The MPLS WG Archive

[neil.2.harrison@bt.com]

Please see below, regards Neil.

Eric Osborne wrote 03 June 2002 19:21
<snipped>
> > The remote peering mode of LDP is useful, but you could use 
> BGP or even RSVP-TE/CR-LDP too with the same scalability 
> level. The question was regarding the mp2p mode of LDP not 
> the p2p remote peering.
> > 
> 
> I don't think RSVP is appropriate for service label signalling in this
> case.
NH=> Agree
> 
> Sure, one could use BGP, but there's all sorts of things to consider
> there, too.  We could use BGP between every pair of directly connected
> neighbors (a la rfc3107) and not use LDP there, either.  But we don't;
> BGP and LDP are different.
NH=> Agree
> 
> Why are you trying to get rid of LDP?  It certainly seems like
> that...:)
NH=> Can't get rid of it....its already here.  We have to live with it.
What I think we are trying to find out is whether there is any strong reason
to use LDP in the core of a network when used to create mp2p constructs.
Remote peering of LDP is useful for certain application such as what you
described in PWE3 VC label distribution. But the general application of mp2p
LDP used in the core of networks creates lots of management problems that
don't exist in IP. 

The following is a comparison between LDP-MPLS (mp2p core) and IP, based on
the comments seen so far:

LDP-MPLS:
- Need another control-plane protocol (i.e., LDP signalling) on top of IP
control-plane (routing only)
- Packets could be mis-routed to wrong destination now because of LDP
failures. Therefore needs an LSP maintenance tool (such as LSP-ping) to run
continuously to detect defects (operators need automatic defect detection
and handling, waiting for a customer complaint is simply not good enough).
- Prevents spoofing

IP:

- Needs only IP control-plane (rouing only)
- Packets can't be mis-routed to wrong destination (except in rare cases)
due to use of IP addressing which is network unique rather than just
hop/node unique
- Is subject to spoofing, therefore needs filtering at the edge
- Requires 16 bytes more header per packet in VPN applications

So the question is which one is better to choose:
-	LDP software on all routers + periodic connectivity test, or
-	Filtering IP addresses (ACL) at the edge + 16 bytes more
header/packet for VPN
 
Note that doing a connectivity test on a constantly moving target (LDP-based
LSPs) is very difficult.
<snipped to end NH>