The MPLS WG Archive

["Vach Kompella" <vkompella@timetra.com>]

Suppose there are two adjacent LSRs A and B which try to set up both regular
link-level LDP, as well as targeted LDP.

   -----         -----
   | A |---------| B |
   -----         -----

Only a single LDP session/TCP connection is established, but are two hello
adjacencies set up?  Having only a single one would mean that if the interface
was brought down, the session would terminate.

Also, the existence of a targeted adjacency is typically for VC label exchange
or for tunneled next hop resolution, whereas link adjacency is typically used
for LSPs following the IGP.

Does one exchange VC labels over a session over which link adjacency only has
been set up?  That seems silly.  E.g., A and B exchange VC labels over a
targeted session, but X and B have a (link) adjacency and an LDP session.  You
wouldn't want B and X to exchange labels for martini VCs.

   -----         -----          -----
   | A |---------| X |----------| B |
   -----         -----          -----

Finally, in the upper configuration, if A and B only have a targeted adjacency,
then shouldn't B avoid sending address FECs to A which doesn't really care for
them?

Since I couldn't find this behavior described in the RFCs, I propose a default
behavior for adjacency creation and scoping of FEC signaling so that:
1. parallel hello adjacencies are set up, even though only one session is used
to exchange labels, when both link and targeted adjacencies are requested
2. if only a link adjacency is established between a pair of LSRs, then address
FECs SHOULD be exchanged over the concomitant session, and no other FECs
3. if only a targeted adjacency is established between a pair of LSRs, then
non-address FECs SHOULD be exchanged over the session, and no other FECs
4. if both adjacencies are set up, then all known FECs SHOULD be exchanged

Similarly, when adjacencies are torn down:
1. if no link adjacency exists, then all address FECs should be withdrawn
2. if no targeted adjacency exists, then all non-address FECs should be
withdrawn

Does this make sense?  I have run across some behavior that doesn't quite follow
these rules, so if others are operating on other rules of behavior, could we
have some discussion on how to resolve this?  Thanks.

-Vach