The MPLS WG Archive

[Eric Osborne <eosborne@cisco.com>]

On Tue, Aug 06, 2002 at 12:15:18AM +0530, Mayank Kumar wrote:
> 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 ???

DoD ordered means 
	- you only get a label (VPI/VCI) when you need one
	- you know that when you get one, things are all set to go
downstream of you

> My doubt arose because i have been reading a book from cisco press named
> "MPLS And VPN architectures" by  Ivan Pepelnjak and Jim Guichard.

...which isn't a book that pretends to be particularly authoritative
on ATM LSR stuff, IIRC..:)

> 	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."
> 

This is really explaining how things are done, not why they're done
that way.  I recommend you read the RFCs that are out there (rfc3031)
to get another perspective on the different modes.

> 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 ???
> 

Not sure; I think it means "go away, this is a frame-mode book"...:)


> 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 ???
> 

The advantage of Ordered is that if you receive a label binding
upstream, you are 100% certain that the downstream forwarding topology
is set up, and that if you forward packets down that LSP, they will
arrive at their destination (modulo congestion, etc), and that you
don't have to worry about needing to do a route lookup in an ATM
cloud.  But I think you might be reading too much into it.


eric


> 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
> 
> 
> 
>