Network Working Group CY. Lee Internet-Draft Alcatel Expires: April 19, 2005 October 19, 2004 Partial Mesh in VPLS draft-lee-l2vpn-vpls-partial-mesh-00.txt Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. By submitting this Internet-Draft, we certify that any applicable patent or other IPR claims of which we are aware have been disclosed, and any of which we become aware will be disclosed, in accordance with RFC 3668. 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 April 19, 2005. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. Abstract Some standard network devices may not be able to communicate with each other as if they were connected to a common LAN segment in the event of partial mesh connectivity in a VPLS. Unless this problem is addressed, the deployment of VPLS may eventually be limited to sites not using link state routing or bridges. Lee Expires April 19, 2005 [Page 1] Internet-Draft Partial Mesh in VPLS October 2004 Table of Contents 1. Examples of Partial Mesh Connectivity . . . . . . . . . . . . 3 2. Consequences of Partial Mesh Connectivity . . . . . . . . . . 4 3. Security Considerations . . . . . . . . . . . . . . . . . . . 6 4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7 5. Normative References . . . . . . . . . . . . . . . . . . . . . 7 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 7 Intellectual Property and Copyright Statements . . . . . . . . 8 Lee Expires April 19, 2005 [Page 2] Internet-Draft Partial Mesh in VPLS October 2004 1. Examples of Partial Mesh Connectivity Some standard network devices may not be able to communicate with each other as if they were connected to a common LAN segment in the event of partial mesh connectivity in a VPLS. An example of partial mesh connectivity is :- in a full-meshed of tunnels connecting node A, B, C, the tunnel between A and B may be missing while the tunnels between A and C, and B and C are working. A tunnel shall be referred to as a Pseudo-Wire (PW) in this document. Partial mesh connectivity may occur due to various reasons, for instance, a PW not being configured or established properly, failed PWs are not restored, mal-functioning PE, a VPLS instance being enabled before all the PWs from/to a new site have been established or a PW of the full-mesh is being disabled. As an example, node A, B, C and D belong to a VPLS and are supposed to be fully-meshed. If B has no PW to C and B does not know C is a member of a VPLS, then the network operator cannot be alerted of the partial mesh connectivity in a VPLS. In contrast, if node A, B, C and D span a tree, and there is supposed to be a branch from B to C, but B does not know C is a member of a VPLS and hence has no PW to C, then the tree is simply partitioned with B on one partition and C on the other partition. Lee Expires April 19, 2005 [Page 3] Internet-Draft Partial Mesh in VPLS October 2004 2. Consequences of Partial Mesh Connectivity In a tree-based emulated LAN architecture, if a link is not working and not protected or restored, the emulated LAN or tree is partitioned. Routers and bridges work in this situation as on a LAN. If a VPLS is partitioned like a tree such that all all members cannot reach another member, routers and bridges function as they are on a LAN as well. However, if a VPLS is not partitioned in that manner, such that only a pair of PEs cannot reach each other, while other PEs can reach each other, aka partial mesh connectivity; routers and bridges may not behave as expected. Some examples where routers and bridges cannot work as on a LAN (when there is partial mesh connectivity in VPLS) are : 1. If a customer router B has no corresponding working PW to one of the sites of a VPLS, say site C, but has corresponding working PWs to all other sites including the site where the Designated Router is, the customer router may black-hole traffic to the site C where the corresponding PW is not working. The customer router B black-hole traffic because it can still receive link states advertised by the router at site C via the Designated Router at A. To illustrate this issue, assume three routers R1, R2, and R3, which is using OSPF or IS-IS with broadcast mode, are connected in a VPLS. R1 is DR (OSPF) or DIS (IS-IS), and the other routers are BDR/DROther (OSPF) or non-DIS (IS-IS). Routing information is propagated between R1-R2 and R1-R3. In this situation, if a PW between R2 and R3 is missing, the OSPF/IS-IS protocol cannot notify R1 DR/DIS. This is because the broadcast mode assumes routers are interconnected with a single medium and is not designed to handle this kind of error. Therefore routing information is not changed and as a result, a black-hole is formed between R2 and R3. Note that the NBMA mode has the same problem. This problem can be avoided by using the point-to-multipoint mode of OSPF [On VPLS and Routing Protocols]. However, there is no need for a VPLS in that case. The customer subscribes point-to-point links from the provider and the customer uses the point-to-multipoint mode in OSPF. Further, it does not seem to make sense if the provider has a VPLS solution but provide point-to-point connectivity instead to allow the customer to get around this partial mesh issue. On the other hand, if a customer uses point-to-point links :- - the number of router "adjacencies that need to be established is O(N^2) (vs O(N) in the broadcast network case) which affects the Lee Expires April 19, 2005 [Page 4] Internet-Draft Partial Mesh in VPLS October 2004 complexity of the routing protocol operation, most notably", in terms of the link-state database size, the amount of routing protocol traffic and SPF computation. - the customer may have to manually configure neighbors. 2. If a customer bridge has no corresponding working PW to one of the sites of a VPLS, but has corresponding working PWs to all other sites including the site where the Root Bridge is located, the customer bridge cannot failover to a backup link. Hence bridges and link state routers experience loss of connectivity (that persist as long as the PW is missing) to a site even if the customer devices have alternate paths to the site. On the other hand, a customer device cannot simply use alternate paths since the customer device is still partially connected to the emulated LAN. If a customer uses the point-to-multipoint mode in OSPF to get around this problem, there is no need for a VPLS then. If this issue is not addressed, the deployment of VPLS may eventually be limited to sites which do not use bridges or link state routing. Lee Expires April 19, 2005 [Page 5] Internet-Draft Partial Mesh in VPLS October 2004 3. Security Considerations This draft does not introduce any new security issues in VPLS. Lee Expires April 19, 2005 [Page 6] Internet-Draft Partial Mesh in VPLS October 2004 4. Acknowledgements We would like to thank the IS-IS and OSPF WG and IEEE 802.1 for clarifying some of the issues routers and bridges have with partial mesh connectivity in an emulated LAN. The draft has also benefited from valuable suggestions by Muneyoshi Suzuki, Vach Kompella, Dimitri Papadimitriou, Italo Busi and Chris Liljenstolpe. 5 Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997. [VPLS] Lasseurre, M. and V. Kompella, "Virtual Private LAN Service", draft-ietf-l2vpn-vpls-ldp-05.txt, 2004. [On VPLS and Routing Protocols] Zinin, A., "On VPLS and Routing Protocols", http://www1.ietf.org/mail-archive/web/l2vpn/current/msg000 65.html. Author's Address Cheng-Yin Lee Alcatel 600 March Rd Ottawa Canada Phone: EMail: Cheng-Yin.Lee@alcatel.com Lee Expires April 19, 2005 [Page 7] Internet-Draft Partial Mesh in VPLS October 2004 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. 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