Re: [mpls-linux-general] LDP question

[James R. L. <jl...@mi...>]

On Wed, May 16, 2001 at 02:09:00PM -0700, Scott Lee wrote:
> Hi James,
> 
> Can I ask you a question on LDP? If you have the
> following topology:
> 
> A-----B
> |     |
> C-----D
> 
> >From A's perspective, there are two equal cost paths
> to D. That means when A receives label mappings from
> B and C, it can use the label value learnt from B when
> forwarding the packet to D via B and use the label
> value learnt from C when forwarding packet to D via C.
> Let's assume that A can perform load-balancing based
> on source IP address so that you would not get out
> of order packets at the final destination.
> 
> Question1: Is this supported in LDP?
> LDP mentioned somthing like multipath is not supported
> in this version. What does that mean?

The draft say no. BUT... In unsolicited mode A will recieve labels from
B and C for FECs reachably by D.  It is upto A how it would like to handle
these labels.  It is a local descision so as long as A can add labels to
the route for each next hop it should work.

Does my code support this?  No.  Can I add it, yeah.  Up until now I've
been ignoring multipath.

> Question2: In ordered control mode, when A receives
> the label mapping from B, does A sends another label
> mapping message to C (because the mapping came from
> the next hop)? I guess C would not send another
> mapping
> message to D because the mapping message did not
> come from the next hop?

Correct.  A has no way of knowing whether C has a route to D that doesn not
go through A.  So A must send forward the mappings.  As you said C will
realize that the mapping isn't along the shortest path and will not
forward it.

> Question3: Does B advertise label for FEC whose next
> hop is C to C also? (I think in the ind control mode,
> B does and the ordered control mode, B does not.)

When running LDP over ethernet with Cisco I see this behavior.  It is quite
odd, I'm not of the rational, but it doesn't break anything.

Jim

> Thanks again, James.
> 
> Scott
> 
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-- 
James R. Leu