RFC 3480 (rfc3480) - Page 2 of 8

Signalling Unnumbered Links in CR-LDP (Constraint-Routing Label Distribution Protocol)

RFC 3480         Signalling Unnumbered Links in CR-LDP     February 2003


   Current signalling used by MPLS TE does not provide support for
   unnumbered links because the current signalling does not provide a
   way to indicate an unnumbered link in its Explicit Route Objects.
   This document proposes simple procedures and extensions that allow
   CR-LDP signalling [CR-LDP] to be used with unnumbered links.

2. Link Identifiers

   An unnumbered link has to be a point-to-point link.  An LSR at each
   end of an unnumbered link assigns an identifier to that link.  This
   identifier is a non-zero 32-bit number that is unique within the
   scope of the LSR that assigns it.  If one is using OSPF or ISIS as
   the IGP in support of traffic engineering, then the IS-IS and/or OSPF
   and CR-LDP modules on an LSR must agree on the identifiers.

   There is no a priori relationship between the identifiers assigned to
   a link by the LSRs at each end of that link.

   LSRs at the two end points of an unnumbered link exchange with each
   other the identifiers they assign to the link.  Exchanging the
   identifiers may be accomplished by configuration, by means of a
   protocol such as LMP ([LMP]), by means of CR-LDP (especially in the
   case where a link is a Forwarding Adjacency, see below), or by means
   of IS-IS or OSPF extensions ([ISIS-GMPLS], [OSPF-GMPLS]).

   Consider an (unnumbered) link between LSRs A and B.  LSR A chooses an
   identifier for that link.  So does LSR B.  From A's perspective, we
   refer to the identifier that A assigned to the link as the "link
   local identifier" (or just "local identifier"), and to the identifier
   that B assigned to the link as the "link remote identifier" (or just
   "remote identifier").  Likewise, from B's perspective, the identifier
   that B assigned to the link is the local identifier, and the
   identifier that A assigned to the link is the remote identifier.

   In the context of this document, the term "Router ID" means a stable
   IP address of an LSR that is always reachable if there is any
   connectivity to the LSR.  This is typically implemented as a
   "loopback address"; the key attribute is that the address does not
   become unusable if an interface on the LSR is down.  In some cases,
   this value will need to be configured.  If one is using OSPF or ISIS
   as the IGP in support of traffic engineering, then it is RECOMMENDED
   for the Router ID to be set to the "Router Address" as defined in
   [OSPF-TE], or "Traffic Engineering Router ID" as defined in [ISIS-
   TE].

   This section is equally applicable to the case of unnumbered
   component links (see [LINK-BUNDLE]).




Kompella, et al.            Standards Track