Subject: Dest IP addr in MPLS

From: Eyad Saheb <[email-masked]>
Date: Fri, 23 Mar 2001 17:19:24 -0500

Hello,

	I am unclear on the proper IP address to use as IP_DEST in RSVP-TE
packets, especially when switches are involved.  Unfortunately, the
"standards" docs aren't too clear on this point (IMHO).  I will illustrate
my point:

1 - 2 - 3 - 4
  \        /  
   5 -----/

In the above topology, if Router 1 wants to establish a path to Router 4
through 2 & 3 it will create an RSVP-TE packet with IP_DEST = 4 & and ERO =
2,3,4.  Correct ?  This should override the IGP routing table which would
probably indicate router 5 as the 'correct' next-hop from router 1.

However, consider the following:

     +-+
     | |- 2
  1 -| |- 3
     | |- 4
     +-+

In this case, all routers are connected via a switch.  If the same RSVP-TE
packet is sent over this network, it would reach router 4 directly.  It
would then reject the packet since it'snot the first node in the ERO.  Is
this expected ?

Was RSVP-TE designed to be run over 'switched' networks ?

Regards,

	Eyad

---

From: Martin Machacek <[email-masked]>
Date: Sat, 24 Mar 2001 15:47:03 +0100

On Fri, Mar 23, 2001 at 05:19:24PM -0500, Eyad Saheb wrote:
>         I am unclear on the proper IP address to use as IP_DEST in
> RSVP-TE packets, especially when switches are involved.  Unfortunately,
> the "standards" docs aren't too clear on this point (IMHO).  I will
> illustrate my point:
> 
> 1 - 2 - 3 - 4 
>   \        /  
>    5 -----/ 
> 
> through 2 & 3 it will create an RSVP-TE packet with IP_DEST = 4 & and
> ERO = 2,3,4.  Correct ? 

Yes.

> This should override the IGP routing table
> which would probably indicate router 5 as the 'correct' next-hop from
> router 1.
> 
> In this case, all routers are connected via a switch.  If the same
> RSVP-TE packet is sent over this network, it would reach router 4
> directly.  It would then reject the packet since it'snot the first node
> in the ERO.  Is this expected ?

Well, you are mixing L3 and L2 topology. Your first figure shows L3 (IP)
topology, so lines are links in the IP sense (i.e they can be implemented by any
combination of compatible L2 links - there may be several LAN switchech, ATM
switchech, FrameRelay switches on every link). Your second figure shows L2
topology so lines are links in Ethernet sense (i.e. cables eventually with
repeaters). IP topology of your second figure would look like this:

   1    2    3    4
   |    |    |    |
   ----------------

4 nodes connected to one shared media segment. Nevetheless, assuming that ERO
processing is implemented correctly in all nodes, using the above explicit route
(2,3,4) and target address 4 you should be able to force LSP between 1 and 4 to
traverse 2 and 3 (instead of going directly to 4). Each node should send the
RSVP packet explicitly to the next node in the ERO (by doing L2 address
resolution) instead to node 4 (which is the target and is directly reachable).
Disclaimer: I've tried this setup.

> Was RSVP-TE designed to be run over 'switched' networks ? 

RSVP-TE does not care which L2 topology it runs on. The only requirement is IP
connectivity.

        Martin 

--
[PGP KeyID F3F409C4]

---

From: David Charlap <[email-masked]>
Date: Mon, 26 Mar 2001 11:08:09 -0500

Eyad Saheb wrote:
> "standards" docs aren't too clear on this point (IMHO).  I will
> illustrate my point:
> 
> 1 - 2 - 3 - 4
>   \        /
>    5 -----/
> 
> In the above topology, if Router 1 wants to establish a path to
> Router 4 through 2 & 3 it will create an RSVP-TE packet with IP_DEST
> = 4 & and ERO = 2,3,4.  Correct ?  This should override the IGP
> routing table which would probably indicate router 5 as the 'correct'
> next-hop from router 1.

Yes.

> directly.  It would then reject the packet since it'snot the first
> node in the ERO.  Is this expected ?

as Martin wrote, you're confusing layer-2 with layer-3.

In this topology, router 1 should send the Path message to 2, which will
then send it to 3, which will then send it to 4.  As per the ERO.  The
fact that 1 is directly conneted to 4 changes nothing.

1 will use 2 as it's next-hop router, and therefore will use 2's
Ethernet address when sending the packet.  The fact that the routing
table shows 4 as the next hop on the shortest path doesn't matter - an
ERO is supposed to override the routing table.

-- David