Signalling Unnumbered Links in CR-LDP (Constraint-Routing Label Distribution Protocol)
RFC 3480

Document Type RFC - Proposed Standard (February 2003; No errata)
Last updated 2015-10-14
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Network Working Group                                        K. Kompella
Request for Comments: 3480                                    Y. Rekhter
Category: Standards Track                               Juniper Networks
                                                             A. Kullberg
                                                        NetPlane Systems
                                                           February 2003

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

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.


   Current signalling used by Multi-Protocol Label Switching Traffic
   Engineering (MPLS TE) does not provide support for unnumbered links.
   This document defines procedures and extensions to Constraint-Routing
   Label Distribution Protocol (CR-LDP), one of the MPLS TE signalling
   protocols that are needed in order to support unnumbered links.

Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in BCP 14, RFC 2119

1. Overview

   Supporting MPLS TE over unnumbered links (i.e., links that do not
   have IP addresses) involves two components: (a) the ability to carry
   (TE) information about unnumbered links in IGP TE extensions (ISIS or
   OSPF), and (b) the ability to specify unnumbered links in MPLS TE
   signalling.  The former is covered in [GMPLS-ISIS, GMPLS-OSPF].  The
   focus of this document is on the latter.

Kompella, et al.            Standards Track                     [Page 1]
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-
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