The MPLS WG Archive

[Eric Osborne <eosborne@cisco.com>]

On Mon, Aug 05, 2002 at 01:59:06PM -0400, Gray, Eric wrote:
> Mayank,
> 
> 	The standards say that an ATM LSR does have L3
> lookup capability.  Specifically, RFC 3035 says about
> ATM LSRs: 
> 
> " ... ATM switches run network layer routing algorithms 
>   (such as Open Shortest Path First (OSPF), Intermediate
>   System to Intermediate System (IS-IS), etc.), and their 
>   data forwarding is based on the results of these routing 
>   algorithms.  No ATM-specific routing or addressing is 
>   needed.  ATM switches used in this way are known as 
>   ATM-LSRs (Label Switching Routers)."

To me, that doesn't say L3 lookup capability.  I could certainly see
an ATM LSR implementation that used L3 for two things:
- maintaining IGP adjacencies
- deciding which interfaces to signal on-demand labels

...but none of that implies that an ATM LSR should be prepared to
forward received data packets.

> 
> I am not sure what standards you were referring to.
> 
> If an ATM Switch is to participate in label switching, it
> must be able to unambiguously determine how to forward 
> labeled packets.  It is possible to implement ATM switches
> that participates in label switching without themselves
> participating in routing by ensuring that the determination
> on how to forward labeled packets is made for them.  As an
> example, this might be done by always using strict explicit 
> routing relative to the ATM switch in question and immediate 
> downstream neighbors.  Such ATM switches are not ATM LSRs.

I disagree; I think there's a difference between "L3 lookup" (which to
me implies forwarding) and simply running a local IP stack for things
which originate/terminate on that router.



eric

> 
> Eric W. Gray
> Systems Architect
> Celox Networks, Inc.
> egray@celoxnetworks.com
> 508 305 7214
> 
> 
> -----Original Message-----
> From: Mayank Kumar [mailto:mkumar@aplion.stpn.soft.net] 
> Sent: Monday, August 05, 2002 1:32 PM
> To: Mpls (E-mail)
> Subject: Layer 3 Lookup in ATM LSR's
> 
> hi
> i have a query which , you all might be able to answer:-
> 
> it is said that atm switches in the core atm networks do not have layer 3
> lookup capability.
> Now the standards say that when atm lsr's do not have a layer 3 lookup
> capability then they operate in Ordered control with down stream on demand
> mode of label distribution.
> 
> lets say that i have the following mpls domain :-
> 
> a--------b---------c-------d
>           |
>           |-------k
> 
> 'b' and 'c' are atm switches with no layer 3 lookup capabilities and are
> operating in Ordered Control with DownStream on Demand Label Distribution
> method.
> Suppose a lsp has to travel the path 'a'----'b'---'c'---'d'
> 
> 'a' requests a mapping for a particular fec from 'b'. 'b' upon receving a
> label request for a particular fec , sees that it has no downstream mapping
> for that fec. Since 'b' is operating in ordered control , it further sends a
> label request to c (and not to k) since it cannot allocate a label for the
> requested fec . 'c' would also further send the label request to 'd' and
> then 'd' would reply with a label mapping message.
> 
> I hope that it's correct till this point.
> 
> My question is, how does 'b' know , that in order to send a label request
> for a particular fec, what is the next hop for that fec when it does not
> have a layer 3 capability.
> why would 'b' send a label request to 'c' and not to 'k' ????
> 
> 
> i hope i have made my point  clear
> 
> 
> Please tell me where am i wrong
> 
> thanks and regards
> mayank
> 
> 
>