North American Network Operators Group

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Re: MPLS VPNs or not?

  • From: Scott Brim
  • Date: Wed Aug 08 06:18:35 2001

On Tue, Aug 07, 2001 at 06:55:09PM -0400, Craig Partridge apparently wrote:
> There are three separate issues (at least) here, so let's tease them out:
> * Current routing protocols don't do policy.  Very right and a known
>   defect in IP routing (though in part, they don't do it because in
>   the general case, policy is hard)

And policy-based routing everywhere is not scalable.  OK, we could argue
about the future, but I suspect that no matter how much power we give
router owners, they'll come up with policies that use it all.

> * Per hop policy decisions can be made more effectively in MPLS than
>   in IP.  Not true in theory unless you want to look very deep in
>   the packet to identify the policy association, though it may be
>   true in practice on certain current systems.

MPLS doesn't require per-hop policy decisions.  Policy decisions only
need to be made at the edge, re FEC inclusion.  Intelligence at the edge
etc.  Parallels with the diffserv model of classifying & marking packets
at the edge so you only need to look at PHBs in the middle.

> * Instantiation of per-hop policy information via MPLS is more scalable
>   than it would be in IP (not quite said above but an implied issue).
>   Almost certainly not true (see above about general policy being hard
>   being why IP doesn't do it).

Instantiation of per-hop policy in MPLS consists of forwarding by LSP,
except at the edge router.

..Scott (at the IETF)