Working Group U. Chunduri Internet-Draft J. Tantsura Intended status: Informational Ericsson Inc. Expires: September 10, 2015 C. Bowers Juniper Networks March 9, 2015 Extended procedures and considerations for evaluating Loop-Free Alternates draft-chunduri-rtgwg-lfa-extended-procedures-02 Abstract This document provide few clarifications and extended procedures to IP Fast Reroute using Loop-Free Alternates as defined in RFC 5286. 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 September 10, 2015. Copyright Notice Copyright (c) 2015 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. Chunduri, et al. Expires September 10, 2015 [Page 1] Internet-Draft Extended procedures for LFAs March 2015 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2 1.2. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . 2 2. LFA Extended Procedures . . . . . . . . . . . . . . . . . . . 3 2.1. Multi Homed Prefixes . . . . . . . . . . . . . . . . . . 3 2.1.1. IS-IS ATT Bit considerations . . . . . . . . . . . . 5 2.2. Links with IGP MAX_METRIC . . . . . . . . . . . . . . . . 5 2.3. Multi Topology Considerations . . . . . . . . . . . . . . 6 3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 4. Security Considerations . . . . . . . . . . . . . . . . . . . 7 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 6.1. Normative References . . . . . . . . . . . . . . . . . . 7 6.2. Informative References . . . . . . . . . . . . . . . . . 8 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 1. Introduction Loop Free Alternatives (LFAs) as defined in [RFC5286] have been widely deployed, and the operational and manageability considerations are described in great detail in [I-D.ietf-rtgwg-lfa-manageability]. This document intends to provide clarifications, additional considerations to [RFC5286], to address a few coverage and operational observations. These observations are in the area of handling Muti-homed prefixes (MHPs), IS-IS attach (ATT) bit in L1 area, links provisioned with MAX_METRIC for traffic engineering (TE) purposes and in the area of Multi Topology (MT) IGP deployments. All these are elaborated in detail in Section 2. 1.1. Requirements 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 [RFC2119]. 1.2. Acronyms AF - Address Family ATT - IS-IS Attach Bit ECMP - Equal Cost Multi Path IGP - Interior Gateway Protocol Chunduri, et al. Expires September 10, 2015 [Page 2] Internet-Draft Extended procedures for LFAs March 2015 IS-IS - Intermediate System to Intermediate System OSPF - Open Shortest Path First MHP - Multi-homed Prefix MT - Multi Topology SPF - Shortest Path First PDU 2. LFA Extended Procedures This section explains the additional considerations in various aspects as listed below to the base LFA specification [RFC5286]. 2.1. Multi Homed Prefixes LFA base specification [RFC5286] Section 6.1 recommends that a router compute the alternate next-hop for an IGP multi-homed prefix by considering alternate paths via all routers that have announced that prefix. However, it also allows for the router to simplify the multi-homed prefix calculation by assuming that the MHP is solely attached to the router that was its pre-failure optimal point of attachment, at the expense of potentially lower coverage. If an implementation chooses to simplify the multi-homed prefix calculation by assuming that the MHP is solely attached to the router that was its pre-failure optimal point of attachment, the procedure described in this memo can potentially improve coverage for equal cost multi path (ECMP) MHPs without incurring extra computational cost. The approach as specified in [RFC5286] Section 6.1 last paragraph, is to simplify the MHP is solely attached to the router that was its pre-failure optimal point of attachment. While this is very scalable approach and simplifies computation, as [RFC5286] notes this may result in little less coverage. This memo improves the above approach to provide loop-free alternatives without any additional cost for equal cost multi path MHPs as described through the below example network. The approach specified here MAY also applicable for handling default routes as explained in Section 2.1.1. Chunduri, et al. Expires September 10, 2015 [Page 3] Internet-Draft Extended procedures for LFAs March 2015 5 +---+ 8 +---+ 5 +---+ +-----| S |------| A |-----| B | | +---+ +---+ +---+ | | | | 5 | 5 | | | | +---+ 5 +---+ 4 +---+ 1 +---+ | C |---| E |-----| M |-------| F | +---+ +---+ +---+ +---+ | 10 5 | +-----------p---------+ Figure 1: MHP with same ECMP Next-hop In the above network a prefix p, is advertised from both Node E and Node F. With simplified approach taken as specified in [RFC5286] Section 6.1, prefix p will get only link protection LFA through the neighbor C while a node protection path is available through neighbor A. In this scenario, E and F both are pre-failure optimal points of attachment and share the same primary next-hop. Hence, an implementation MAY compare the kind of protection A provides to F (link-and-node protection) with the kind of protection C provides to E (link protection) and inherit the better alternative to prefix p and here it is A. However, in the below network prefix p has an ECMP through both node E and node F with cost 20. Though it has 2 pre-failure optimal points of attachment, the primary next-hop to each pre-failure optimal point of attachment is different. In this case, prefix p shall inherit corresponding LFA to each primary next-hop calculated for the router advertising the same respectively (node E's and node F's LFA). +---+ 3 +---+ | S |----------------| B | +---+ +---+ | | 10 | 1 | | | +---+ 6 +---+ | E |-----------------| F | +---+ +---+ | 10 16 | +-----------p---------+ Figure 2: MHP with different ECMP Next-hops Chunduri, et al. Expires September 10, 2015 [Page 4] Internet-Draft Extended procedures for LFAs March 2015 In summary, if there are multiple pre-failure points of attachment for a MHP and primary next-hop of a MHP is same as that of the primary next-hop of the router that was pre-failure optimal point of attachment, an implementation MAY provide the better protection to MHP without incurring any additional computation cost. 2.1.1. IS-IS ATT Bit considerations Per [RFC1195] a default route needs to be added in Level1 (L1) router to the closest reachable Level1/Level2 (L1/L2) router in the network advertising ATT (attach) bit in its LSP-0 fragment. All L1 routers in the area would do this during the decision process with the next- hop of the default route set to the adjacent router through which the closest L1/L2 router is reachable. The base LFA specification [RFC5286] does not specify any procedure for computing LFA for a default route in IS-IS L1 area. Potentially one MAY consider a default route is being advertised from the boarder L1/L2 router where ATT bit is set and can do LFA computation for the default route. But, when multiple ECMP L1/L2 routers are reachable in an L1 area corresponding best LFAs SHOULD be given for each primary next-hop associated with default route. Considerations as specified in Section 2.1 are applicable for default routes, if the default route is considered as ECMP MHP. 2.2. Links with IGP MAX_METRIC Section 3.5 and 3.6 of [RFC5286] describes procedures for excluding nodes and links from use in alternate paths based on the maximum link metric (as defined in for IS-IS in [RFC5305] or as defined in [RFC3137] for OSPF). If these procedures are strictly followed, there are situations, as described below, where the only potential alternate available which satisfies the basic loop-free condition will not be considered as alternative. Chunduri, et al. Expires September 10, 2015 [Page 5] Internet-Draft Extended procedures for LFAs March 2015 +---+ 10 +---+ 10 +---+ | S |------|N1 |-----|D1 | +---+ +---+ +---+ | | 10 | 10 | |MAX_MET(N2 to S) | | | | +---+ | +-------|N2 |--------+ +---+ 10 | +---+ |D2 | +---+ Figure 3: Link with IGP MAX_METRIC In the simple example network, all the link costs have a cost of 10 in both directions, except for the link between S and N2. The S-N2 link has a cost of 10 in the direction from S to N2, and a cost of MAX_METRIC in the direction from N2 to S (0xffffff /2^24 - 1 for IS- IS and 0xffff for OSPF) for a specific end to end Traffic Engineering (TE) requirement of the operator. At node S, D1 is reachable through N1 with cost 20, and D2 is reachable through N2 with cost 20. Even though neighbor N2 satisfies basic loop-free condition (inequality 1 of [RFC5286]) for D1 this could be excluded as potential alternative because of the current exclusions as specified in section 3.5 and 3.6 procedure of [RFC5286]. But, as the primary traffic destined to D2 is continue to use the link and hence irrespective of the reverse metric in this case, the same link MAY be used as a potential LFA for D1. Alternatively, reverse metric of the link MAY be configured with MAX_METRIC-1, so that the link can be used as an alternative while meeting the TE requirements. 2.3. Multi Topology Considerations Section 6.2 and 6.3.2 of [RFC5286] state that multi-topology OSPF and ISIS are out of scope for that specification. This memo clarifies and describes the applicability. In Multi Topology (MT) IGP deployments, for each MT ID, a separate shortest path tree (SPT) is built with topology specific adjacencies, the LFA principles laid out in [RFC5286] are actually applicable for MT IS-IS [RFC5120] LFA SPF. The primary difference in this case is, identifying the eligible-set of neighbors for each LFA computation Chunduri, et al. Expires September 10, 2015 [Page 6] Internet-Draft Extended procedures for LFAs March 2015 which is done per MT ID. The eligible-set for each MT ID is determined by the presence of IGP adjacency from Source to the neighboring node on that MT-ID apart from the administrative restrictions and other checks laid out in [RFC5286]. The same is also applicable for OSPF [RFC4915] [MT-OSPF] or different AFs in multi instance OSPFv3 [RFC5838]. However for MT IS-IS, if a default topology is used with MT-ID 0 [RFC5286] and both IPv4 [RFC5305] and IPv6 routes/AFs [RFC5308] are present, then the condition of network congruency is applicable for LFA computation as well. Network congruency here refers to, having same address families provisioned on all the links and all the nodes of the network with MT-ID 0. Here with single decision process both IPv4 and IPv6 next-hops are computed for all the prefixes in the network and similarly with one LFA computation from all eligible neighbors per [RFC5286], all potential alternatives can be computed. 3. IANA Considerations This document defines no new namespaces and no actions for IANA. 4. Security Considerations This document does not introduce any new security issues or any change in security considerations as noted in the LFA base specification [RFC5286]. 5. Acknowledgements Authors would like to thank Alia Atlas for detailed review of initial document and providing valuable suggestions. We also thank Bruno Decreane, Stephane Litkowski for their initial review and feedback on the document. 6. References 6.1. Normative References [RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and dual environments", RFC 1195, December 1990. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC5286] Atlas, A. and A. Zinin, "Basic Specification for IP Fast Reroute: Loop-Free Alternates", RFC 5286, September 2008. Chunduri, et al. Expires September 10, 2015 [Page 7] Internet-Draft Extended procedures for LFAs March 2015 6.2. Informative References [I-D.ietf-rtgwg-lfa-manageability] Litkowski, S., Decraene, B., Filsfils, C., Raza, K., Horneffer, M., and P. Sarkar, "Operational management of Loop Free Alternates", draft-ietf-rtgwg-lfa- manageability-08 (work in progress), March 2015. [RFC3137] Retana, A., Nguyen, L., White, R., Zinin, A., and D. McPherson, "OSPF Stub Router Advertisement", RFC 3137, June 2001. [RFC4915] Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P. Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF", RFC 4915, June 2007. [RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi Topology (MT) Routing in Intermediate System to Intermediate Systems (IS-ISs)", RFC 5120, February 2008. [RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic Engineering", RFC 5305, October 2008. [RFC5308] Hopps, C., "Routing IPv6 with IS-IS", RFC 5308, October 2008. [RFC5838] Lindem, A., Mirtorabi, S., Roy, A., Barnes, M., and R. Aggarwal, "Support of Address Families in OSPFv3", RFC 5838, April 2010. Authors' Addresses Uma Chunduri Ericsson Inc. 300 Holger Way, San Jose, California 95134 USA Phone: 408 750-5678 Email: uma.chunduri@ericsson.com Chunduri, et al. Expires September 10, 2015 [Page 8] Internet-Draft Extended procedures for LFAs March 2015 Jeff Tantsura Ericsson Inc. 300 Holger Way, San Jose, California 95134 USA Email: jeff.tantsura@ericsson.com Chris Bowers Juniper Networks 1194 N. Mathilda Ave. Sunnyvale, California 94089 USA Email: cbowers@juniper.net Chunduri, et al. Expires September 10, 2015 [Page 9]