Extended procedures and considerations for evaluating Loop-Free Alternates
draft-chunduri-rtgwg-lfa-extended-procedures-00
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Replaced".
|
|
|---|---|---|---|
| Authors | Uma Chunduri , Jeff Tantsura | ||
| Last updated | 2014-10-15 | ||
| Replaced by | draft-ietf-rtgwg-multihomed-prefix-lfa, draft-ietf-rtgwg-multihomed-prefix-lfa, RFC 8518 | ||
| RFC stream | (None) | ||
| Formats | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
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| Send notices to | (None) |
draft-chunduri-rtgwg-lfa-extended-procedures-00
Working Group U. Chunduri
Internet-Draft J. Tantsura
Intended status: Informational Ericsson Inc.
Expires: April 18, 2015 October 15, 2014
Extended procedures and considerations for evaluating Loop-Free
Alternates
draft-chunduri-rtgwg-lfa-extended-procedures-00
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.
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This Internet-Draft will expire on April 18, 2015.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2
1.2. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . 2
2. LFA Misc. extensions . . . . . . . . . . . . . . . . . . . . 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 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
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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 Misc. extensions
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, this 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 multi-homed prefix (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 incurring extra cost for equal cost multi path
(ECMP) MHPs.
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5 +---+ 8 +---+ 5 +---+
+-----| S |------| A |-----| B |
| +---+ +---+ +---+
| | |
| 5 | 5 |
| | |
+---+ 5 +---+ 4 +---+ 1 +---+
| C |---| E |-----| M |-------| F |
+---+ +---+ +---+ +---+
| 10 5 |
+-----------p---------+
Figure 1: MHP with same ECMP Nexthop
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 nexthop. 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 nexthop to each pre-failure optimal
point of attachment is different. In this case, prefix p shall
inherit corresponding LFA to each primary nexthop 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 Nexthops
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In summary, if there are multiple pre-failure points of attachment
for a MHP and primary nexthop of a MHP is same as that of the primary
nexthop 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 L1 router to the
closest reachable 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
potentially 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 nexthop 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.
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+---+ 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
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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 nexthops 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.
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6.2. Informative References
[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
Jeff Tantsura
Ericsson Inc.
300 Holger Way,
San Jose, California 95134
USA
Email: jeff.tantsura@ericsson.com
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