Subject: [[email-masked]: Re: Comments on BGP Draft 20.....]

From: Jeffrey Haas <[email-masked]>
Date: Tue, 20 May 2003 14:55:57 -0400

Yakov,

----- Forwarded message from Russ White <[email-masked]> -----

Date: Tue, 20 May 2003 14:33:38 -0400 (EDT)
From: Russ White <[email-masked]>
To: Jeffrey Haas <[email-masked]>
Subject: Re: Comments on BGP Draft 20.....
Reply-To: Russ White <[email-masked]>
X-Virus-Scanned: by AMaViS perl-11
X-OriginalArrivalTime: 20 May 2003 18:33:51.0941 (UTC) FILETIME=[5C876750:01C31EFE]

Yeah, this sounds better....

:-)

Russ

On Tue, 20 May 2003, Jeffrey Haas wrote:

> [off-list]
>
> Howzabout:
>    The primary function of a BGP speaking system is to exchange network
>    reachability information with other BGP systems. This network reacha-
>    bility information includes information on the list of Autonomous
>    Systems (ASs) that reachability information traverses.  This informa-
>    tion is sufficient to construct a graph of AS connectivity
> +  for this reachability
>    from which
>    routing loops may be pruned and some policy decisions at the AS level
>    may be enforced.
>
> --
> Jeff Haas
> NextHop Technologies
>

__________________________________
[email-masked] CCIE <>< Grace Alone

----- End forwarded message -----

-- 
Jeff Haas 
NextHop Technologies

---

From: "Parag Deshpande" <[email-masked]>
Date: Fri, 23 May 2003 11:45:38 -0500

Hi,

I have a doubt regarding setting of nexthop value in the following scenario:

Router R has 2 ebgp peers A and B, all on same subnet S1.
A - Sends a prefix to R with nexthop N1 where N1 = A.
R - Installs and then forwards the prefix to B with N1 = ?

In this case should R set N1 = A or N1 = R (on S1).

I saw a major vendor setting it to R. Is that a preffered practice?
If yes why?

Reference:
5.1.3 NEXT_HOP
      2) When sending a message to an external peer X, and the peer is
      one IP hop away from the speaker:
      ........
       - Otherwise, if the route being announced was learned from an
         external peer, the speaker can use in the NEXT_HOP attribute an
         IP address of any adjacent router (known from the received
         NEXT_HOP attribute) that the speaker itself uses for local
         route calculation, provided that peer X shares a common subnet
         with this address. This is a second form of "third party"
         NEXT_HOP attribute.
>> N1 = A
         - Otherwise, if the external peer to which the route is being
         advertised shares a common subnet with one of the interfaces of
         the announcing BGP speaker, the speaker MAY use the IP address
         associated with such an interface in the NEXT_HOP attribute.
         This is known as a "first party" NEXT_HOP attribute.
>> N1 = R
       .....

Thanks
Parag

---

From: Danny McPherson <[email-masked]>
Date: Fri, 23 May 2003 13:03:27 -0600

> In this case should R set N1 = A or N1 = R (on S1).

It's a matter of policy, really.  For instance, perhaps A and B don't peer
directly and A doesn't want to accept packets directly from B, so setting a
third party NEXT_HOP may break things (e.g., Link Layer filtering is
implemented or no Layer 2 connection exists directly between A and B, even
though A, B & R share a common subnet) or violate some policy (In a previous
job we peered with a network at a multi-access exchange point -- purely out
of goodwill.  They began sending us lots of traffic and after some
investigation we realized they were selling transit across the local
exchange point via readvertising our routes to their transit customers and
preserving the NEXT_HOP, such that their customers were sending traffic
directly to us -- they never touched the outbound traffic!  Needless to say,
MAC-Layer filtering was deployed shortly thereafter).

On the other hand, perhaps they're all in agreement that this is a fine
thing in order to optimize the forwarding path AND connectivity exists such
that B can send traffic directly to A -- so it makes sense for R to preserve
the NEXT_HOP.  

> I saw a major vendor setting it to R. Is that a preffered practice?

Again, it's all a matter of policy, and all the "major vendors" I'm familiar
with provide the knobs to set it pretty much however you prefer, though I
have seen some variances in default behaviors.

-danny

---

From: "Parag Deshpande" <[email-masked]>
Date: Fri, 23 May 2003 15:19:30 -0500

Thanks Danny,

Then what I get is that it really doesn't matter what the default behavior
is since
knobs (policies) can be used to modify NEXT_HOP as needed. (?)

Parag

---

From: Danny McPherson <[email-masked]>
Date: Fri, 23 May 2003 14:51:16 -0600

On 5/23/03 2:19 PM, "Parag Deshpande" <[email-masked]> wrote:

> Thanks Danny,
> 
> Then what I get is that it really doesn't matter what the default
> behavior is since knobs (policies) can be used to modify NEXT_HOP
> as needed. (?)

Correct.  Both on the transmit and receive (e.g., enforce next-hop)
sides.

-danny