Subject: RE: Active Route

From: "Natale, Jonathan" <[email-masked]>
Date: Wed, 2 Oct 2002 15:58:43 -0400

Dennis,

Thank you very much for taking the time to respond and for
actually understanding what I am [trying] to say.  I agree with
everything you say, particularly "it is the way some of the world works"
and this is why I am proposing the change. Also, the existing text does
not describe Juniper either since they "strictly conform to the
quoted behavior for both EBGP and IBGP", whereas the existing text
only applies EBGP.

I am not sure if you agree with the change or not.  On the one hand you
say that Cisco's behavior is a bug, but on the other hand you say that
it would be hard to find practical cases where the above algorithm
would actually break much of anything.

Do you agree or not?

Thank you.
-Jonathan

-----Original Message-----
From: Dennis Ferguson [mailto:[email-masked]] 
Sent: Wednesday, October 02, 2002 3:29 PM
To: Natale, Jonathan
Cc: [email-masked]
Subject: Re: Active Route 

Jon,

> "one must focus on the rule that a BGP speaker advertises to its peers
> (other BGP speakers which it communicates with) in neighboring ASs only
> those routes that it itself uses."
>
> This means that if the speaker's routing table has a non-bgp route to a
> destination but does not have bgp route to the same destination (for
> example, based on administrative distance), then the speaker may not
> advertise that route to it's peers. This is true on Juniper by default for
> EBGP, but is NOT TRUE [ever?] on Juniper for IBGP, and is NOT TRUE on
> Juniper if ~= "advertise inactive" is enabled.  This is NOT TRUE ever on
> Cisco.

I would note that, for implementation reasons, it requires a moderately
unnatural act for Juniper routers to ever advertise a route to any
BGP peer, whether internal or external, which is not the route it is
using for forwarding.  I suspect that if you don't add "advertise inactive",
you'll find that the routers strictly conform to the quoted behaviour for
both EBGP and IBGP.  The knob is a relatively late addition added as a
concession to the fact that people are used to the Cisco behaviour, and some
have built configurations which depend on it.  I personally consider the
Cisco behaviour to be a bug, but it is the way some of the world works.

> Now we are proposing that the quoted clause above be completely removed.
> This means that it will be allowable to advertise a route to a destination
> that is not locally reachable.  This is obviously (to me, but not to all--
> which is why it should not be removed) bad. 

I think the algorithm Juniper routers use when the knob is enabled is that
there are two routes potentially available for readvertisement, the route
which the router is using for forwarding and the best BGP route which would
be usable for forwarding if the preferred (non-BGP) route(s) wasn't present.
If policy prohibits readvertisement of the first route but permits
readvertisement of the second route, the second route is readvertised.
While telling lies about the route you are using seems theoretically
fragile it is, however, hard to find practical cases where the above
algorithm would actually break much of anything.

Dennis Ferguson

---

From: Dennis Ferguson <[email-masked]>
Date: Wed, 02 Oct 2002 14:14:34 -0700

Jonathan,

> quoted behavior for both EBGP and IBGP", whereas the existing text

I think the existing text says you must only advertise routes you
are using for forwarding to your EBGP neighbours, but doesn't place
much of a requirement at all on what you do or don't advertise to your
IBGP neighbours.  Given the first constraint I don't think it actually
matters all that much how you handle the second, there are several
ways to end up at the same end result.  What Juniper routers prefer to
do is something the existing text gives them the freedom to do.

> would actually break much of anything.

I have this view that routing protocols should be constrained to be basically
correct (i.e. do what is necessary to prevent loops and otherwise ensure
that they can actually reach the destinations they say they can), but
that anything else is fair game.  I don't like what Cisco does, and
certainly wouldn't have wanted the original BGP spec to say this, since
it is hard to think through a theoretical assurance of basic correctness
compared to just constraining routers to advertise only the routes they
are using.  On the other hand, they did it this way anyway and we now have
a lot of experience which says that routers can behave like this without
causing any particular damage, so I don't object to a change which makes
the document more closely reflect the practical reality.

Dennis Ferguson