The MPLS WG Archive

["Thomas D. Nadeau" <tnadeau@cisco.com>]

         Comments inline.

>Hi Tom! This is a follow-up question based on your reply.
>In your reply, you pointed-out "the XCindex notes the
>"group" of LSPs used for the tunnel".

         It may denote a group of associated LSPs. It depends
on how you want to implement tunnels and LSPs. See below
for more information.

>If this was the main reason for having the mplsXCIndex in
>the mplsXCTable, then I might have mis-interpreted the MIB
>definitions related to the mplsXCTable. I will appreciate,
>if you can clarify the following.
>
>Given the following context:
>---------------------------
>(1) mplsLspId is not a part of the index into the mplsXCTable
>and is optional for implementation [LSR-MIB mplsXCTable
>definition and conformance statements]
>
>(1) "It supports point-to-point, point-to-multipoint and
>multipoint-to-point connections" [mplsXCTable description,
>in LSR-MIB]
>
>(2) "mplsTunnelTable does not define the in and out segments
>    forming the tunnel.  Instead, these are defined by creating
>    rows in the in-segment and out-segment tables, defining
>    relationships in the cross-connect table and referring to
>    these rows in the mplsTunnelTable using a cross-connect
>    index, mplsTunnelXCIndex. These segment and cross-connect
>    related objects are defined in LSRMIB." [MPLS-TE-MIB Section
>    6.1]
>
>(3)"Uniquely identifies an instance of a tunnel. It is
>     useful to identify multiple instances of tunnels
>     for the purposes of backup and parallel tunnels." [Description
>     of mplsTunnelInstance in MPLS-TE-MIB]
>
>I interpreted the following statement:
>-------------------------------------
>"-cross-connect index mplsXCIndex that uniquely identifies
>   a group of cross-connect entries" [Description in the
>   mplsXCEntry]
>
>as:
>---
>(a) cross-connect index mplsXCINdex uniquely identifies a group of
>     cross-connects entries, that belong to the __same LSP___.
>     [NOT a group of independent LSPs, as you pointed out in your
>     reply.]

         The LSR MIB exists at all points along an LSP and
identifies the local in-out label swapping for any
particular hop of the LSP. Remember that a tunnel
exists only if at least one LSP which has been signaled
to carry the traffic for that tunnel. To that end, if the
XC pointer in the tunnel entry does not point to anything,
then no LSP exists. What may be confusing is that
each tunnel instance has one LSP per instance and that
it *may* be convenient to associate a "group" of LSPs
in the LSR MIB together using this primary index, but that is
not mandatory. This is not necessarily mandatory, since you
can have distinct XCindexes for each underlying LSP as well.

>(b) The main (and only) reason for having the mplsXCIndex, is to
>     represent a LSP (inside the switch)with different cast types:
>     point-to-point point-to-multipoint and multipoint-to-point.

         The main point for the XCindex is to represent the
cross-connection between some set of incoming labels to
some set of outgoing labels.

>(c) mplsTunnelXCPointer is used to associated a particular LSP
>     with the mpls tunnel defined in the mplsTunnelTable.

         Yes.

>(d) Parallel tunnels are abstracted in mplsTunnelTable using the
>     mplsTunnelIndex and mplsTunnelInstance.

         Yes, but as I mentioned above, each instance needs its
own LSP.

         --Tom