draft-ietf-rtgwg-ipfrr-spec-base-11.txt   draft-ietf-rtgwg-ipfrr-spec-base-12.txt 
Routing Area Working Group A. Atlas, Ed. Routing Area Working Group A. Atlas, Ed.
Internet-Draft BT Internet-Draft BT
Intended status: Standards Track A. Zinin, Ed. Intended status: Standards Track A. Zinin, Ed.
Expires: August 27, 2008 Alcatel Expires: September 28, 2008 Alcatel
Feb 24, 2008 Mar 27, 2008
Basic Specification for IP Fast-Reroute: Loop-free Alternates Basic Specification for IP Fast-Reroute: Loop-free Alternates
draft-ietf-rtgwg-ipfrr-spec-base-11 draft-ietf-rtgwg-ipfrr-spec-base-12
Status of this Memo Status of this Memo
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This Internet-Draft will expire on August 27, 2008. This Internet-Draft will expire on September 28, 2008.
Copyright Notice Copyright Notice
Copyright (C) The IETF Trust (2008). Copyright (C) The IETF Trust (2008).
Abstract Abstract
This document describes the use of loop-free alternates to provide This document describes the use of loop-free alternates to provide
local protection for unicast traffic in pure IP and MPLS/LDP networks local protection for unicast traffic in pure IP and MPLS/LDP networks
in the event of a single failure, whether link, node or shared risk in the event of a single failure, whether link, node or shared risk
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maintenance which is not permitting traffic across that link unless maintenance which is not permitting traffic across that link unless
there is no other path. there is no other path.
If a link or router which is costed out was the only possible If a link or router which is costed out was the only possible
alternate to protect traffic from a particular router S to a alternate to protect traffic from a particular router S to a
particular destination, then there should be no alternate provided particular destination, then there should be no alternate provided
for protection. for protection.
3.5.1. Interactions with ISIS Link Attributes 3.5.1. Interactions with ISIS Link Attributes
[I-D.ietf-isis-link-attr] describes several flags whose interactions [RFC5029] describes several flags whose interactions with LFAs needs
with LFAs needs to be defined. A router SHOULD NOT specify the to be defined. A router SHOULD NOT specify the "local protection
"local protection available" flag as a result of having LFAs. A available" flag as a result of having LFAs. A router SHOULD NOT use
router SHOULD NOT use an alternate next-hop that is along a link for an alternate next-hop that is along a link for which the link has
which the link has been advertised with the attribute "link excluded been advertised with the attribute "link excluded from local
from local protection path" or with the attribute "local maintenance protection path" or with the attribute "local maintenance required".
required".
3.6. Selection Procedure 3.6. Selection Procedure
A router supporting this specification SHOULD attempt to select at A router supporting this specification SHOULD attempt to select at
least one loop-free alternate next-hop for each primary next-hop used least one loop-free alternate next-hop for each primary next-hop used
for a given prefix. A router MAY decide to not use an available for a given prefix. A router MAY decide to not use an available
loop-free alternate next-hop. A reason for such a decision might be loop-free alternate next-hop. A reason for such a decision might be
that the loop-free alternate next-hop does not provide protection for that the loop-free alternate next-hop does not provide protection for
the failure scenario of interest. the failure scenario of interest.
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change, or change, or
b. if a configured hold-down, which represents a worst-case bound on b. if a configured hold-down, which represents a worst-case bound on
the length of the network convergence transition, has expired, or the length of the network convergence transition, has expired, or
c. if notification of an unrelated topological change in the network c. if notification of an unrelated topological change in the network
is received. is received.
5. Requirements on LDP Mode 5. Requirements on LDP Mode
Since LDP [RFC3036] traffic will follow the path specified by the Since LDP [RFC5036] traffic will follow the path specified by the
IGP, it is also possible for the LDP traffic to follow the loop-free IGP, it is also possible for the LDP traffic to follow the loop-free
alternates indicated by the IGP. To do so, it is necessary for LDP alternates indicated by the IGP. To do so, it is necessary for LDP
to have the appropriate labels available for the alternate so that to have the appropriate labels available for the alternate so that
the appropriate out-segments can be installed in the forwarding plane the appropriate out-segments can be installed in the forwarding plane
before the failure occurs. before the failure occurs.
This means that a Label Switched Router (LSR) running LDP must This means that a Label Switched Router (LSR) running LDP must
distribute its labels for the FECs it can provide to all its distribute its labels for the FECs it can provide to all its
neighbors, regardless of whether or not they are upstream. neighbors, regardless of whether or not they are upstream.
Additionally, LDP must be acting in liberal label retention mode so Additionally, LDP must be acting in liberal label retention mode so
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metric of 1 and to F with a metric of 3, then that prefix X could use metric of 1 and to F with a metric of 3, then that prefix X could use
the same alternate next-hop as was computed for prefix p. the same alternate next-hop as was computed for prefix p.
A router SHOULD compute the alternate next-hop for an IGP multi-homed A router SHOULD compute the alternate next-hop for an IGP multi-homed
prefix by considering alternate paths via all routers that have prefix by considering alternate paths via all routers that have
announced that prefix. announced that prefix.
6.2. ISIS 6.2. ISIS
The applicability and interactions of LFAs with multi-topology ISIS The applicability and interactions of LFAs with multi-topology ISIS
[I-D.ietf-isis-wg-multi-topology] is out of scope for this [RFC5120] is out of scope for this specification.
specification.
6.3. OSPF 6.3. OSPF
OSPF introduces certain complications because it is possible for the OSPF introduces certain complications because it is possible for the
traffic path to exit an area and then re-enter that area. This can traffic path to exit an area and then re-enter that area. This can
occur whenever a router considers the same route from multiple areas. occur whenever a router considers the same route from multiple areas.
There are several cases where issues such as this can occur. They There are several cases where issues such as this can occur. They
happen when another area permits a shorter path to connect two ABRs happen when another area permits a shorter path to connect two ABRs
than is available in the area where the LFA has been computed. To than is available in the area where the LFA has been computed. To
clarify, an example topology is given in Appendix A. clarify, an example topology is given in Appendix A.
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10.1. Normative References 10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998. [RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[RFC2740] Coltun, R., Ferguson, D., and J. Moy, "OSPF for IPv6", [RFC2740] Coltun, R., Ferguson, D., and J. Moy, "OSPF for IPv6",
RFC 2740, December 1999. RFC 2740, December 1999.
[RFC3036] Andersson, L., Doolan, P., Feldman, N., Fredette, A., and [RFC5036] Andersson, L., Minei, I., and B. Thomas, "LDP
B. Thomas, "LDP Specification", RFC 3036, January 2001. Specification", RFC 5036, October 2007.
10.2. Informative References 10.2. Informative References
[I-D.francois-ordered-fib] [I-D.francois-ordered-fib]
Francois, P., "Loop-free convergence using oFIB", Francois, P., "Loop-free convergence using oFIB",
draft-francois-ordered-fib-02 (work in progress), draft-francois-ordered-fib-02 (work in progress),
October 2006. October 2006.
[I-D.ietf-isis-link-attr]
Vasseur, J. and S. Previdi, "Definition of an IS-IS Link
Attribute sub-TLV", draft-ietf-isis-link-attr-03 (work in
progress), February 2007.
[I-D.ietf-isis-wg-multi-topology]
Przygienda, T., "M-ISIS: Multi Topology (MT) Routing in
IS-IS", draft-ietf-isis-wg-multi-topology-12 (work in
progress), November 2007.
[I-D.ietf-ospf-mt-ospfv3] [I-D.ietf-ospf-mt-ospfv3]
Mirtorabi, S. and A. Roy, "Multi-topology routing in Mirtorabi, S. and A. Roy, "Multi-topology routing in
OSPFv3 (MT-OSPFv3)", draft-ietf-ospf-mt-ospfv3-03 (work in OSPFv3 (MT-OSPFv3)", draft-ietf-ospf-mt-ospfv3-03 (work in
progress), July 2007. progress), July 2007.
[I-D.ietf-ospf-ospfv3-update] [I-D.ietf-ospf-ospfv3-update]
Ferguson, D., "OSPF for IPv6", Ferguson, D., "OSPF for IPv6",
draft-ietf-ospf-ospfv3-update-18 (work in progress), draft-ietf-ospf-ospfv3-update-18 (work in progress),
November 2007. November 2007.
[I-D.ietf-rtgwg-ipfrr-framework] [I-D.ietf-rtgwg-ipfrr-framework]
Shand, M. and S. Bryant, "IP Fast Reroute Framework", Shand, M. and S. Bryant, "IP Fast Reroute Framework",
draft-ietf-rtgwg-ipfrr-framework-07 (work in progress), draft-ietf-rtgwg-ipfrr-framework-08 (work in progress),
July 2007. February 2008.
[I-D.ietf-rtgwg-microloop-analysis] [I-D.ietf-rtgwg-microloop-analysis]
Zinin, A., "Analysis and Minimization of Microloops in Zinin, A., "Analysis and Minimization of Microloops in
Link-state Routing Protocols", Link-state Routing Protocols",
draft-ietf-rtgwg-microloop-analysis-01 (work in progress), draft-ietf-rtgwg-microloop-analysis-01 (work in progress),
October 2005. October 2005.
[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and [RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, December 1990. dual environments", RFC 1195, December 1990.
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[RFC4205] Kompella, K. and Y. Rekhter, "Intermediate System to [RFC4205] Kompella, K. and Y. Rekhter, "Intermediate System to
Intermediate System (IS-IS) Extensions in Support of Intermediate System (IS-IS) Extensions in Support of
Generalized Multi-Protocol Label Switching (GMPLS)", Generalized Multi-Protocol Label Switching (GMPLS)",
RFC 4205, October 2005. RFC 4205, October 2005.
[RFC4915] Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P. [RFC4915] Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF", Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
RFC 4915, June 2007. RFC 4915, June 2007.
[RFC5029] Vasseur, JP. and S. Previdi, "Definition of an IS-IS Link
Attribute Sub-TLV", RFC 5029, September 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.
Appendix A. OSPF Example Where LFA Based on Local Area Topology is Appendix A. OSPF Example Where LFA Based on Local Area Topology is
Insufficient Insufficient
This appendix provides an example scenario where the local area This appendix provides an example scenario where the local area
topology does not suffice to determine that an LFA is available. As topology does not suffice to determine that an LFA is available. As
described in Section 6.3, one problem scenario is for ASBR summaries described in Section 6.3, one problem scenario is for ASBR summaries
where the ASBR is available in two areas via intra-area routes and where the ASBR is available in two areas via intra-area routes and
there is at least one ABR or alternate ABR that is in both areas. there is at least one ABR or alternate ABR that is in both areas.
The following Figure 7 illustrates this case. The following Figure 7 illustrates this case.
5 5
[ F ]-----------[ C ] [ F ]-----------[ C ]
| | | |
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| 40 | # 50 * 2 | 40 | # 50 * 2
| | 5 # 2 * | | 5 # 2 *
| [ S ]-----[ B ]*****[ G ] | [ S ]-----[ B ]*****[ G ]
| | * | | *
| 5 | * 15 | 5 | * 15
| | * | | *
| [ E ] [ H ] | [ E ] [ H ]
| | * | | *
| 5 | * 10** | 5 | * 10**
| | * | | *
|---[ X ]-----[ASBR] |---[ X ]----[ ASBR ]
5 5
---- Link in Area 1 ---- Link in Area 1
**** Link in Area 2 **** Link in Area 2
#### Link in Backbone Area 0 #### Link in Backbone Area 0
Figure 7: Topology with Multi-area ASBR Causing Area Transiting Figure 7: Topology with Multi-area ASBR Causing Area Transiting
In Figure 7, the ASBR is also an ABR and is announced into both area In Figure 7, the ASBR is also an ABR and is announced into both area
1 and area 2. A and B are both ABRs that are also connected to the 1 and area 2. A and B are both ABRs that are also connected to the
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