The MPLS WG Archive

[Kireeti Kompella <kireeti@juniper.net>]

Hi,

> > When filling in the ERO, there are three options:
> > 
> > 1) Put in the router ID of the routers. This does not work for cases
> > when routers have multiple links and for unnumbered interfaces.

Technically this always works.  You lose control of exactly which
interface to use, but that is a perfectly valid ERO.

> > 2) Put in the egress interface of the routers on the path and finish it
> > with the router ID of the final router. So that from router A to router
> > B, we represent the hop as the IP address/interface index of the
> > interface from A to B on router A.

I interpret this as "remote IP address on each link", i.e., put B's
address (or outgoing interface index from A's point of view) for the
A->B link.

Note that "finishing with router ID of final router" is not needed.

> > 3) Put in the ingress interface of the routers on the path starting with
> > the router ID of ingress and finishing with the router ID of the egress
> > LSRs. This is kind of opposite of (2).

I read: "put the local address of each link, i.e., A's address for
the A->B link".  In the unnumbered case, you are stuck -- you could
somehow put B's outgoing interface id for the A-B link, but it would
do A no good in finding the next hop.

Note that while either (2) or (3) works for point-to-point links,
only (2) works for multipoint links.

FWIW, there is yet another option: put both the local and remote
interface addresses in the ERO.  Yes, the ERO size is doubled, but
this works, and there are even some situations where this makes sense.

> > I believe that (2) is the only option that works with unnumbered
> > interfaces.

>From the unnumbered draft:

6.1. Interpreting the Unnumbered Interface ID Subobject

   The Interface ID is the outgoing interface identifier with respect to
   the previous node in the path (i.e., the PHOP).

You are saying the same thing -- use outgoing indices in the ERO.
(Which I believe is Markus's point.)  In any case, we're in sync :-)

Kireeti.