MPLS VPNs or not?
Scott Brim
sbrim at cisco.com
Wed Aug 8 09:38:21 UTC 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)
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