Re: MPLS VPNs or not?

[Craig Partridge <craig () aland bbn com>]

In message <200108072211.WAA10474 () i-14 isi edu>, Walter Prue writes:

> An aspect of MPLS routing that this may be overlooking is the fact that
> MPLS tunnels can be designed with certain attributes which implement a
> form of policy routing that normal least cost routing does not implement.
> That is, blue packets can route over this link but not red packets unless
> there is an outage of the S.F to N.Y. Link.  Another policy might say that
> traffic from this customer may exit the backbone at only a few points. 
> These engineered routing decisions are configured into an MPLS network so
> these routing decisions are not made on a packet by packet basis.  Once
> data is stuffed into an MPLS tunnel that was set up with certain policies
> in mind you know that the data comes out at the other end.  Intermediate
> nodes don't need to be convinced to "non-optimally" route this data but only
> this data.

Hi Walt:

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)

* 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.

* 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).

Craig