Requirements for the Graceful Shutdown of BGP Sessions
The information below is for an old version of the document that is already published as an RFC.
This is an older version of an Internet-Draft that was ultimately published as RFC 6198.
|Authors||Bruno Decraene , Zubair Ahmad , Tomonori Takeda , Pierre Francois|
|Last updated||2015-10-14 (Latest revision 2011-01-31)|
|RFC stream||Internet Engineering Task Force (IETF)|
|Additional resources||Mailing list discussion|
|IESG||IESG state||RFC 6198 (Informational)|
|Responsible AD||Ron Bonica|
|Send notices to||(None)|
GROW Working Group B. Decraene Internet-Draft France Telecom Intended status: Informational P. Francois UCL C. Pelsser IIJ Z. Ahmad Orange Business Services A. J. Elizondo Armengol Telefonica I+D T. Takeda NTT January 28, 2011 Requirements for the graceful shutdown of BGP sessions draft-ietf-grow-bgp-graceful-shutdown-requirements-07.txt Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. 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. This Internet-Draft will expire on July 27, 2011. Decraene, et al. Expires July 2011 [Page 1] Internet-Draft Requirements for the graceful shutdown of BGP sessions Copyright Notice Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Abstract The Border Gateway Protocol(BGP) is heavily used in Service Provider networks both for Internet and BGP/MPLS VPN services. For resiliency purposes, redundant routers and BGP sessions can be deployed to reduce the consequences of an AS Border Router or BGP session breakdown on customers' or peers' traffic. However simply taking down or even bringing up a BGP session for maintenance purposes may still induce connectivity losses during the BGP convergence. This is not satisfactory any more for new applications (e.g. voice over IP, on line gaming, VPN). Therefore, a solution is required for the graceful shutdown of a (set of) BGP session(s) in order to limit the amount of traffic loss during a planned shutdown. This document expresses requirements for such a solution. Table of Contents 1. Conventions used in this document...........................3 2. Introduction................................................3 3. Problem statement...........................................4 3.1. Example of undesirable BGP routing behavior.................4 3.2. Causes of packet loss.......................................5 4. Terminology.................................................6 5. Goals and requirements......................................7 6. Security Considerations.....................................9 7. IANA Considerations........................................10 8. References.................................................10 8.1. Normative References.......................................10 8.2. Informative References.....................................10 9. Acknowledgments............................................10 10. Appendix: Reference BGP Topologies.........................12 10.1. EBGP topologies............................................12 10.2. IBGP topologies............................................14 10.3. Routing decisions..........................................17 Decraene, et al. Expires July 2011 [Page 2] Internet-Draft Requirements for the graceful shutdown of BGP sessions 1. Conventions used in this document 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 RFC 2119 [RFC2119]. 2. Introduction The Border Gateway Protocol(BGP) [RFC4271] is heavily used in Service Provider networks both for Internet and BGP/MPLS VPN services [RFC4364]. For resiliency purposes, redundant routers and BGP sessions can be deployed to reduce the consequences of an AS Border Router or BGP session breakdown on customers' or peers' traffic. We place ourselves in the context where a Service Provider performs a maintenance operation and needs to shut down one or multiple BGP peering link(s) or a whole ASBR. If an alternate path is available within the AS, the requirement is to avoid or reduce customer or peer traffic loss during the BGP convergence. Indeed, as an alternate path is available in the Autonomous System (AS), it should be made possible to reroute the customer or peer traffic on this backup path before the BGP session(s) is/are torn down, the nominal path is withdrawn and the forwarding is stopped. The requirements also cover the subsequent re-establishment of the BGP session as even this "UP" case can currently trigger route loss and thus traffic loss at some routers. BGP [RFC4271] and MP-BGP [RFC4760] do not currently have a mechanism to gracefully migrate traffic from one BGP next hop to another without interrupting the flow of traffic. When a BGP session is taken down, BGP behaves as if it was a sudden link or router failure and withdraws the prefixes learnt over that session, which may trigger traffic loss. There is no mechanism to advertise to its BGP peers that the prefix will soon be unreachable, while still being reachable. When applicable, such mechanism would reduce or prevent traffic loss. It would typically be applicable in case of a maintenance operation requiring the shutdown of a forwarding resource. Typical examples would be a link or line card maintenance, replacement or upgrade. It may also be applicable for a software upgrade as it may involve a firmware reset on the line cards and hence forwarding interruption. The introduction of Route Reflectors as per [RFC4456] to solve scalability issues bound to IBGP full-meshes has worsened the duration of routing convergence as some route reflectors may hide the back up path. Thus depending on RR topology more IBGP hops may be involved in the IBGP convergence. Decraene, et al. Expires July 2011 [Page 3] Internet-Draft Requirements for the graceful shutdown of BGP sessions Note that these planned maintenance operations cannot be addressed by Graceful Restart extensions [RFC4724] as GR only applies when the forwarding is preserved during the control plane restart. On the contrary, Graceful Shutdown applies when the forwarding is interrupted. Note also that some protocols are already considering such graceful shutdown procedure (e.g. GMPLS in [RFC5817]). A metric of success is the degree to which such a mechanism eliminates traffic loss during maintenance operations. 3. Problem statement As per [RFC4271], when one (or many) BGP session(s) are shut down, a BGP NOTIFICATION message is sent to the peer and the session is then closed. A protocol convergence is then triggered both by the local router and by the peer. Alternate paths to the destination are selected, if known. If those alternates paths are not known prior to the BGP session shutdown, additional BGP convergence steps are required in each AS to search for an alternate path. This behavior is not satisfactory in a maintenance situation because the traffic that was directed towards the removed next-hops may be lost until the end of the BGP convergence. As it is a planned operation, a make before break solution should be made possible. As maintenance operations are frequent in large networks [Reliable], the global availability of the network is significantly impaired by this BGP maintenance issue. 3.1. Example of undesirable BGP routing behavior To illustrate these problems, let us consider the following simple example where one customer router "CUST" is dual-attached to two SP routers "ASBR1" and "ASBR2". ASBR1 and ASBR2 are in the same AS and owned by the same service provider. Both are IBGP client of the route reflector R1. Decraene, et al. Expires July 2011 [Page 4] Internet-Draft Requirements for the graceful shutdown of BGP sessions ' AS1 ' AS2 ' /-----------ASBR1--- / \ / \ CUST R1 \ / Z/z \ / \-----------ASBR2--- ' AS1 ' AS2 ' Figure 1. Dual attached customer Before the maintenance, packets for destination Z/z use the ASBR1- CUST link because R1 selects ASBR1's route based on the IGP cost. Let's assume the service provider wants to shutdown the ASBR1-CUST link for maintenance purposes. Currently, when the shutdown is performed on ASBR1, the following steps are performed: 1. ASBR1 withdraw its prefix Z/z to its route reflector R1. 2. R1 runs its decision process, selects the route from ASBR2 and advertises the new path to ASBR1. 3. ASBR1 runs its decision process and recovers the reachability of Z/z. Traffic is lost between step 1 when ASBR1 looses its route and step 3 when it discovers a new path. Note that this is a simplified description for illustrative purpose. In a bigger AS, multiple steps of BGP convergence may be required to find and select the best alternate path (e.g. ASBR1 is chosen based on a higher local pref, hierarchical route reflectors are used...). When multiple BGP routers are involved and plenty of prefixes are affected, the recovery process can take longer than applications requirements. 3.2. Causes of packet loss The loss of packets during the maintenance has two main causes: - lack of an alternate path on some routers, - transient routing inconsistency. Decraene, et al. Expires July 2011 [Page 5] Internet-Draft Requirements for the graceful shutdown of BGP sessions Some routers may lack an alternate path because another router is hiding the backup path. This router can be: - a route reflector only propagating its best path; - the backup ASBR not advertising the backup path because it prefers the nominal path. This lack of knowledge of the alternate path is the first target of this requirement draft. Transient routing inconsistencies happen during IBGP convergence because routers do not simultaneously update their RIBs and hence do not simultaneously update their FIBs entries. This can lead to forwarding loops which result in both link congestion and packet drops. The duration of these transient micro-loops is dependent on the IBGP topology (e.g. number of Route Reflectors between ingress and egress ASBR), implementation differences among router platforms which result in differences in the time taken to update specific prefix in the FIB, forwarding mode (hop by hop IP forwarding versus tunneling). Note that when an IP lookup is only performed on entry to the AS, for example prior to entry into a tunnel across the AS, micro-loops will not occur. An example of this is when BGP is being used to as the routing protocol for MPLS VPN as defined in [RFC4364]. Note that [RFC5715] defines a framework for loop-free convergence. It has been written in the context of IP Fast ReRoute for link state IGP [RFC5714] but some concepts are also of interest for BGP convergence. 4. Terminology g-shut: Graceful SHUTdown. A method for explicitly notifying the BGP routers that a BGP session (and hence the prefixes learnt over that session) is going to be disabled. g-noshut: Graceful NO SHUTdown. A method for explicitly notifying the BGP routers that a BGP session (and hence the prefixes learnt over that session) is going to be enabled. g-shut initiator: the router on which the session(s) shutdown is (are) performed for the maintenance. g-shut neighbor: a router that peers with the g-shut initiator via (one of) the session(s) undergoing maintenance. Affected prefixes: a prefix initially reached via the peering link(s) undergoing maintenance. Affected router: a router reaching an affected prefix via a peering link undergoing maintenance. Initiator AS: the autonomous system of the g-shut initiator router. Decraene, et al. Expires July 2011 [Page 6] Internet-Draft Requirements for the graceful shutdown of BGP sessions Neighbor AS(es): the autonomous system(s) of the g-shut neighbor router(s). 5. Goals and requirements Currently, when a BGP session of the router under maintenance is shut down, the router removes the routes and then triggers the BGP convergence on its BGP peers by withdrawing its route. The goal of BGP graceful shutdown of a (set of) BGP session(s) is to minimize traffic loss during a planned shutdown. Ideally a solution should reduce this traffic loss to zero. Another goal is to minimize and preferably to eliminate packet loss when the BGP session is re-established following the maintenance. As the event is known in advance, a make before break solution can be used in order to initiate the BGP convergence, find and install the alternate paths before the nominal paths are removed. As a result, before the nominal BGP session is shut down, all affected routers learn and use the alternate paths. Those alternate paths are computed by BGP taking into account the known status of the network which includes known failures that the network is processing concurrently with the BGP session graceful shutdown and possibly known other graceful shutdown under way. Therefore multiple BGP graceful shutdowns overlapping within a short timeframe are gracefully handled. Indeed a given graceful shutdown takes into account all previous ones and previous graceful shutdown are given some time to adapt to this new one. Then the nominal BGP session can be shut down. As a result, provided an alternate path with enough remaining capacity is available, the packets are rerouted before the BGP session termination and fewer packets (possibly none) are lost during the BGP convergence process since at any time, all routers have a valid path. From the above goals we can derive the following requirements: a) A mechanism to advertise the maintenance action to all affected routers is REQUIRED. Such mechanism may be either implicit or explicit. Note that affected routers can be located both in the local AS and in neighboring ASes. Note also that the maintenance action can either be the shutdown of a BGP session or the establishment of a BGP session. The mechanism SHOULD allow BGP routers to minimize and preferably to eliminate packet loss when a path is removed or advertised. In particular, it SHOULD be ensured that the old path is not removed from the routing tables of the affected routers before the new path is known. The solution mechanism MUST significantly reduce and ideally Decraene, et al. Expires July 2011 [Page 7] Internet-Draft Requirements for the graceful shutdown of BGP sessions eliminate packet loss. A trade off may be made between the degree of packet loss and the simplicity of the solution. b) An Internet wide convergence is OPTIONAL. However if the initiator AS and the neighbor AS(es) have a backup path, they SHOULD be able to gracefully converge before the nominal path is shut down. c) The proposed solution SHOULD be applicable to any kind of BGP sessions (EBGP, IBGP, IBGP route reflector client, EBGP confederations, EBGP multi hop, MultiProtocol BGP extension...) and any address family. If a BGP implementation allows closing or enabling a sub-set of AFIs carried in a MP-BGP session, this mechanism MAY be applicable to this sub-set of AFIs. Depending on the kind of session, there may be some variations in the proposed solution in order to fulfill the requirements. The following cases should be handled in priority: - The shutdown of an inter-AS link and therefore the shutdown of an eBGP session; - The shutdown of an AS Border Router and therefore the shutdown of all its BGP sessions. Service Providers and platforms implementing a graceful shutdown solution should note that in BGP/MPLS VPN as per [RFC4364], the PE-CE routing can be performed by other protocols than BGP (e.g. static routes, RIPv2, OSPF, IS-IS). This is out of scope of this document. d) The proposed solution SHOULD NOT change the BGP convergence behavior for the ASes exterior to the maintenance process, namely ASes other than the initiator AS and it(s) neighbor AS(es). e) An incremental deployment on a per AS or per BGP session basis MUST be made possible. In case of partial deployment the proposed solution SHOULD incrementally improve the maintenance process. It should be noted that in an inter domain relation, one AS may have more incentive to use graceful shutdown than the other. Similarly, in a BGP/MPLS VPN environment, it's much easier to upgrade the PE routers than the CE mainly because there is at least an order of magnitude more CE and CE locations than PE and PE locations. As a consequence, when splitting the cost of the solution between the g- shut initiator and the g-shut neighbour the solution SHOULD favour a low cost solution on the neighbour AS side in order to reduce the impact on the g-shut neighbour. Impact should be understood as a generic term which includes first hardware, then software, then configuration upgrade. f) Redistribution or advertisement of (static) IP routes into BGP SHOULD also be covered. Decraene, et al. Expires July 2011 [Page 8] Internet-Draft Requirements for the graceful shutdown of BGP sessions g) The proposed solution MAY be designed in order to avoid transient forwarding loops. Indeed, forwarding loops increase packet transit delay and may lead to link saturation. h) The specific procedure SHOULD end when the BGP session is closed following the g-shut and once the BGP session is gracefully opened following the g-noshut. In the end, once the planned maintenance is finished the nominal BGP routing MUST be reestablished. The duration of the g-shut procedure, and hence the time before the BGP session is safely closed SHOULD be discussed by the solution document. Examples of possible solutions are the use of a pre- configured timer, of a message to signal the end of the BGP convergence or monitoring the traffic on the g-shut interface. i) The solution SHOULD be simple and simple to operate. Hence it MAY only cover a subset of the cases. As a consequence, most of the above requirements are expressed as "SHOULD" rather than "MUST". The metrics to evaluate and compare the proposed solutions are: - The duration of the remaining loss of connectivity when the BGP session is brought down or up; - The applicability to a wide range of BGP and network topologies; - The simplicity; - The duration of transient forwarding loops; - The additional load introduced in BGP (e.g. BGP messages sent to peer routers, peer ASes, the Internet). 6. Security Considerations At the requirements stage, this graceful shutdown mechanism is expected to not affect the security of the BGP protocol, especially if it can be kept simple. No new sessions are required and the additional ability to signal the graceful shutdown is not expected to bring additional attack vector as BGP neighbors already have the ability to send incorrect or misleading information or even shut down the session. Security considerations MUST be addressed by the proposed solutions. In particular they SHOULD address the issues of bogus g-shut messages and how they would affect the network(s), as well as the impact of hiding a g-shut message so that g-shut is not performed. The solution SHOULD NOT increase the ability for one AS to selectively influence routing decision in the peer AS (inbound Traffic Engineering) outside the case of the BGP session shutdown. Otherwise, the peer AS SHOULD have means to detect such behavior. Decraene, et al. Expires July 2011 [Page 9] Internet-Draft Requirements for the graceful shutdown of BGP sessions 7. IANA Considerations This document has no actions for IANA. 8. References 8.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4271] Rekhter, Y. and T. Li, "A Border Gateway protocol 4 (BGP)", RFC 4271, January 2006. [RFC4760] Bates, T., Chandra, R., Katz, D. and Y. Rekhter, "Multiprotocol Extensions for BGP-4", RFC 4760 January 2007. [RFC4456] Bates, T., Chen E. and R. Chandra "BGP Route Reflection: An Alternative to Full Mesh Internal BGP (IBGP)", RFC 4456 April 2006. [RFC4364] Rosen, E. and Y. Rekhter "BGP/MPLS IP Virtual Private Networks (VPNs)", RFC 4364 February 2006. 8.2. Informative References [RFC5817] Ali, Z., Vasseur, J.P., Zamfir, A. and J. Newton, "Graceful Shutdown in MPLS and Generalized MPLS Traffic Engineering Networks", RFC 5817, April 2010. [RFC5715] Shand, M. and S. Bryant, "A Framework for Loop-Free Convergence", RFC 5715, January 2010. [RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework", RFC 5714, January 2010. [RFC4724] Sangli, S., Chen, E., Fernando, R., Scudder, J. and Y. Rekhter, "Graceful Restart Mechanism for BGP", RFC 4724, January 2007. [Reliable] Network Strategy Partners, LLC. "Reliable IP Nodes: A prerequisite to profitable IP services", November 2002. http://www.nspllc.com/NewPages/Reliable_IP_Nodes.pdf 9. Acknowledgments Authors would like to thank Nicolas Dubois, Benoit Fondeviole, Christian Jacquenet, Olivier Bonaventure, Steve Uhlig, Xavier Vinet, Vincent Gillet, Jean-Louis le Roux, Pierre Alain Coste and Decraene, et al. Expires July 2011 [Page 10] Internet-Draft Requirements for the graceful shutdown of BGP sessions Ronald Bonica for the useful discussions on this subject, their review and comments. This draft has been partly sponsored by the European project IST AGAVE. Decraene, et al. Expires July 2011 [Page 11] Internet-Draft Requirements for the graceful shutdown of BGP sessions 10. Appendix: Reference BGP Topologies This section describes some frequent BGP topologies used both within the AS (IBGP) and between ASes (EBGP). Solutions should be applicable to the following topologies and their combinations. 10.1. EBGP topologies This section describes some frequent BGP topologies used between ASes. In each figure, a line represents a BGP session. 10.1.1. 1 ASBR in AS1 connected to two ASBRs in the neighboring AS2 In this topology we have an asymmetric protection scheme between AS1 and AS2: - On AS2 side, two different routers are used to connect to AS1. - On AS1 side, one single router with two BGP sessions is used. ' AS1 ' AS2 ' /----------- ASBR2.1 / ' / ' ASBR1.1 ' \ ' \ ' \----------- ASBR2.2 ' ' AS1 ' AS2 ' Figure 2. EBGP topology with redundant ASBR in one of the AS. BGP graceful shutdown is expected to be applicable for the maintenance of: - one of the routers of AS2; - one link between AS1 and AS2, performed either on an AS1 or AS2 router. Note that in case of maintenance of the whole router, all its BGP sessions need to be gracefully shutdown at the beginning of the maintenance and gracefully brought up at the end of the maintenance. Decraene, et al. Expires July 2011 [Page 12] Internet-Draft Requirements for the graceful shutdown of BGP sessions 10.1.2. 2 ASBRs in AS1 connected to 2 ASBRs in AS2 In this topology we have a symmetric protection scheme between AS1 and AS2: on both sides, two different routers are used to connect AS1 to AS2. ' AS1 ' AS2 ' ASBR1.1----------- ASBR2.1 ' ' ' ' ' ASBR1.2----------- ASBR2.2 ' AS1 ' AS2 ' Figure 3. EBGP topology with redundant ASBRs in both ASes BGP graceful shutdown is expected to be applicable for the maintenance of: - any of the ASBR routers (in AS1 or AS2); - one link between AS1 and AS2 performed either on an AS1 or AS2 router. 10.1.3. 2 ASBRs in AS2 each connected to two different ASes In this topology at least three ASes are involved. ' AS1 ' AS2 ' ASBR1.1----------- ASBR2.1 | ' | ' '''''|'''''''''' | ' | ' ASBR3.1----------- ASBR2.2 ' AS3 ' AS2 Figure 4. EBGP topology of a dual homed customer Decraene, et al. Expires July 2011 [Page 13] Internet-Draft Requirements for the graceful shutdown of BGP sessions As the requirements expressed in section 5 is to advertise the maintenance only within the initiator and neighbor ASes, but not Internet wide, BGP graceful shutdown solutions may not be applicable to this topology. Depending on which routes are exchanged between these ASes, some protection for some of the traffic may be possible. For instance if ASBR2.2 performs a maintenance affecting ASBR3.1 then ASBR3.1 will be notified. However ASBR1.1 may not be notified of the maintenance of the eBGP session between ASBR3.1 and ASBR2.2. 10.2. IBGP topologies This section describes some frequent BGP topologies used within an AS. In each figure, a line represents a BGP session. 10.2.1. IBGP Full-Mesh In this topology we have a full mesh of IBGP sessions: P1 ----- P2 | \ / | | \ / | | \/ | AS1 | /\ | | / \ | | / \ | ASBR1.1--ASBR1.2 \ / \ / ''''''\'''/'''''''''''' \ / AS2 ASBR2.1 Figure 5. IBGP full mesh When the session between ASBR1.1 and ASBR2.1 is gracefully shutdown, it is required that all affected routers of AS1 reroute traffic to ASBR1.2 before the session between ASBR1.1 and ASBR2.1 is shut down. Similarly, when the session between ASBR1.1 and ASBR2.1 is gracefully brought up, all affected routers of AS1 preferring ASBR1.1 over ASBR1.2 need to reroute traffic to ASBR1.1 before the less preferred path through ASBR1.2 is possibly withdrawn. 10.2.2. Route Reflector In this topology, route reflectors are used to limit the number of IBGP sessions. There is a single level of route reflectors and the route reflectors are fully meshed. Decraene, et al. Expires July 2011 [Page 14] Internet-Draft Requirements for the graceful shutdown of BGP sessions P1 (RR)-- P2 (RR) | \ / | | \ / | | \ / | AS1 | \/ | | /\ | | / \ | | / \ | | / \ | ASBR1.1 ASBR1.2 \ / \ / ''''''\''''''/'''''''''''' \ / \ / AS2 ASBR2.1 Figure 6. Route Reflector When the session between ASBR1.1 and ASBR2.1 is gracefully shutdown, all BGP routers of AS1 need to reroute traffic to ASBR1.2 before the session between ASBR1.1 and ASBR2.1 is shut down. Similarly, when the session between ASBR1.1 and ASBR2.1 is gracefully brought up, all affected routers of AS1 preferring ASBR1.1 over ASBR1.2 need to reroute traffic to ASBR1.1 before the less preferred path through ASBR1.2 is possibly withdrawn. 10.2.3. hierarchical Route Reflector In this topology, hierarchical route reflectors are used to limit the number of IBGP sessions. There could me more than two levels of route reflectors and the top level route reflectors are fully meshed. Decraene, et al. Expires July 2011 [Page 15] Internet-Draft Requirements for the graceful shutdown of BGP sessions P1 (RR) -------- P2 (RR) | | | | | | AS1 | | | | P3 (RR) P4 (RR) | | | | | | AS1 | | | | ASBR1.1 ASBR1.2 \ / \ / ''''''\'''''''''/'''''''''''' \ / \ / AS2 ASBR2.1 Figure 7. Hierarchical Route Reflector When the session between ASBR1.1 and ASBR2.1 is gracefully shutdown, all BGP routers of AS1 need to reroute traffic to ASBR1.2 before the session between ASBR1.1 and ASBR2.1 is shut down. Similarly, when the session between ASBR1.1 and ASBR2.1 is gracefully brought up, all affected routers of AS1 preferring ASBR1.1 over ASBR1.2 need to reroute traffic to ASBR1.1 before the less preferred path through ASBR1.2 is possibly withdrawn. 10.2.4. Confederations In this topology, a confederation of ASs is used to limit the number of IBGP sessions. Moreover, RRs may be present in the member ASs of the confederation. Confederations may be run with different sub-options. Regarding the IGP, each member AS can run its own IGP or they can all share the same IGP. Regarding BGP, local_pref may or may not cross the member AS boundaries. A solution should support the graceful shutdown and graceful bring up of EBGP sessions between member-ASs in the confederation in addition to the graceful shutdown and graceful bring up of EBGP sessions between a member-AS and an AS outside of the confederation. Decraene, et al. Expires July 2011 [Page 16] Internet-Draft Requirements for the graceful shutdown of BGP sessions ASBR1C.1 ---------- ASBR1C.2 | | | | | AS1C | | | | | """|"""""""""""""""""""|""" | " | ASBR1A.2 " ASBR1B.2 | " | | " | | AS1A " AS1B | AS1 | " | | " | ASBR1A.1 " ASBR1B.1 \ " / \ " / ''''''\'''''''''''''/'''''''''''' \ / \ / AS2 ASBR2.1 Figure 8. Confederation In the above figure, member-AS AS1A, AS1B, AS1C belong to a confederation of ASes in AS1. AS1A and AS1B are connected to AS2. In normal operation, for the traffic toward AS2, . AS1A sends the traffic directly to AS2 through ASBR1A.1 . AS1B sends the traffic directly to AS2 through ASBR1B.1 . AS1C load balances the traffic between AS1A and AS1B When the session between ASBR1A.1 and ASBR2.1 is gracefully shutdown, all BGP routers of AS1 need to reroute traffic to ASBR1B.1 before the session between ASBR1A.1 and ASBR2.1 is shut down. Similarly, when the session between ASBR1A.1 and ASBR2.1 is gracefully brought up, all affected routers of AS1 preferring ASBR1A.1 over ASBR1.2 need to reroute traffic to ASBR1A.1 before the less preferred path through ASBR1.2 is possibly withdrawn. 10.3. Routing decisions We describe here some routing engineering choices that are frequently used in ASes and that should be supported by the solution. Decraene, et al. Expires July 2011 [Page 17] Internet-Draft Requirements for the graceful shutdown of BGP sessions 10.3.1. Hot potato (IGP cost) Ingress router selects the nominal egress ASBR (AS exit point) based on the IGP cost to reach the BGP next-hop. 10.3.2. Cold potato (BGP local preference) Ingress router selects the nominal egress ASBR based on the BGP local LOCAL_PREF value set and advertised by the exit point. 10.3.3. Cold potato (BGP preference set on ingress) Ingress router selects the nominal egress ASBR based on preconfigured policy information. (Typically by locally setting the BGP local pref based on the BGP communities attached on the routes). As per [RFC4271], note that if tunnels are not used to forward packets between ingress and egress ASBR, this can lead to persistent forwarding loops. Authors' Addresses Bruno Decraene France Telecom 38-40 rue du General Leclerc 92794 Issy Moulineaux cedex 9 France Email: firstname.lastname@example.org Pierre Francois Universite catholique de Louvain Place Ste Barbe, 2 Louvain-la-Neuve 1348 BE Email: email@example.com Cristel Pelsser Internet Initiative Japan Jinbocho Mitsui Building 1-105 Kanda jinbo-cho Chiyoda-ku, Tokyo 101-0051 Japan Email: firstname.lastname@example.org Decraene, et al. Expires July 2011 [Page 18] Internet-Draft Requirements for the graceful shutdown of BGP sessions Zubair Ahmad Orange Business Services 13775 McLearen Road, Oak Hill VA 20171 USA Email: email@example.com Antonio Jose Elizondo Armengol Division de Analisis Tecnologicos Technology Analysis Division Telefonica I+D C/ Emilio Vargas 6 28043, Madrid E-mail: firstname.lastname@example.org Tomonori Takeda NTT Corporation 9-11, Midori-Cho 3 Chrome Musashino-Shi, Tokyo 180-8585 Japan Email: email@example.com Decraene, et al. Expires July 2011 [Page 19]