The MPLS WG Archive

[David Charlap <David.Charlap@marconi.com>]

Mayank Kumar wrote:
>
> 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 ???

ATM switches don't have a concept of the "per platform" label space. 
ATM forwarding hardware is designed around ATM capabilities, which only 
allocated VCIDs on a per-interface basis.  multipoint-to-point 
connections may not be possible either.

This can quickly lead to resource exhaustion on the line cards and 
switching fabric, since each interface would need to allocate a label 
for evey IGP prefix, and you'd need to create fabric connections for 
every interface/prefix combination - which can be tremendous.

> 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.

This description is assuming connectionless hardware.  A connection- 
oriented layer-3 router that doesn't support per-platform label spaces 
and multipoint-to-point connections can hit the same resource exhaustion 
problems that an ATM switch is likely to hit.

> --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."

See above.  They're talking about layer-3 hardware-forwarding 
capabilities, not a lack of ability to receive an IP packet.

On-demand/ordered operation is better for these kind of switches because 
you only create connections that are explicitly requested, which will 
usually be less than the product of all prefixes and all interfaces. 
This behavior comes at the cost of longer convergence times.

> 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.

No.  You're confusing what "lookup capable" means.

A lookup capable router (or interface) is one that can receive an 
unlabeled packet, determine the next-hop (which may be into an LSP), 
push an appropriate label, and forward the packet to that next-hop. 
With modern equipment, this mean that this is done in hardware, since no 
processor is fast enough to do this when there are a lot of high-speed 
interfaces.

Lookup capability is also needed for a router (or interface) that must 
pop the label stack.  It must do a label-lookup (which all switches can 
do), but it must then pop the stack and then do an IP lookup on the 
result before forwarding.  This is beyond the capability of most ATM 
forwarding hardware.

> 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 ???

See above.  It helps to prevent the resource exhaustion that will happen 
if you have a lot of prefixes and a lot of interfaces, wihtout a 
per-platform label space or mp2p connection.

-- David