BGP Bestpath Selection Criteria Enhancement
Author(s): Rajiv Asati
BGP specification [RFC4271] prescribes 'BGP next-hop reachability' as one of the key 'Route Resolvability Condition' that must be satisfied before the BGP bestpath candidate selection. This condition, however, may not be sufficient (as explained in the Appendix section)...
IDR Working Group Rajiv Asati Internet Draft Cisco Systems Intended status: Standards Track Expires: August 25, 2013 February 25, 2013 BGP Bestpath Selection Criteria Enhancement draft-ietf-idr-bgp-bestpath-selection-criteria-06.txt Abstract BGP specification [RFC4271] prescribes 'BGP next-hop reachability' as one of the key 'Route Resolvability Condition' that must be satisfied before the BGP bestpath candidate selection. This condition, however, may not be sufficient (as explained in the Appendix section) and desire further granularity. This document defines enhances the "Route Resolvability Condition" to facilitate the next-hop to be resolved in the chosen data plane. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on August 25, 2013. Copyright Notice Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved. Asati, Rajiv Expires August 25, 2013 [Page 1] Internet-Draft BGP Bestpath Selection Criteria February 25, 2013 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. Table of Contents 1. Introduction...................................................2 2. Specification Language.........................................3 3. Route Resolvability Condition - Modification...................3 4. Conclusions....................................................4 5. Security Considerations........................................4 6. IANA Considerations............................................5 7. Acknowledgments................................................5 8. Appendix.......................................................5 9. References.....................................................7 Author's Addresses................................................8 1. Introduction As per BGP specification [RFC4271], when a router receives a BGP path, BGP must qualify it as the valid candidate prior to the BGP bestpath selection using the 'Route Resolvability Condition' (section#184.108.40.206 of RFC4271]. After the path gets qualified as the bestpath candidate, it becomes eligible to be the bestpath, and may get advertised out to the neigbhor(s), if it became the bestpath. However, in BGP networks that utilize data plane protocol other than IP, such as MPLS [RFC3031] etc. to forward the received traffic towards the next-hop, the above qualification condition may not be sufficient. In fact, this may expose the BGP networks to experience traffic blackholing i.e. traffic loss, due to malfunctioning of the chosen data plane protocol to the next-hop. This is explained further in the Appendix section. This document defines further granularity to the "Route Resolvability Condition" by (a) resolving the BGP next-hop Asati, Rajiv Expires August 25, 2013 [Page 2] Internet-Draft BGP Bestpath Selection Criteria February 25, 2013 reachability in the forwarding database of a particular data plane protocol, and (b) optionally including the BGP next-hop "path availability" check. The goal is to enable BGP to select the bestpaths based on whether or not the corresponding nexthop can be resolved in the valid data plane. 2. Specification Language 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]. 3. Route Resolvability Condition - Modification This document proposes two amendments to 'Route Resolvability Condition', which is defined in RFC4271, in consideration for a particular data plane protocol: 1) The next-hop reachability (check) SHOULD be resolved in a forwarding database of a particular data plane protocol. For example, if a BGP IPv4/v6 or VPNv4/v6 path wants to use MPLS data plane to the next-hop, as determined by the policy, then the BGP 'next-hop reachability' should be resolved using the MPLS forwarding database. In another example, if BGP path wants to use the IP data plane to the next-hop, as determined by the policy, then BGP 'next-hop reachability' should be resolved using the IP forwarding database. The latter example relates to MPLS-in-IP encapsulation techniques such as [RFC4817], [RFC4023] etc. The selection of particular data plane is a matter of a policy, and is outside the scope of this document. It is envisioned that the policy would exist for either per-neighbor or per-SAFI or both. A dynamic signaling such as BGP encapsulation SAFI (or tunnel encap attribute) [RFC5512] may be used to convey the data plane protocol chosen by the policy. This check is about confirming the availability of the valid forwarding entry for the next-hop in the forwarding database of the chosen data plane protocol. Asati, Rajiv Expires August 25, 2013 [Page 3] Internet-Draft BGP Bestpath Selection Criteria February 25, 2013 2) The 'path availability' check for the BGP next-hop MAY be performed. This criterion checks for the functional data plane path to the next-hop in a particular data plane protocol. The path availability check may be performed by any of the OAM data- plane liveness mechanisms associated with the data plane that is used to reach the Next Hop. The data plane protocol for this criterion MUST be the same as the one selected by the previous criterion (#1). The mechanism(s) to perform the "path availability" check and the selection of particular data plane are a matter of a policy and outside the scope of this document. For example, if a BGP VPNv4 path wants to use the MPLS as the data plane protocol to the next-hop, then MPLS path availability to the next-hop should be evaluated i.e. liveness of MPLS LSP to the next-hop should be validated. This check is about confirming the availability of functioning path to the next-hop. Note that it is not necessary to trigger the data- plane liveness mechanism for a given next-hop as a consequence of this check, though it may be an option. Another option is to do it a priori. The selection of a particular option is deemed deployment specific and outside the scope of this document. 4. Conclusions Both amendments discussed in section 2 provide further clarity and granularity to help the BGP speaker to either continue to advertise a BGP path's reachability or withdraw the BGP path's reachability, based on the consideration for the path's next-hop reachability and/or availability in a particular data plane. It is not expected that the proposed amendments would negatively impact BGP convergence, barring any implementation specifics. The intention of this document is to help operators to build BGP networks that can avoid self-blackholing. 5. Security Considerations This draft doesn't impose any additional security constraints. Asati, Rajiv Expires August 25, 2013 [Page 4] Internet-Draft BGP Bestpath Selection Criteria February 25, 2013 6. IANA Considerations None. 7. Acknowledgments Yakov Rekhter provided critical suggestions and feedback to improve this document. Thanks to John Scudder and Chandrashekhar Appanna for contributing to the discussions that formed the basis of this document. Thanks to Ilya Varlashkin and Michael Benjamin, who made the case to revive this document and provided useful feedback. Also thanks to Keyur Patel for his feedback. This document was prepared using 2-Word-v2.0.template.dot. 8. Appendix 8.1. Problem Applicability In IP networks using BGP, a router would continue to attract traffic by advertising the BGP prefix reachability to neighbor(s) as long as the router had a route to the next-hop in its routing table, but independent of whether the router has a functional forwarding path to the next-hop. This may cause the forwarded traffic to be dropped inside the IP network. In MPLS or MPLS VPN networks [RFC4364], the same problem is observed if the functional MPLS LSP to the next-hop is not available (due to the forwarding path error on any node along the path to the next- hop). The following MPLS/VPN topology clarifies the problem - <-eBGP/IGP-> <-------MP-BGP------> <-eBGP/IGP-> CE1~~~~~~~~~PE1~~~MPLS Network~~~PE2~~~~~~~~CE2~~ ^ ======PE1-PE2 LSP==> ^ ^ a.b.c.d Figure 1 MPLS VPN Network Asati, Rajiv Expires August 25, 2013 [Page 5] Internet-Draft BGP Bestpath Selection Criteria February 25, 2013 In the network illustrated in Figure 1, the PE1 to PE2 LSP may be non-functional due to any reason such as corrupted MPLS Forwarding Table entry, or the missing MPLS Forwarding table entry, or LDP binding defect, or down LDP session between the P routers (with independent label distribution control) etc. In such a situation, it is clear that the CE1->CE2 traffic inserted into the MPLS network by PE1 will get dropped inside the MPLS network. It is undesirable to have PE1 continue to convey to the CE1 router that PE1 (and the MPLS network) is still the next-hop for the remote VPN reachability, without being sure of the corresponding LSP health. 8.1.1. Multi-Homed VPN Site If the remote VPN site is dual-homed to both PE2 and PE3, then PE1 may learn two VPNv4 paths to the prefix a.b.c.d. via PE2 and PE3 routers, as shown below in Figure 2. PE1 may select the bestpath for the prefix a.b.c.d via PE2 (say, for which the PE1->PE2 LSP is mal- functioning) and advertise that bestpath to CE1 in the context of figure 2. <------MP-BGP------> CE1~~~~~~~~~PE1~~~MPLS Network~~~PE2~~~~~~~~CE2~~ \ / ^ \~~~~~~~~~~PE3~~~~~~~/ ^ ^ a.b.c.d Figure 2 MPLS VPN Network - CE2 Dual-Homing This causes CE1 to likely send the traffic destined to prefix a.b.c.d to the PE1 router, which forwards the traffic over the malfunctioning LSP to PE2. It is clear that this MPLS encapsulated VPN traffic ends up getting dropped or blackholed somewhere inside the MPLS network. It is desirable to force PE1 to select an alternate bestpath via that next-hop (such as PE3), whose LSP is correctly functioning. Asati, Rajiv Expires August 25, 2013 [Page 6] Internet-Draft BGP Bestpath Selection Criteria February 25, 2013 8.1.2. Single-Homed VPN Site with Site-to-Site Backup Connectivity The local VPN site may have a backup/dial-up link available at the CE router, but the backup link will not even be activated as long as the CE's routing table continues to point to the PE router as the next-hop (over the MPLS/VPN network). <------MP-BGP------> CE1~~~~~~~~~PE1~~~MPLS Network~~~PE2~~~~~~~~CE2~~ \ / ^ \~~~~~~~~~~~~~~backup path~~~~~~~~~~~~~~/ ^ ^ a.b.c.d Figure 3 MPLS VPN Network - CE1-CE2 Backup connection Unless PE2 withdraws the route via the routing protocol used on the PE-CE link, CE1 will not be able to activate the backup link (barring any tracking functionality) to the remote VPN site. In summary, if PE1 could appropriately qualify the BGP VPNv4 bestpath, then the VPN traffic outage could likely be avoided. Even if the VPN site was not multi-homed, it is desirable to force PE1 to withdraw the path from CE1 to improve the CE-to-CE convergence. This document proposes a mechanism to achieve the optimal BGP behavior at PE. 8.1.3. 6PE or 6VPE This problem is very much applicable to the MPLS network that is providing either 6PE [RFC4978] or 6VPE [RFC4659] service to transport IPv6 packets over the MPLS network. 9. References 9.1. Normative References [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. Asati, Rajiv Expires August 25, 2013 [Page 7] Internet-Draft BGP Bestpath Selection Criteria February 25, 2013 [RFC4364] Rosen E. and Rekhter Y., "BGP/MPLS IP Virtual Private Networks (VPNs)", RFC4364, February 2006. [RFC4271] Rekhter, Y., Li T., and Hares S.(editors), "A Border Gateway Protocol 4 (BGP-4)", RFC4271, January 2006 9.2. Informative References [RFC3031] Rosen, et al., "Multiprotocol Label Switching Architecture", RFC3031, Jan 2001. [RFC5512] Rosen, E., Mohapatra, P., "BGP Encapsulation SAFI and BGP Tunnel Encapsulation Attribute", RFC5512, April 2009. [RFC4023] Rosen, et al., "Encapsulating MPLS in IP or Generic Routing Encapsulation", RFC4023, March 2005. [RFC4817] Townsley, et al., "Encapsulation of MPLS over Layer 2 Tunneling Protocol Version 3", RFC4817, Nov 2006. [RFC4978] De Clercq, et al., "Connecting IPv6 Islands over IPv4 MPLS Using IPv6 Provider Edge Routers", RFC4978, Feb 2007. [RFC4659] De Clercq, et al., "BGP-MPLS IP VPN Extension for IPv6 VPN", RFC4659, Sep 2006. Author's Addresses Rajiv Asati Cisco Systems 7025 Kit Creek Road RTP, NC 27560 USA Email: email@example.com Asati, Rajiv Expires August 25, 2013 [Page 8]