Subject: Re: the nexthop of RSVP_TE.

From: David Charlap <[email-masked]>
Date: Tue, 23 Oct 2001 10:15:43 -0400

Marshall Ma Shaowen wrote:
> 
> hi, all
> I have a question about the nexthop of RSVP_TE.
> as the draft said that we should follow the ERO to forward the
> packet.
> but the rfc2205 says that the destination address is the egress ip
> address.
> so there is some problem about this

What's the problem?  You pick your next-hop based on the ERO.  You write
the destination address in the IP header and send the packet to your
chosen next-hop.

(Note that a stock IP stack may not let you send to a next-hop that is
not chosen by the destination address and local routing table.  If you
have such a stack, you may need to upgrade or modify it.)

> RTA
>  |
> -+--------------+---------------+----------------
>                 |               |
>                 |               |
>                RTB             RTC
>                 |               |
>                 |               |
> ----------------+---------------+--------------+-
>                                                |
>                                               RTD

(I cleaned up your drawing a bit.  ASCII-art with variable-width fonts
does not result in readable diagrams.)

> if the CSPF caclulate a path next hop by RTB and then RTD.
> but if the RTA send out the packet with the destiantion of RTD,
> both the RTB and RTC will forward this packet. so How can ensure
> the LSP establish through RTA-RTB-RTD ?

Why will both routers forward the packet?  Only one of them is your next
hop.  The packet is only going to be sent to one of them.

If you are using a broadcast network (as your diagram implies), RTA will
ARP for the next-hop (RTB).  It will put that address in the layer-2
header.  RTC should ignore the packet, since its layer-2 address
indicates a different router.

This is no different from sending an unlabeled packet without an ERO
over this network topology.

(If you have point to point links, then the entire question is moot,
since there is only one neighbor per interface, by definition.)

-- David