The MPLS WG Archive

["Gray, Eric" <egray@celoxnetworks.com>]

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)."

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.

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