L2VPN Working Group B. Kothari Internet Draft K. Kompella Intended status: Standards Track Juniper Networks Expires: January 2010 W. Henderickx F. Balus Alcatel-Lucent July 13, 2009 BGP based Multi-homing in Virtual Private LAN Service draft-kothari-henderickx-l2vpn-vpls-multihoming-01.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. 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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 January 13, 2010. Kothari, et al. Expires January 13, 2010 [Page 1] Internet-Draft BGP based Multi-homing in VPLS July 2009 Copyright Notice Copyright (c) 2009 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 in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Abstract Virtual Private LAN Service (VPLS) is a Layer 2 Virtual Private Network (VPN) that gives its customers the appearance that their sites are connected via a Local Area Network (LAN). It is often required for the Service Provider (SP) to give the customer redundant connectivity to some sites, often called "multi-homing". This memo shows how BGP-based multi-homing can be offered in the context of LDP and BGP VPLS solutions. Table of Contents 1. Introduction...................................................3 1.1. General Terminology.......................................4 1.2. Conventions used in this document.........................4 2. Background.....................................................4 2.1. Scenarios.................................................4 2.2. VPLS Multi-homing Considerations..........................5 3. Multi-homing Operation.........................................6 3.1. Provisioning Model........................................6 3.2. Multi-homing NLRI.........................................6 3.3. Designated Forwarder Election.............................7 3.3.1. Attributes...........................................7 3.3.2. Variables Used.......................................7 3.3.2.1. RD..............................................7 3.3.2.2. MH-ID...........................................7 3.3.2.3. VBO.............................................8 3.3.2.4. DOM.............................................8 3.3.2.5. ACS.............................................8 3.3.2.6. PREF............................................8 3.3.2.7. PE-ID...........................................9 3.3.3. Election Procedures..................................9 3.3.3.1. Bucketization for BGP DF Election..............10 3.3.3.2. Bucketization for VPLS DF Election.............10 3.3.3.3. Tie-breaking Rules.............................10 3.4. DF Election on PEs.......................................11 Kothari, et al. Expires January 13, 2010 [Page 2] Internet-Draft BGP based Multi-homing in VPLS July 2009 4. Multi-AS VPLS.................................................12 4.1. Route Origin Extended Community..........................12 4.2. VPLS Preference..........................................12 4.3. Use of BGP-MH attributes in Inter-AS Methods.............13 4.3.1. Inter-AS Method (b): EBGP Redistribution of VPLS Information between ASBRs..................................14 4.3.2. Inter-AS Method (c): Multi-Hop EBGP Redistribution of VPLS Information between ASes..............................15 5. MAC Flush Operations..........................................15 5.1. MAC List Flush...........................................16 5.2. Implicit MAC Flush.......................................16 6. Backwards Compatibility.......................................16 6.1. BGP based VPLS...........................................17 6.2. LDP VPLS with BGP Auto-discovery.........................17 7. Security Considerations.......................................17 8. IANA Considerations...........................................17 9. References....................................................17 9.1. Normative References.....................................17 9.2. Informative References...................................18 10. Acknowledgments..............................................18 1. Introduction Virtual Private LAN Service (VPLS) is a Layer 2 Virtual Private Network (VPN) that gives its customers the appearance that their sites are connected via a Local Area Network (LAN). It is often required for a Service Provider (SP) to give the customer redundant connectivity to one or more sites, often called "multi-homing". [RFC4761] explains how VPLS can be offered using BGP for auto- discovery and signaling; section 3.5 of that document describes how multi-homing can be achieved in this context. [I-D.ietf-l2vpn- signaling] explains how VPLS can be offered using BGP for auto- discovery (BGP-AD) and [RFC4762] explains how VPLS can be offered using LDP for signaling. This document provides a BGP-based multi- homing solution applicable to both BGP and LDP VPLS technologies. Note that BGP MH can be used for LDP VPLS without the use of the BGP- AD solution. Section 2 lays out some of the scenarios for multi-homing, other ways that this can be achieved, and some of the expectations of BGP-based multi-homing. Section 3 defines the components of BGP-based multi- homing, and the procedures required to achieve this. Section 7 may someday discuss security considerations. Kothari, et al. Expires January 13, 2010 [Page 3] Internet-Draft BGP based Multi-homing in VPLS July 2009 1.1. General Terminology Some general terminology is defined here; most is from [RFC4761], [RFC4762] or [RFC4364]. Terminology specific to this memo is introduced as needed in later sections. A "Customer Edge" (CE) device, typically located on customer premises, connects to a "Provider Edge" (PE) device, which is owned and operated by the SP. A "Provider" (P) device is also owned and operated by the SP, but has no direct customer connections. A "VPLS Edge" (VE) device is a PE that offers VPLS services. A VPLS domain represents a bridging domain per customer. A Route Target community as described in [RFC4360] is typically used to identify all the PE routers participating in a particular VPLS domain. A VPLS site is a grouping of ports on a PE that belong to the same VPLS domain. A Multi-homed (MH) site is uniquely identified by a MH site ID (MH-ID). Sites are referred to as local or remote depending on whether they are configured on the PE router in context or on one of the remote PE routers (network peers). 1.2. 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 [RFC2119]. 2. Background This section describes various scenarios where multi-homing may be required, and the implications thereof. It also describes some of the singular properties of VPLS multi-homing, and what that means from both an operational point of view and an implementation point of view. There are other approaches for providing multi-homing such as Spanning Tree Protocol, and this document specifies use of BGP for multi-homing. Comprehensive comparison among the approaches is outside the scope of this document. 2.1. Scenarios The most basic scenario is shown in Figure 1. CE1 is a VPLS CE that is dual-homed to both PE1 and PE2 for redundant connectivity. Kothari, et al. Expires January 13, 2010 [Page 4] Internet-Draft BGP based Multi-homing in VPLS July 2009 ............... . . ___ CE2 ___ PE1 . / / : PE3 __/ : Service : CE1 __ : Provider PE4 \ : : \___ CE3 \___ PE2 . . . ............... Figure 1 Scenario 1 CE1 is a VPLS CE that is dual-homed to both PE1 and PE2 for redundant connectivity. However, CE4, which is also in the same VPLS domain, is single-homed to just PE1. CE4 ------- ............... \ . . ___ CE2 ___ PE1 . / / : PE3 __/ : Service : CE1 __ : Provider PE4 \ : : \___ CE3 \___ PE2 . . . ............... Figure 2 Scenario 2 2.2. VPLS Multi-homing Considerations The first (perhaps obvious) fact about a multi-homed VPLS CE, such as CE1 in Figure 1 is that if CE1 is an Ethernet switch or bridge, a loop has been created in the customer VPLS. This is a dangerous situation for an Ethernet network, and the loop must be broken. Even if CE1 is a router, it will get duplicates every time a packet is flooded, which is clearly undesirable. The next is that (unlike the case of IP-based multi-homing) only one of PE1 and PE2 can be actively sending traffic, either towards CE1 or Kothari, et al. Expires January 13, 2010 [Page 5] Internet-Draft BGP based Multi-homing in VPLS July 2009 into the SP cloud. That is to say, load balancing techniques will not work. All other PEs MUST choose the same designated forwarder for a multi-homed site. Call the PE that is chosen to send traffic to/from CE1 the "designated forwarder". In Figure 2, CE1 and CE4 must be dealt with independently, since CE1 is dual-homed, but CE4 is not. 3. Multi-homing Operation This section describes procedures for electing a designated forwarder among the set of PEs that are multi-homed to a customer site. The procedures described in this section are applicable to BGP based VPLS, LDP based VPLS with BGP-AD or a VPLS that contains a mix of both BGP and LDP signaled PWs. 3.1. Provisioning Model Figure 1 shows a customer site, CE1, multi-homed to two VPLS PEs, PE1 and PE2. In order for all VPLS PEs within the same VPLS domain to elect one of the multi-homed PEs as the designated forwarder, an indicator that the PEs are multi-homed to the same customer site is required. This is achieved by assigning the same multi-homed site ID (MH-ID) on PE1 and PE2 for CE1. When remote VPLS PEs receive NLRI advertisement from PE1 and PE2 for CE1, the two NLRI advertisements for CE1 are identified as candidates for designated forwarder selection due to the same MH-ID. Thus, same MH-ID SHOULD be assigned on all VPLS PEs that are multi-homed to the same customer site. Note that a MH-ID=0 is invalid and a PE should discard such an advertisement. 3.2. Multi-homing NLRI Section 3.2.2 in [RFC4761] describes the encoding of the BGP VPLS NLRI. This NLRI contains fields VE-ID, VE block offset, VE block size and label base. For multi-homing operation, the same NLRI is used for identifying the multi-homed customers sites. The VE-ID field in the NLRI is set to MH-ID; the VE block offset, VE block size and label base are set to zero. Thus, the NLRI contains 2 octets indicating the length, 8 octets for Route Distinguisher, 2 octets for MH-ID and 7 octets with value zero. Figure 2 shows two customer sites, CE1 and CE4, connected to PE1 with CE1 multi-homed to PE1 and PE2. CE4 does not require special addressing, being associated with the base VPLS instance identified Kothari, et al. Expires January 13, 2010 [Page 6] Internet-Draft BGP based Multi-homing in VPLS July 2009 by the VSI-ID for LDP VPLS and VE-ID for BGP VPLS. However, CE1 which is multi-homed to PE1 and PE2 requires configuration of MH-ID and both PE1 and PE2 MUST be provisioned with the same MH-ID for CE1. It is valid to have non-zero VE block offset, VE block size and label base in the VPLS NLRI for a multi-homed site. However, multi-homing operations in such a case are outside the scope of this document. 3.3. Designated Forwarder Election BGP-based multi-homing for VPLS relies on BGP DF election and VPLS DF election. The net result of doing both BGP and VPLS DF election is that of electing a single designated forwarder (DF) among the set of PEs to which a customer site is multi-homed. All the PEs that are elected as non-designated forwarders MUST keep their attachment circuit to the multi-homed CE in blocked status (no forwarding). These election algorithms operate on VPLS advertisements, which include both the NLRI and attached BGP attributes. In order to simplify the explanation of these algorithms, we will use a number of variables derived from fields in the VPLS advertisement. These variables are: RD, MH-ID, VBO, DOM, ACS, PREF and PE-ID. The notation ADV -> means that from a received VPLS advertisement ADV, the respective variables were derived. The following sections describe two attributes needed for DF election, then describe the variables and how they are derived from fields in VPLS advertisement ADV, and finally describe how DF election is done. 3.3.1. Attributes The procedures below refer to two attributes: the Route Origin community (see Section 4.1) and the L2-info community (see Section 4.2.1). These attributes are required for inter-AS operation; for generality, the procedures below show how they are to be used. The procedures also say how to handle the case that either or both are not present. 3.3.2. Variables Used 3.3.2.1. RD RD is simply set to the Route Distinguisher field in the NLRI part of ADV. 3.3.2.2. MH-ID MH-ID is simply set to the VE-ID field in the NLRI part of ADV. Kothari, et al. Expires January 13, 2010 [Page 7] Internet-Draft BGP based Multi-homing in VPLS July 2009 3.3.2.3. VBO VBO is simply set to the VE Block Offset field in the NLRI part of ADV. This field will typically be zero. 3.3.2.4. DOM This variable, indicating the VPLS domain to which ADV belongs, is derived by applying BGP policy to the Route Target extended communities in ADV. The details of how this is done are outside the scope of this document. 3.3.2.5. ACS ACS is the status of the attachment circuits for a given site of a VPLS. ACS = 1 if all attachment circuits for the site are down, and 0 otherwise. For BGP-based Multi-homing, ADV MUST contain an L2-info extended community; within this community are control flags. One of these flags is the 'D' bit, described in [I-D.kothari-l2vpn-auto-site-id]. ACS is set to the value of the 'D' bit in ADV. 3.3.2.6. PREF PREF is derived from the Local Preference (LP) attribute in ADV as well as the VPLS Preference field (VP) in the L2-info extended community. If the Local Preference attribute is missing, LP is set to 0; if the L2-info community is missing, VP is set to 0. The following table shows how PREF is computed from LP and VP. Kothari, et al. Expires January 13, 2010 [Page 8] Internet-Draft BGP based Multi-homing in VPLS July 2009 +---------+---------------+----------+------------------------------+ | VP | LP Value | PREF | Comment | | Value | | Value | | +---------+---------------+----------+------------------------------+ | 0 | 0 | 0 | malformed advertisement, | | | | | unless ACS=1 | | | | | | | 0 | 1 to (2^16-1) | LP | backwards compatibility | | | | | | | 0 | 2^16 to | (2^16-1) | backwards compatibility | | | (2^32-1) | | | | | | | | | >0 | LP same as VP | VP | Implementation supports VP | | | | | | | >0 | LP != VP | 0 | malformed advertisement | +---------+---------------+----------+------------------------------+ Figure 3 PREF table 3.3.2.7. PE-ID If ADV contains a Route Origin (RO) community (see Section 4.1) with type 0x01, then PE-ID is set to the Global Administrator sub-field of the RO. Otherwise, if ADV has an ORIGINATOR_ID attribute, then PE-ID is set to the ORIGINATOR_ID. Otherwise, PE-ID is set to the BGP Identifier. 3.3.3. Election Procedures The election procedures described in this section apply equally to BGP VPLS and LDP VPLS. Election occurs in two stages. The first stage divides all received VPLS advertisements into buckets of relevant and comparable advertisements. Distinction MUST NOT be made on whether the NLRI is a multi-homing NLRI or not. In this stage, advertisements may be discarded as not being relevant to DF election. The second stage picks a single "winner" from each bucket by repeatedly applying a tie-breaking algorithm on a pair of advertisements from that bucket. The tie-breaking rules are such that the order in which advertisements are picked from the bucket does not affect the final result. Note that this is a conceptual description of the process; an implementation MAY choose to realize this differently as long as the semantics are preserved. Kothari, et al. Expires January 13, 2010 [Page 9] Internet-Draft BGP based Multi-homing in VPLS July 2009 Note: these procedures supersede the tie breaking rules described in (Section 9.1.2.2) [RFC4271] 3.3.3.1. Bucketization for BGP DF Election An advertisement ADV -> is discarded if DOM is not of interest to the BGP speaker. Otherwise, ADV is put into the bucket for . In other words, the information in BGP DF election consists of and only advertisements with exact same are candidates for DF election. 3.3.3.2. Bucketization for VPLS DF Election An advertisement ADV -> is discarded if DOM is not of interest to the VPLS PE. Otherwise, ADV is put into the bucket for . In other words, all advertisements for a particular VPLS domain that have the same MH-ID are candidates for VPLS DF election. 3.3.3.3. Tie-breaking Rules This section describes the tie-breaking rules for both BGP and VPLS DF election. Tie-breaking rules for BGP DF election are applied to candidate advertisements by any BGP speaker. Since RD must be same for advertisements to be candidates for BGP DF election, use of unique RDs will result in no candidate advertisements for BGP tie- breaking rules and thus, a BGP speaker in such a case will simply not do BGP DF election. Tie-breaking rules for VPLS DF election are applied to candidate advertisements by all VPLS PEs and the actions taken by VPLS PEs based on the VPLS DF election result are described in Section 3.4. Given two advertisements ADV1 and ADV2 from a given bucket, first compute the variables needed for DF election: ADV1 -> ADV2 -> Kothari, et al. Expires January 13, 2010 [Page 10] Internet-Draft BGP based Multi-homing in VPLS July 2009 Note that MH-ID1 = MH-ID2 and DOM1 = DOM2, since ADV1 and ADV2 came from the same bucket. If this is for BGP DF election, RD1 = RD2 and VBO1 = VBO2 as well. Then the following tie-breaking rules MUST be applied in the given order. 1. if (ACS1 != 1) AND (ACS2 == 1) ADV1 wins; stop if (ACS1 == 1) AND (ACS2 != 1) ADV2 wins; stop else continue 2. if (PREF1 > PREF2) ADV1 wins; stop; else if (PREF1 < PREF2) ADV2 wins; stop; else continue 3. if (PE-ID1 < PE-ID2) ADV1 wins; stop; else if (PE-ID1 > PE-ID2) ADV2 wins; stop; else ADV1 and ADV2 are from the same VPLS PE For BGP DF election, if there is no winner and ADV1 and ADV2 are from the same PE, BGP DF election should simply consider this as an update. For VPLS DF election, if there is no winner and ADV1 and ADV2 are from the same PE, a VPLS PE MUST retain both ADV1 and ADV2. 3.4. DF Election on PEs DF election algorithm MUST be run by all multi-homed VPLS PEs. In addition, all other PEs SHOULD also run the DF election algorithm. As a result of the DF election, multi-homed PEs that loose the DF election for a MH-ID MUST put the ACs associated with the MH-ID in non-forwarding state. DF election result on the egress PEs can be used in traffic forwarding decision. Figure 2 shows two customer sites, CE1 and CE4, connected to PE1 with CE1 multi-homed to PE1 and PE2. If PE1 is the designated forwarder for CE1, based on the DF election result, PE3 can chose to not send unknown unicast and multicast traffic to PE2 as Kothari, et al. Expires January 13, 2010 [Page 11] Internet-Draft BGP based Multi-homing in VPLS July 2009 PE2 is not the designated forwarder for any customer site and it has no other single homed sites connected to it. 4. Multi-AS VPLS This section describes multi-homing in an inter-AS context. 4.1. Route Origin Extended Community Due to lack of information about the PEs that originate the VPLS NLRIs in inter-AS operations, Route Origin Extended Community [RFC4360] is used to carry the source PE's IP address. To use Route Origin Extended Community for carrying the originator VPLS PE's loopback address, the type field of the community MUST be set to 0x01 and the Global Administrator sub-field MUST be set to the PE's loopback IP address. 4.2. VPLS Preference When multiple PEs are assigned the same site ID for multi-homing, it is often desired to be able to control the selection of a particular PE as the designated forwarder. Section 3.5 in [RFC4761] describes the use of BGP Local Preference in path selection to choose a particular NLRI, where Local Preference indicates the degree of preference for a particular VE. The use of Local Preference is inadequate when VPLS PEs are spread across multiple ASes as Local Preference is not carried across AS boundary. A new field, VPLS preference (VP), is introduced in this document that can be used to accomplish this. VPLS preference indicates a degree of preference for a particular customer site. VPLS preference is not mandatory for intra-AS operation; the algorithm explained in Section 3.3 will work with or without the presence of VPLS preference. Section 3.2.4 in [RFC4761] describes the Layer2 Info Extended Community that carries control information about the pseudowires. The last two octets that were reserved now carries VPLS preference as shown in Figure 3. Kothari, et al. Expires January 13, 2010 [Page 12] Internet-Draft BGP based Multi-homing in VPLS July 2009 +------------------------------------+ | Extended community type (2 octets) | +------------------------------------+ | Encaps Type (1 octet) | +------------------------------------+ | Control Flags (1 octet) | +------------------------------------+ | Layer-2 MTU (2 octet) | +------------------------------------+ | VPLS Preference (2 octets) | +------------------------------------+ Figure 4 Layer 2 Info Extended Community A VPLS preference is a 2-octets unsigned integer. A value of zero indicates absence of a VP and is not a valid preference value. This interpretation is required for backwards compatibility. Implementations using Layer2 Info Extended Community as described in (Section 3.2.4) [RFC4761] MUST set the last two octets as zero since it was a reserved field. For backwards compatibility, if VP is used, then BGP Local Preference MUST be set to the value of VP. Note that a Local Preference value of zero for a MH-Site is not valid unless 'D' bit in the control flags is set (see [I-D.kothari-l2vpn-auto-site-id]). In addition, Local Preference value greater than or equal to 2^16 for VPLS advertisements is not valid. 4.3. Use of BGP-MH attributes in Inter-AS Methods Section 3.4 in [RFC4761] and section 4 in [I-D.ietf-l2vpn-signaling] describe three methods (a, b and c) to connect sites in a VPLS to PEs that are across multiple AS. Since VPLS advertisements in method (a) do not cross AS boundaries, multi-homing operations for method (a) remain exactly the same as they are within as AS. However, for method (b) and (c), VPLS advertisements do cross AS boundary. This section describes the VPLS operations for method (b) and method (c). Consider Figure 5 for inter-AS VPLS with multi-homed customer sites. Kothari, et al. Expires January 13, 2010 [Page 13] Internet-Draft BGP based Multi-homing in VPLS July 2009 4.3.1. Inter-AS Method (b): EBGP Redistribution of VPLS Information between ASBRs AS1 AS2 ........ ........ CE2 _______ . . . . ___ PE1 . . PE3 --- CE3 / : . . : __/ : : : : CE1 __ : ASBR1 --- ASBR2 : \ : : : : \___ PE2 . . PE4 ---- CE4 . . . . ........ ........ Figure 5 Inter-AS VPLS A customer has four sites, CE1, CE2, CE3 and CE4. CE1 is multi-homed to PE1 and PE2 in AS1. CE2 is single-homed to PE1. CE3 and CE4 are also single homed to PE3 and PE4 respectively in AS2. Assume that in addition to the base LDP/BGP VPLS addressing (VSI-IDs/VE-IDs), MH ID 1 is assigned for CE1. After running DF election algorithm, all four VPLS PEs must elect the same designated forwarder for CE1 site. Since BGP Local Preference is not carried across AS boundary, VPLS preference as described in Section 4.2 MUST be used for carrying site preference in inter-AS VPLS operations. For Inter-AS method (b) ASBR1 will send a VPLS NLRI received from PE1 to ASBR2 with itself as the BGP nexthop. ASBR2 will send the received NLRI from ASBR1 to PE3 and PE4 with itself as the BGP nexthop. Since VPLS PEs use BGP Local Preference in DF election, for backwards compatibility, ASBR2 MUST set the Local Preference value in the VPLS advertisements it sends to PE3 and PE4 to the VPLS preference value contained in the VPLS advertisement it receives from ASBR1. ASBR1 MUST do the same for the NLRIs it sends to PE1 and PE2. If ASBR1 receives a VPLS advertisement without a valid VPLS preference from a PE within its AS, then ASBR1 MUST set the VPLS preference in the advertisements to the Local Preference value before sending it to ASBR2. Similarly, ASBR2 must do the same for advertisements without VPLS Preference it receives from PEs within its AS. Thus, in method (b), ASBRs MUST update the VPLS and Local Preference based on the advertisements they receive either from an ASBR or a PE within their AS. Kothari, et al. Expires January 13, 2010 [Page 14] Internet-Draft BGP based Multi-homing in VPLS July 2009 In Figure 5, PE1 will send the VPLS advertisements, including the ones for MH site CE1, with Route Origin Extended Community containing its loopback address. PE2 will do the same. Even though PE3 receives the VPLS advertisements from the same BGP nexthop, ASBR2, the source PE address contained in the Route Origin Extended Community is different for the VPLS advertisements received from PE1 and PE2, and thus, PE3 can apply correctly the DF Election algorithm as the resulting PE-IDs are different. 4.3.2. Inter-AS Method (c): Multi-Hop EBGP Redistribution of VPLS Information between ASes In this method, there is a multi-hop E-BGP peering between the PEs or Route Reflectors in AS1 and the PEs or Route Reflectors in AS2. There is no VPLS state in either control or data plane on the ASBRs. The multi-homing operations on the PEs in this method are exactly the same as they are in intra-AS scenario. However, since Local Preference is not carried across AS boundary, the translation of LP to VP and vice versa MUST be done by RR, if RR is used to reflect VPLS advertisements to other ASes. This is exactly the same as what a ASBR does in case of method (b). A RR must set the VP to the LP value in an advertisement before sending it to other ASes and must set the LP to the VP value in an advertisement that it receives from other ASes before sending to the PEs within the AS. 5. MAC Flush Operations In a service provider VPLS network, customer MAC learning is confined to PE devices and any intermediate nodes, such as a Route Reflector, do not have any state for MAC addresses. Topology changes either in the service provider's network or in customer's network can result in the movement of MAC addresses from one PE device to another. Such events can result into traffic being dropped due to stale state of MAC addresses on the PE devices. Age out timers that clear the stale state will resume the traffic forwarding, but age out timers are typically in minutes, and convergence of the order of minutes can severely impact customer's service. To handle such events and expedite convergence of traffic, flushing of affected MAC addresses is highly desirable. This section describes the scenarios where VPLS flush is desirable and the specific VPLS Flush TLVs that provide capability to flush the affected MAC addresses on the PE devices. All operations described in this section are in context of a particular VPLS domain and not across multiple VPLS domains. Mechanisms for MAC flush are described Kothari, et al. Expires January 13, 2010 [Page 15] Internet-Draft BGP based Multi-homing in VPLS July 2009 in [I-D.kothari-l2vpn-vpls-flush] for BGP based VPLS and in [RFC4762] for LDP based VPLS. 5.1. MAC List Flush If multiple customer sites are connected to the same PE, PE1 as shown in Figure 2, and redundancy per site is desired when multi-homing procedures described in this document are in effect, then it is desirable to flush just the relevant MAC addresses from a particular site when the site connectivity is lost. To flush particular set of MAC addresses, a PE SHOULD originate a flush message with MAC list that contains a list of MAC addresses that needs to be flushed. In Figure 2, if connectivity between CE1 and PE1 goes down and if PE1 was the designated forwarder for CE1, PE1 SHOULD send a list of MAC addresses that belong to CE1 to all its BGP peers. It is RECOMMENDED that in case of excessive link flap of customer attachment circuit in a short duration, a PE should have a means to throttle advertisements of flush messages so that excessive flooding of such advertisements do not occur. 5.2. Implicit MAC Flush When connectivity to a customer site is lost, remote PEs learn that a particular site is no longer reachable. The local PE either withdraws the VPLS NLRI that it previously advertised for the site or it sends a BGP update message for the site's VPLS NLRI with the 'D' bit set. If a remote PE detects that a multi-homed PE has transitioned from being a DF to a non-DF, then the remote PE can choose to flush all MAC addresses that it learned from the multi-homed PE transitioning from DF to non-DF. Alternatively the remote PE may chose to react when detecting the non-DF to DF transition for a multi-homed PE by flushing in the related VPLS context all the MACs learned with the exception of the MACs associated with the new DF PE. 6. Backwards Compatibility No forwarding loops are formed when PEs or Route Reflectors that do not support procedures defined in this section co exist in the network with PEs or Route Reflectors that do support. Kothari, et al. Expires January 13, 2010 [Page 16] Internet-Draft BGP based Multi-homing in VPLS July 2009 6.1. BGP based VPLS As explained in this section, multi-homed PEs to the same customer site MUST assign the same MH-ID and related NLRI SHOULD contain the block offset, block size and label base as zero. Remote PEs that lack support of multi-homing operations specified in this document will fail to create any PWs for the multi-homed MH-IDs due to the label value of zero and thus, the multi-homing NLRI should have no impact on the operation of Remote PEs that lack support of multi-homing operations specified in this document. 6.2. LDP VPLS with BGP Auto-discovery The BGP-AD NLRI has a prefix length of 12 containing only a 8 bytes RD and a 4 bytes VSI-ID. If a LDP VPLS PEs running BGP AD lacks support of multi-homing operations specified in this document, it SHOULD ignore a MH NLRI with the length field of 17. As a result it will not ask LDP to create any PWs for the multi-homed Site-ID and thus, the multi-homing NLRI should have no impact on LDP VPLS operation. 7. Security Considerations No new security issues are introduced beyond those that are described in [RFC4761] and [RFC4762]. 8. IANA Considerations At this time, this memo includes no request to IANA. 9. References 9.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4761] Kompella, K. and Y. Rekhter, "Virtual Private LAN Service (VPLS) Using BGP for Auto-Discovery and Signaling", RFC 4761, January 2007. [RFC4447] Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G. Heron, "Pseudowire Setup and Maintenance Using the Label Distribution Protocol (LDP)", RFC 4447, April 2006. [RFC4446] Martini, L., "IANA Allocations for Pseudowire Edge to Edge Emulation (PWE3)", BCP 116, RFC 4446, April 2006. Kothari, et al. Expires January 13, 2010 [Page 17] Internet-Draft BGP based Multi-homing in VPLS July 2009 [I-D.ietf-l2vpn-signaling] Rosen, E., "Provisioning, Autodiscovery, and Signaling in L2VPNs", draft-ietf-l2vpn-signaling-08 (work in progress), May 2006. [I-D.kothari-l2vpn-vpls-flush] Kothari, B. and R. Fernando, "VPLS Flush in BGP-based Virtual Private LAN Service", draft- kothari-l2vpn-vpls-flush-00 (work in progress), October 2008. [I-D.kothari-l2vpn-auto-site-id] Kothari, B., Kompella, K., and T. IV, "Automatic Generation of Site IDs for Virtual Private LAN Service", draft-kothari-l2vpn-auto-site-id-01 (work in progress), October 2008. [I-D.ietf-pwe3-redundancy-bit] Muley, P., Bocci, M., and L. Martini, "Preferential Forwarding Status bit definition", draft- ietf-pwe3-redundancy-bit-01 (work in progress), September 2008. 9.2. Informative References [RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended Communities Attribute", RFC 4360, February 2006. [RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private Networks (VPNs)", RFC 4364, February 2006. [RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route Reflection: An Alternative to Full Mesh Internal BGP (IBGP)", RFC 4456, April 2006. [RFC4762] Lasserre, M. and V. Kompella, "Virtual Private LAN Service (VPLS) Using Label Distribution Protocol (LDP) Signaling", RFC 4762, January 2007. [RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, January 2006. 10. Acknowledgments The authors would like to thank Yakov Rekhter, Nischal Sheth, Mitali Singh, Deven Raut and Nehal Bhau for their insightful comments and probing questions. Kothari, et al. Expires January 13, 2010 [Page 18] Internet-Draft BGP based Multi-homing in VPLS July 2009 Authors' Addresses Bhupesh Kothari Juniper Networks 1194 N. Mathilda Ave. Sunnyvale, CA 94089 US Email: bhupesh@juniper.net Kireeti Kompella Juniper Networks 1194 N. Mathilda Ave. Sunnyvale, CA 94089 US Email: kireeti@juniper.net Wim Henderickx Alcatel-Lucent Copernicuslaan 50 2018 Antwerp, Belgium Email: wim.henderickx@alcatel-lucent.be Florin Balus Alcatel-Lucent 701 E. Middlefield Road Mountain View, CA, USA 94043 Email: florin.balus@alcatel-lucent.com Kothari, et al. Expires January 13, 2010 [Page 19]