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Versions: 00 01 02 03 04 05 RFC 4781

Network Working Group               Yakov Rekhter (Juniper Networks)
Internet Draft                     Rahul Aggarwal (Redback Networks)
Expiration Date: July 2002


              Graceful Restart Mechanism for BGP with MPLS

                draft-ietf-mpls-bgp-mpls-restart-00.txt


1. Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
   groups may also distribute working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as ``work in progress.''

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.


2. Abstract

   [1] describes a mechanism for BGP that would help minimize the
   negative effects on routing caused by BGP restart. This document
   extends this mechanism to also minimize the negative effects on MPLS
   forwarding when BGP is used to carry MPLS labels [2]. The mechanism
   described in this document is agnostic with respect to the types of
   the addresses carried in the BGP NLRI. As such it works in
   conjunction with any of the address famililies that could be carried
   in BGP (e.g., IPv4, IPv6, etc...)










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3. Summary for Sub-IP Area


3.1. Summary

   This document describes a mechanism that helps to minimize the
   negative effects on MPLS forwarding caused by LSR's control plane
   restart, and specifically by the restart of its BGP component in the
   case where BGP is used to carry MPLS labels and LSR is capable of
   preserving its MPLS forwarding state across the restart.


3.2. Related documents

   See the Reference Section


3.3. Where does it fit in the Picture of the Sub-IP Work

   This work fits squarely in MPLS box.


3.4. Why is it Targeted at this WG

   The specifications on carrying MPLS Labels in BGP is a product of the
   MPLS WG. This document specifies procedures to minimize the negative
   effects on MPLS forwarding caused by the restart of the control plane
   BGP module in the case where BGP is used to carry MPLS labels. Since
   the procedures described in this document are directly related to
   MPLS forwarding and carrying MPLS labels in BGP, it would be logical
   to target this document at the MPLS WG.


3.5. Justification

   The WG should consider this document, as it allows to minimize the
   negative effects on MPLS forwarding caused by the restart of the
   control plane BGP module in the case where BGP is used to carry MPLS
   labels.












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4. Motivation

   In the case where an LSR could preserve its MPLS forwarding state
   across restart of its control plane, and specifically its BGP
   component, it may be desirable not to perturb the LSPs going through
   that LSR (and specifically, the LSPs established by BGP). In this
   document, we describe a mechanism that allows to accomplish this
   goal. The mechanism described in this document works in conjunction
   with the mechanism specified in [1]. The mechanism described in this
   document places no restrictions on the types of addresses (address
   families) that it can support.


5. Assumptions

   First of all we assume that an LSR implements the Graceful Restart
   Mechanism for BGP, as specified in [1]. Second, we assume that the
   LSR is capable of preserving its MPLS forwarding state across the
   restart of its control plane (including the restart of BGP).


6. Capability Advertisement

   An LSR that supports the mechanism described in this document
   advertises this to its peer by using the Graceful Restart Capability,
   as specified in [1]. The SAFI in the advertised capability should
   indicate that NLRI carries not just address prefixes but labels as
   well.


7. Procedures for the restarting LSR

   After the LSR restarts, it follows the procedures as specified in
   [1]. In addition, if the LSR is able to preserve its MPLS forwarding
   state across the restart, the LSR advertises this to its neighbors by
   appropriately setting the Flag field in the Graceful Restart
   Capability for all applicable AFI/SAFI pairs.

   For the sake of brevity in the context of this document by "MPLS
   forwarding state" we mean either <incoming label -> (outgoing label,
   next hop)>, or <address prefix -> (outgoing label, next hop)>
   mapping. In the context of this document the forwarding state that is
   referred to in [1] means MPLS forwarding state. This document doesn't
   require the restarting LSR to preserve its IP forwarding state across
   the restart.

   Once the restarting LSR completes its route selection (as specified
   in Section 6.1 of [1]), then in addition to the procedures specified



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   in [1], the restarting LSR performs one of the following:


7.1. Case 1

   The following applies when (a) the best route selected by the
   restarting LSR was received with a label, (b) that label is not an
   Implicit NULL, and (c) the LSR advertises this route with itself as
   the next hop.

   In this case the restarting LSR searches its MPLS forwarding state
   (the one preserved across the restart) for an entry with <outgoing
   label, Next-Hop> equal to the one in the received route. If such an
   entry is found, the LSR no longer marks the entry as stale. In
   addition if the entry is of type <incoming label, (outgoing label,
   next hop)> rather than <prefix, (outgoing label, next hop)>, the LSR
   uses the incoming label from the entry when advertising the route to
   its neighbors. If the found entry has no incoming label, or if no
   such entry is found, the LSR just picks up some unused label when
   advertising the route to its neighbors (assuming that there are
   neighbors to which the LSR has to advertise the route with a label).


7.2. Case 2

   The following applies when (a) the best route selected by the
   restarting LSR was received either without a label, or with an
   Implicit NULL label, or the route is originated by the restarting
   LSR, (b) the LSR advertises this route with itself as the next hop,
   and (c) the LSR has to generate a (non Implicit NULL) label for the
   route.

   In this case the LSR searches its MPLS forwarding state for an entry
   that indicates that the LSR has to perform label pop, and the next
   hop equal to the next hop of the route in consideration. If such an
   entry is found, then the LSR uses the incoming label from the entry
   when advertising the route to its neighbors. If no such entry is
   found, the LSR just picks up some unused label when advertising the
   route to its neighbors.

   The description in the above paragraph assumes that the restarting
   LSR generates the same label for all the routes with the same next
   hop. If this is not the case, and the restarting LSR generates a
   unique label per each such route, then the LSR needs to preserve
   across the restart not just <incoming label, (outgoing label, next
   hop)> mapping, but also the prefix associated with this mapping.  In
   such case the LSR would search its MPLS forwarding state for an entry
   that (a) indicates Label pop (means no outgoing label), (b) the next



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   hop equal to the next hop of the route and (c) has the same prefix as
   the route. If such an entry is found, then the LSR uses the incoming
   label from the entry when advertising the route to its neighbors. If
   no such entry is found, the LSR just picks up some unused label when
   advertising the route to its neighbors.


7.3. Case 3

   The following applies when the restarting LSR does not set BGP Next
   Hop to self.

   In this case the restarting LSR, when advertising its best route for
   a particular NLRI just uses the label that was received with that
   route. And if the route was received with no label, the LSR
   advertises the route with no label as well.


8. Alternative procedures for the restarting LSR

   In this section we describe an alternative to the procedures
   described in Section 7.

   The procedures described in this section assume that the restarting
   LSR has (at least) as many unallocated as allocated labels.  The
   latter forms the MPLS forwarding state that the LSR managed to
   preserve across the restart. The former is used for allocating labels
   after the restart.

   After the LSR restarts, it follows the procedures as specified in
   [1]. In addition, if the LSR is able to preserve its MPLS forwarding
   state across the restart, the LSR advertises this to its neighbors by
   appropriately setting the Flag field in the Graceful Restart
   Capability.

   To create local label bindings the LSR uses unallocated labels (this
   is pretty much the normal procedure). That means that as long as the
   LSR retains the MPLS forwarding state that the LSR preserved across
   the restart, the labels from that state are not used for creating
   local label bindings.

   The restarting LSR should retain the MPLS forwarding state that the
   LSR preserved across the restart at least until the LSR sends End-of-
   RIB marker to all of its neighbors (by that time the LSR already
   completed its route selection process, and also advertised its Adj-
   RIB-Out to its neighbors). It may be desirable to retain the
   forwarding state even a bit longer, as to allow the neighbors to
   receive and process the routes that have been advertised by the



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   restarting LSR. After that, the restarting LSR may delete the MPLS
   forwarding state that it preserved across the restart.

   Note that while an LSR is in the process of restarting, the LSR may
   have not one, but two local label bindings for a given BGP route -
   one that was retained from prior to restart, and another that was
   created after the restart. Once the LSR completes its restart, the
   former will be deleted. Both of these bindings though would have the
   same outgoing label (and the same next hop).


9. Procedures for a neighbor of a restarting LSR

   The neighbor of a restarting LSR (the receiving router in terminology
   used in [1]) follows the procedures specified in [1].  In addition,
   the neighbor should treat the MPLS labels received from the
   restarting LSR the same way as it treats the routes received from the
   restarting LSR (both prior and after the restart).

   Replacing the stale routes by the routing updates received from the
   restarting LSR involves replacing/updating the appropriate MPLS
   labels.

   In addition, if the Flags in the Graceful Restart Capability received
   from the restarting LSR indicate that the LSR wasn't able to retain
   its MPLS state across the restart, the neighbor should immediately
   remove all the NLRI and the associated MPLS labels that it previously
   acquired via BGP from the restarting LSR.

   An LSR, once it creates a <label, FEC> binding, should keep the value
   of the label in this binding for as long as the LSR has a route to
   the FEC in the binding. If the route to the FEC disappears, and then
   re-appears again later, then this may result in using a different
   label value, as when the route re-appears, the LSR would create a new
   <label, FEC> binding.

   To minimize the potential mis-routing caused by the label change,
   when creating a new <label, FEC> binding the LSR should pick up the
   least recently used label. Once an LSR releases a label, the LSR
   should not re-use this label for advertising a <label, FEC> binding
   to a neighbor that supports graceful restart for at least the Restart
   Time, as advertised by the neighbor to the LSR.









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10. Security Consideration

   This document does not introduce new security issues. The security
   considerations pertaining to the original BGP protocol remain
   relevant.


11. Intellectual Property Considerations

   Juniper Networks, Inc. is seeking patent protection on some or all of
   the technology described in this Internet-Draft. If technology in
   this document is adopted as a standard, Juniper Networks agrees to
   license, on reasonable and non-discriminatory terms, any patent
   rights it obtains covering such technology to the extent necessary to
   comply with the standard.

   Redback Networks, Inc. is seeking patent protection on some of the
   technology described in this Internet-Draft. If technology in this
   document is adopted as a standard, Redback Networks agrees to
   license, on reasonable and non-discriminatory terms, any patent
   rights it obtains covering such technology to the extent necessary to
   comply with the standard.


12. Acknowledgments

   We would like to thank Chaitanya Kodeboyina for his review and
   comments. The approach described in Section 8 is based on the idea
   suggested by Manoj Leelanivas.


13. References

   [1] "Graceful Restart Mechanism for BGP", draft-ietf-idr-
   restart-01.txt

   [2] "Carrying Label Information in BGP-4", RFC3107














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14. Author Information


Yakov Rekhter
Juniper Networks
1194 N.Mathilda Ave
Sunnyvale, CA 94089
e-mail: yakov@juniper.net

Rahul Aggarwal
Redback Networks
350 Holger Way
San Jose, CA 95134
e-mail: rahul@redback.com





































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