The MPLS WG Archive

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

Eric Gray wrote:
> 
>     Hmmm.  There is the interworking scenario:
> 
>   --- R1 --- R2 --- R3 ---
> 
> in which R1 only "speaks" CR-LDP/LDP and R3 only speaks RSVP-TE (and
> maybe LDP).  In this scenario, it is quite reasonable for R2 to
> "translate" between the two protocols for LSP set-up.

Would you want to have R2 translate one signalling protocol to another? 
This seems unmanagable and needlessly complicated.

In this scenario, I think it would make more sense to use LDP to signal
an LSP that terminates on R2.  R2 can then signal a different LSP that
goes through R3 to the edge of the MPLS cloud.  R2 can then configure
itself to forward packets between the two LSPs.

While the effect is the same, the mechanism is different.  It avoids a
few nasty problems that would result from the differences in the
protocols:

- RSVP-TE uses a different QoS representation from CR-LDP

- RSVP-TE explicltly LSPs from a known ingress router to a known
  egress router.  LDP distributes labels without a firm concept of an
  LSP during the signalling.  CR-LDP appears to be like LDP in this
  respect, although the presence of an explicit route mitigates the
  difference somewhat.

- There is no LDP or CR-LDP way to signal multiple LSPs that share
  resources.  (RSVP's "SE" reservation style)

These problems can all be worked around, of course, but it they can be
avoided altogether if the routers create two LSPs, connected at R2
intead of trying to signal one LSP with two protocols.

> Also, there is very little difference between setting up a single LSP
> through (for example) R2 above and terminating one LSP (from R1, for
> instance), making a new LSP (toward R3) and connecting them
> internally (at R2) - as long as the mapping (might be a FEC) is
> one-to-one from one LSP to the other.

In the data plane, there is no difference.  But the setup procedures
will be different.  IMO, the two-connected-LSP method will be simpler.

-- David