RFC 3478 (rfc3478) - Page 2 of 12


Graceful Restart Mechanism for Label Distribution Protocol



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RFC 3478           Graceful Restart Mechanism for LDP      February 2003


   The procedures described in this document apply to downstream
   unsolicited label distribution.  Extending these procedures to
   downstream on demand label distribution is for further study.

Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14, RFC 2119
   [RFC 2119].

1. Motivation

   For the sake of brevity in the context of this document, by "the
   control plane" we mean "the LDP component of the control plane".

   For the sake of brevity in the context of this document, by "MPLS
   forwarding state" we mean either  (outgoing label,
   next hop)> (non-ingress case), or (outgoing label, next hop)>
   (ingress case) mapping.

   In the case where a Label Switching Router (LSR) could preserve its
   MPLS forwarding state across restart of its control plane,
   specifically its LDP component [LDP], it is desirable not to perturb
   the LSPs going through that LSR (specifically, the LSPs established
   by LDP).  In this document, we describe a mechanism, termed "LDP
   Graceful Restart", that allows the accomplishment of this goal.

   The mechanism described in this document is applicable to all LSRs,
   both those with the ability to preserve forwarding state during LDP
   restart and those without (although the latter need to implement only
   a subset of the mechanism described in this document).  Supporting (a
   subset of) the mechanism described here by the LSRs that can not
   preserve their MPLS forwarding state across the restart would not
   reduce the negative impact on MPLS traffic caused by their control
   plane restart, but it would minimize the impact if their neighbor(s)
   are capable of preserving the forwarding state across the restart of
   their control plane and implement the mechanism described here.

   The mechanism makes minimalistic assumptions on what has to be
   preserved across restart - the mechanism assumes that only the actual
   MPLS forwarding state has to be preserved.  Clearly this is the
   minimum amount of state that has to be preserved across the restart
   in order not to perturb the LSPs traversing a restarting LSR.  The
   mechanism does not require any of the LDP-related states to be
   preserved across the restart.





Leelanivas, et al.          Standards Track


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