The MPLS WG Archive

["Mayank Kumar" <mkumar@aplion.stpn.soft.net>]

hi Eric,
then why is it recommended that in atm backbones running mpls,we should use
Downstream on Demand  with Ordered LSP Control.
What advantages does it provide ???
My doubt arose because i have been reading a book from cisco press named
"MPLS And VPN architectures" by  Ivan Pepelnjak and Jim Guichard.
	the book on pg 57 says that
"the label allocation and distribution process accross the atm-lsr domain
has the following characteristics:-
--Label allocation in devices with layer 3 lookup capabilites (routers ) is
done regardless of whether the router has already received a label for the
same prefix from its next hop router . Label allocation in routers is thus
called independent control.
--label allocation in devices with no layer 3 lookup capabilites  (atm
switches) is performed only if a corresponding downstream label has already
been allocated . Label allocation in atm  swicthes is thus Ordered control."

the book further says that "the downstream lsr could simply allocate a label
and respond to the request from the upstream lsr with a corresponding reply
message. Under some circumstances , this action would require the downstream
lsr to have layer 3 lookup capability (such as if the downstream lsr would
have no further downstream label for the requested destination). Therefore ,
the atm swicthes never respond to a label allocation request unless they
already have a corresponding downstream label allocated"

What does that mean ???

And do u want to say that an LSR always has a layer 3 lookup capabilites
irrespective of whether it is an atm lsr or frame based lsr.

I think Eric is right in his reply ???
I think that Layer 3 Lookup implies , being able to process the IP header
while forwarding packets and thus being able to make a forwarding decision
based on the destination ip address. Atm swicthes do form routing
adjacencies on the control vc but they dont have layer 3 lookup capability.

Why does the Book say then , that label allocation in atm swicthes is
Ordered. What advantage does that provide ???

thanks and reagards
Mayank

-----Original Message-----
From: Gray, Eric [mailto:egray@celoxnetworks.com]
Sent: Monday, August 05, 2002 11:29 PM
To: 'mkumar@aplion.stpn.soft.net'
Cc: Mpls (E-mail)
Subject: RE: Layer 3 Lookup in ATM LSR's


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