Subject: Layer 3 Lookup in ATM LSR's

From: "Mayank Kumar" <[email-masked]>
Date: Mon, 5 Aug 2002 23:02:29 +0530

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

---

From: David Charlap <[email-masked]>
Date: Mon, 05 Aug 2002 14:40:10 -0400

> it is said that atm switches in the core atm networks do not have layer 3

Their line cards can not typically use information other than what's in 
the cell header for forwarding purposes.

If they are to be used with MPLS, their control processors will still 
have to run an IP stack.  A PVC (typically 0:32) is usually configured 
on each interface for the purpose of sending unlabeled IP traffic to a 
switch's processor.

Software forwarding of IP (via the control processor) may even exist, in 
order to facilitate control-plane routing protocols and signaling 
protocols that may require it.  This forwarding will typically be orders 
of magnitude slower than ATM cell switching, so you can't use it in the 
data plane for anything, but it should still exist in the control plane.

Keeping this in mind, the answer to your quetion should now be obvious.

> why would 'b' send a label request to 'c' and not to 'k' ????

This next-hop determination comes from the routing table.

An ATM switch that is participating in an MPLS network will probably be 
running an IP-based routing protocol, like OSPF or IS-IS.  If it isn't 
then it will at least have static routes.  Either way, the routing table 
will be populated with the necessary data.

If the data's not there, of course, then the LSP can't propagate to the 
egress.

-- David