"Synchronise implies that the BGP next_hop is reachable via the IGP
*not* the BGP prefix itself."

Wrong. Synchronized (Cisco Term) implies that the route (set of
destinations) has been learned via an IGP (including connected and direct).
IBGP learned routes must be synchronized to be advertised via EBGP, Post
12.0(6???) added that un-synchronized routes may not be used for local
forwarding. Synch can be config'd off.

NEXT_HOP must always be resolvable, no way to config this off.

-----Original Message-----
From: Curtis Villamizar [mailto:[email-masked]] 
Sent: Wednesday, October 02, 2002 12:33 PM
To: [email-masked]
Cc: BGP mailing list
Subject: Re: Active Route 

In message <[email-masked]>, [email-masked]
writ
es:
> 
> Then I redistribute the routes into IGP, and at the same time I forward
> the routes via IBGP (not full mesh) to all of the other AS border routers
(bu
> t
> to no other routers).

This is an incredibly bad idea.  Only the BGP next_hop needs to be in
the IGP.

> At the other side of my AS, I allow the border router (router B) to pass
thos
> e
> routes learned via IBGP (form router A) to the (next) AS via an EBGP
connecti
> on.
> (I am using the  "synchronise BGP = TRUE" option.)

Synchronise implies that the BGP next_hop is reachable via the IGP
*not* the BGP prefix itself.

There is never a need to put the destination prefix into the IGP if it
is carried in IBGP except for the case where you are originating a
prefix and you want it to go out with specific attributes such as
specific BGP communities or indicate through BGP communities some
special action such as punching a hole in an aggregate.

> Now in this fairly straightforward example, the router B will forward
routes
> learned via IBGP to the next AS using EBGP, whereas for those routes the
> reachability in the FIB is taken care of by IGP (who has typically a
higher
> preference in ther FIB then IBGP).
> 
> So, in theory the rule : "one must focus on the rule that a BGP speaker
> advertises to its peers  in neighboring ASs only those routes that it
itself
> uses." is broken, since it is clearly an IGP route which in the FIB.
> 
> ---
> Hans de Vleeschouwer

The only way to fully address all of the instances of this sort of
objection is to add to the spec "Most BGP implementations allow the
user to do things that violate the protocol specification in minor
ways, sometimes with valid uses, but also sometimes with questionalble
uses and sometimes with dire consequences".  Although this statement
is true, I don't don't think we need to add this.

Curtis

---

From: Curtis Villamizar <[email-masked]>
Date: Wed, 02 Oct 2002 18:06:24 -0400

In message <[email-masked]
> NEXT_HOP must always be resolvable, no way to config this off.

Maybe I'm wrong on how Cisco uses "synchronized".  Last I had the
discussion on whether to enable synchronized was a long time ago and I
quite sure that at that time the Cisco recursive lookup had to resolve
to a next hop before the route could be advertised.  I could have been
wrong then, confused with the corresponding gated restriction, or they
could have changed it.

If so I stand corrected.

Curtis