Network Working Group A. Atlas, Ed.
Internet-Draft Google, Inc.
Expires: January 7, 2008 A. Zinin, Ed.
Alcatel
July 6, 2007
Basic Specification for IP Fast-Reroute: Loop-free Alternates
draft-ietf-rtgwg-ipfrr-spec-base-07
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
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 January 7, 2008.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document describes the use of loop-free alternates to provide
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
link group (SRLG). The goal of this technology is to reduce the
micro-looping and packet loss that happens while routers converge
after a topology change due to a failure. Rapid failure repair is
achieved through use of precalculated backup next-hops that are loop-
Atlas, et al. Expires January 7, 2008 [Page 1]
Internet-Draft draft-ietf-rtgwg-ipfrr-spec-base-07 July 2007
free and safe to use until the distributed network convergence
process completes.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Failure Scenarios . . . . . . . . . . . . . . . . . . . . 5
2. Applicability of Described Mechanisms . . . . . . . . . . . . 8
3. Alternate Next-Hop Calculation . . . . . . . . . . . . . . . . 8
3.1. Basic Loop-free Condition . . . . . . . . . . . . . . . . 10
3.2. Node-Protecting Alternate Next-Hops . . . . . . . . . . . 10
3.3. Broadcast and NBMA Links . . . . . . . . . . . . . . . . . 10
3.4. ECMP and Alternates . . . . . . . . . . . . . . . . . . . 12
3.5. Interactions with ISIS Overload, RFC 3137 and Costed
Out Links . . . . . . . . . . . . . . . . . . . . . . . . 13
3.5.1. Interactions with ISIS Link Attributes . . . . . . . . 14
3.6. Selection Procedure . . . . . . . . . . . . . . . . . . . 14
4. Using an Alternate . . . . . . . . . . . . . . . . . . . . . . 18
4.1. Terminating Use of Alternate . . . . . . . . . . . . . . . 18
5. Requirements on LDP Mode . . . . . . . . . . . . . . . . . . . 20
6. Routing Aspects . . . . . . . . . . . . . . . . . . . . . . . 20
6.1. Multi-Homed Prefixes . . . . . . . . . . . . . . . . . . . 20
6.2. ISIS . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
6.3. OSPF . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
6.3.1. OSPF External Routing . . . . . . . . . . . . . . . . 22
6.3.2. OSPF Multi-Topology . . . . . . . . . . . . . . . . . 23
6.4. BGP Next-Hop Synchronization . . . . . . . . . . . . . . . 23
6.5. Multicast Considerations . . . . . . . . . . . . . . . . . 23
7. Security Considerations . . . . . . . . . . . . . . . . . . . 23
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 24
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
10.1. Normative References . . . . . . . . . . . . . . . . . . . 24
10.2. Informative References . . . . . . . . . . . . . . . . . . 24
Appendix A. OSPF Example Where LFA Based on Local Area
Topology is Insufficient . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26
Intellectual Property and Copyright Statements . . . . . . . . . . 29
Atlas, et al. Expires January 7, 2008 [Page 2]
Internet-Draft draft-ietf-rtgwg-ipfrr-spec-base-07 July 2007
1. Introduction
Applications for interactive multimedia services such as VoIP and
pseudo-wires can be very sensitive to traffic loss, such as occurs
when a link or router in the network fails. A router's convergence
time is generally on the order of hundreds of milliseconds; the
application traffic may be sensitive to losses greater than tens of
milliseconds.
As discussed in [I-D.ietf-rtgwg-ipfrr-framework], minimizing traffic
loss requires a mechanism for the router adjacent to a failure to
rapidly invoke a repair path, which is minimally affected by any
subsequent re-convergence. This specification describes such a
mechanism which allows a router whose local link has failed to
forward traffic to a pre-computed alternate until the router installs
the new primary next-hops based upon the changed network topology.
The terminology used in this specification is given in
[I-D.ietf-rtgwg-ipfrr-framework]. The described mechanism assumes
that routing in the network is performed using a link-state routing
protocol-- OSPF [RFC2328] [RFC2740] [I-D.ietf-ospf-ospfv3-update] or
ISIS [RFC1195] [RFC2966] (for IPv4 or IPv6). The mechanism also
assumes that both the primary path and the alternate path are in the
same routing area.
When a local link fails, a router currently must signal the event to
its neighbors via the IGP, recompute new primary next-hops for all
affected prefixes, and only then install those new primary next-hops
into the forwarding plane. Until the new primary next-hops are
installed, traffic directed towards the affected prefixes is
discarded. This process can take hundreds of milliseconds.
<--
+-----+
/------| S |--\
/ +-----+ \
/ 5 8 \
/ \
+-----+ +-----+
| E | | N_1 |
+-----+ +-----+
\ /
\ \ 4 3 / /
\| \ / |/
-+ \ +-----+ / +-
\---| D |---/
+-----+
Figure 1: Basic Topology
Atlas, et al. Expires January 7, 2008 [Page 3]
Internet-Draft draft-ietf-rtgwg-ipfrr-spec-base-07 July 2007
The goal of IP Fast-Reroute is to reduce failure reaction time to 10s
of milliseconds by using a pre-computed alternate next-hop, in the
event that the currently selected primary next-hop fails, so that the
alternate can be rapidly used when the failure is detected. A
network with this feature experiences less traffic loss and less
micro-looping of packets than a network without IPFRR. There are
cases where micro-looping is still a possibility since IPFRR coverage
varies but in the worst possible situation a network with IPFRR is
equivalent with respect to traffic convergence to a network without
IPFRR.
To clarify the behavior of IP Fast-Reroute, consider the simple
topology in Figure 1. When router S computes its shortest path to
router D, router S determines to use the link to router E as its
primary next-hop. Without IP Fast-Reroute, that link is the only
next-hop that router S computes to reach D. With IP Fast-Reroute, S
also looks for an alternate next-hop to use. In this example, S
would determine that it could send traffic destined to D by using the
link to router N_1 and therefore S would install the link to N_1 as
its alternate next-hop. At some later time, the link between router
S and router E could fail. When that link fails, S and E will be the
first to detect it. On detecting the failure, S will stop sending
traffic destined for D towards E via the failed link, and instead
send the traffic to S's pre-computed alternate next-hop, which is the
link to N_1, until a new SPF is run and its results are installed.
As with the primary next-hop, an alternate next-hop is computed for
each destination. The process of computing an alternate next-hop
does not alter the primary next-hop computed via a standard SPF.
If in the example of Figure 1, the link cost from N_1 to D increased
to 30 from 3, then N_1 would not be a loop-free alternate, because
the cost of the path from N_1 to D via S would be 17 while the cost
from N_1 directly to D would be 30. In real networks, we may often
face this situation. The existence of a suitable loop-free alternate
next-hop is dependent on the topology and the nature of the failure
the alternate is calculated for.
This specification uses the terminology introduced in
[I-D.ietf-rtgwg-ipfrr-framework]. In particular, it uses
Distance_opt(X,Y), abbreviated to D_opt(X,Y), to indicate the
shortest distance from X to Y. S is used to indicate the calculating
router. N_i is a neighbor of S; N is used as an abbreviation when
only one neighbor is being discussed. D is the destination under
consideration.
A neighbor N can provide a loop-free alternate (LFA) if and only if
Atlas, et al. Expires January 7, 2008 [Page 4]
Internet-Draft draft-ietf-rtgwg-ipfrr-spec-base-07 July 2007
Distance_opt(N, D) < Distance_opt(N, S) + Distance_opt(S, D)
Inequality 1: Loop-Free Criterion
A sub-set of loop-free alternate are downstream paths which must meet
a more restrictive condition that is applicable to more complex
failure scenarios:
Distance_opt(N, D) < Distance_opt(S, D)
Inequality 2: Downstream Path Criterion
1.1. Failure Scenarios
The alternate next-hop can protect against a single link failure, a
single node failure, one or more shared risk link group failures, or
a combination of these. Whenever a failure occurs that is more
extensive than what the alternate was intended to protect, there is
the possibility of temporarily looping traffic (note again, that such
a loop would only last until the next complete SPF calculation). The
example where a node fails when the alternate provided only link
protection is illustrated below. If unexpected simultaneous failures
occur, then micro-looping may occur since the alternates are not pre-
computed to avoid the set of failed links.
If only link protection is provided and the node fails, it is
possible for traffic using the alternates to experience micro-
looping. This issue is illustrated in Figure 2. If Link(S->E)
fails, then the link-protecting alternate via N will work correctly.
However, if router E fails, then both S and N will detect a failure
and switch to their alternates. In this example, that would cause S
to redirect the traffic to N and N to redirect the traffic to S and
thus causing a forwarding loop. Such a scenario can arise because
the key assumption, that all other routers in the network are
forwarding based upon the shortest path, is violated because of a
second simultaneous correlated failure - another link connected to
the same primary neighbor. If there are not other protection
mechanisms a node failure is still a concern when only using link
protection.
Atlas, et al. Expires January 7, 2008 [Page 5]
Internet-Draft draft-ietf-rtgwg-ipfrr-spec-base-07 July 2007
<@@@
@@@>
+-----+ +-----+
| S |-------| N |
+-+---+ 5 +-----+
| |
| 5 4 | |
| | | \|/
\|/ | |
| +-----+ |
+----| E |---+
+--+--+
|
|
| 10
|
+--+--+
| D |
+-----+
Figure 2: Link-Protecting Alternates Causing Loop on Node Failure
Micro-looping of traffic via the alternates caused when a more
extensive failure than planned for occurs can be prevented via
selection of only downstream paths as alternates. In Figure 2, S
would be able to use N as an alternate, but N could not use S;
therefore N would have no alternate and would discard the traffic,
thus avoiding the micro-loop. A micro-loop due to the use of
alternates can be avoided by using downstream paths because each
succeeding router in the path to the destination must be closer to
the destination than its predecessor (according to the topology prior
to the failures). Although use of downstream paths ensures that the
micro-looping via alternates does not occur, such a restriction can
severely limit the coverage of alternates.
As shown above, the use of either a node protecting LFA or a
downstream path provides protection against micro-looping in the
event of node failure. There are topologies where there may be
either a node portecting LFA, a downstream path, both or neither. A
node may select either a node protecting LFA or a downstream path
without risk of causing micro-loops in the event of node failure.
While a link-and-node-protecting LFA guarantees protection against
either link or node failure, a downstream path provides protection
only against a link failure and may or may not provide protection
against a node failure depending on the protection available at the
downstream node, but it cannot cause a micro-loop. For example in
Figure 2, if S uses N as a downstream path, although no looping can
occur, the traffic will not be protected in the event of the failure
Atlas, et al. Expires January 7, 2008 [Page 6]
Internet-Draft draft-ietf-rtgwg-ipfrr-spec-base-07 July 2007
of node E because N has no viable repair path, and it will simply
discard the packet. However, if N had a link and node protecting LFA
or downstream path via some other path (not shown), then the repair
may succeed.
Since the functionality of link and node protecting LFAs is greater
than that of downstream paths, a router SHOULD select a link and node
protecting LFA over a downstream path. If there are any destinations
for which a link and node protecting LFA is not available, then by
definition the path to all of those destinations from any neighbor of
the computing router (S) must be through the node (E) being protected
(otherwise there would be a node protecting LFA for that
destination). Consequently, if there exists a downstream path to the
protected node as destination, then that downstream path may be used
for all those destinations for which a link and node protecting LFA
is not available; the existence of a downstream path can be
determined by a single check of the condition Distance_opt(N, E) <
Distance_opt(S, E).
It may be desirable to find an alternate that can protect against
other correlated failures (of which node failure is a specific
instance). In the general case, these are handled by shared risk
link groups (SRLGs) where any links in the network can belong to the
SRLG. General SRLGs may add unacceptably to the computational
complexity of finding a loop-free alternate.
However, a sub-category of SRLGs is of interest and can be applied
only during the selection of an acceptable alternate. This sub-
category is to express correlated failures of links that are
connected to the same router. For example, if there are multiple
logical sub-interfaces on the same physical interface, such as VLANs
on an Ethernet interface, if multiple interfaces use the same
physical port because of channelization, or if multiple interfaces
share a correlated failure because they are on the same line-card.
This sub-category of SRLGs will be referred to as local-SRLGs. A
local-SRLG has all of its member links with one end connected to the
same router. Thus, router S could select a loop-free alternate which
does not use a link in the same local-SRLG as the primary next-hop.
The local-SRLGs belonging to E can be protected against via node-
protection; i.e. picking a loop-free node-protecting alternate.
Where SRLG protection is provided, it is in the context of the
particular OSPF or ISIS area, whose topology is used in the SPF
computations to compute the loop-free alternates. If an SRLG
contains links in multiple areas, then separate SRLG-protecting
alternates would be required in each area that is traversed by the
affected traffic.
Atlas, et al. Expires January 7, 2008 [Page 7]
Internet-Draft draft-ietf-rtgwg-ipfrr-spec-base-07 July 2007
2. Applicability of Described Mechanisms
IP Fast Reroute mechanisms described in this memo cover intra-domain
routing only, with OSPF[RFC2328] or ISIS [RFC1195] [RFC2966] as the
IGP. Specifically, Fast Reroute for BGP inter-domain routing is not
part of this specification.
Certain aspects of OSPF inter-area routing behavior explained in
Section 6.3 and Appendix A impact the ability of the router
calculating the backup next-hops to assess traffic trajectories. In
order to avoid micro-looping and ensure required coverage, certain
constraints are applied to multi-area OSPF networks:
a. Loop-free alternates should not be used in the backbone area if
there are any virtual links configured unless, for each transit
area, there is a full mesh of virtual links between all ABRs in
that area. Loop-free alternates may be used in non-backbone
areas regardless of whether there are virtual links configured.
b. Loop-free alternates should not be used for inter-area routes in
an area that contains more than one alternate ABR. [RFC3509]
c. Loop-free alternates should not be used for AS External routes or
ASBR routes in a non-backbone area of a network where there
exists an ABR that is announced as an ASBR in multiple non-
backbone areas and there exists another ABR that is in at least
two of the same non-backbone areas.
d. Loop-free alternates should not be used in a non-backbone area of
a network for AS External routes where an AS External prefix is
advertised with the same type of external metric by multiple
ASBRs, which are in different non-backbone areas, with a
forwarding address of 0.0.0.0 or by one or more ASBRs with
forwarding addresses in multiple non-backbone areas when an ABR
exists simultaneously in two or more of those non-backbone areas.
3. Alternate Next-Hop Calculation
In addition to the set of primary next-hops obtained through a
shortest path tree (SPT) computation that is part of standard link-
state routing functionality, routers supporting IP Fast Reroute also
calculate a set of backup next hops that are engaged when a local
failure occurs. These backup next hops are calculated to provide the
required type of protection (i.e. link-protecting and/or node-
protecting) and to guarantee that when the expected failure occurs,
forwarding traffic through them will not result in a loop. Such next
hops are called loop-free alternates or LFAs throughout this
Atlas, et al. Expires January 7, 2008 [Page 8]
Internet-Draft draft-ietf-rtgwg-ipfrr-spec-base-07 July 2007
specification.
In general, to be able to calculate the set of LFAs for a specific
destination D, a router needs to know the following basic pieces of
information:
o Shortest-path distance from the calculating router to the
destination (Distance_opt(S, D))
o Shortest-path distance from the router's IGP neighbors to the
destination (Distance_opt(N, D))
o Shortest path distance from the router's IGP neighbors to itself
(Distance_opt(N, S))
o Distance_opt(S, D) is normally available from the regular SPF
calculation performed by the link-state routing protocols.
Distance_opt(N, D) and Distance_opt(N, S) can be obtained by
performing additional SPF calculations from the perspective of
each IGP neighbor (i.e. considering the neighbor's vertex as the
root of the SPT--called SPT(N) hereafter--rather than the
calculating router's one, called SPT(S)).
This specification defines a form of SRLG protection limited to those
SRLGs that include a link that the calculating router is directly
connected to. Only that set of SRLGs could cause a local failure;
the calculating router only computes alternates to handle a local
failure. Information about local link SRLG membership is manually
configured. Information about remote link SRLG membership may be
dynamically obtained using [RFC4205] or [RFC4203]. Define
SRLG_local(S) to be the set of SRLGs that include a link that the
calculating router S is directly connected to. Only SRLG_local(S) is
of interest during the calculation, but the calculating router must
correctly handle changes to SRLG_local(S) triggered by local link
SRLG membership changes.
In order to choose among all available LFAs that provide required
SRLG protection for a given destination, the calculating router needs
to track the set of SRLGs in SRLG_local(S) that the path through a
specific IGP neighbor involves. To do so, each node D in the network
topology is associated with SRLG_set(N, D), which is the set of SRLGs
that would be crossed if traffic to D was forwarded through N. To
calculate this set, the router initializes SRLG_set(N, N) for each of
its IGP neighbors to be empty. During the SPT(N) calculation, when a
new vertex V is added to the SPT, its SRLG_set(N, V) is set to the
union of SRLG sets associated with its parents, and the SRLG sets in
SRLG_local(S) that are associated with the links from V's parents to
V. The union of the set of SRLG associated with a candidate alternate
Atlas, et al. Expires January 7, 2008 [Page 9]
Internet-Draft draft-ietf-rtgwg-ipfrr-spec-base-07 July 2007
next-hop and the SRLG_set(N, D) for the neighbor reached via that
candidate next-hop is used to determine SRLG protection.
The following sections provide information required for calculation
of LFAs. Sections Section 3.1 through Section 3.4 define different
types of LFA conditions. Section 3.5 describes constraints imposed
by the IS-IS overload and OSPF stub router functionality.
Section 3.6 defines the summarized algorithm for LFA calculation
using the definitions in the previous sections.
3.1. Basic Loop-free Condition
Alternate next hops used by implementations following this
specification MUST conform to at least the loop-freeness condition
stated above in Inequality 1. This condition guarantees that
forwarding traffic to an LFA will not result in a loop after a link
failure.
Further conditions may be applied when determining link-protecting
and/or node-protecting alternate next-hops as described in Sections
Section 3.2 and Section 3.3.
3.2. Node-Protecting Alternate Next-Hops
For an alternate next-hop N to protect against node failure of a
primary neighbor E for destination D, N must be loop-free with
respect to both E and D. In other words, N's path to D must not go
through E. This is the case if Inequality 3 is true, where N is the
neighbor providing a loop-free alternate.
Distance_opt(N, D) < Distance_opt(N, E) + Distance_opt(E, D)
Inequality 3: Criteria for a Node-Protecting Loop-Free Alternate
If Distance_opt(N,D) = Distance_opt(N, E) + Distance_opt(E, D), it is
possible that N has equal-cost paths and one of those could provide
protection against E's node failure. However, it is equally possible
that one of N's paths goes through E, and the calculating router has
no way to influence N's decision to use it. Therefore, it SHOULD be
assumed that an alternate next-hop does not offer node protection if
Inequality 3 is not met.
3.3. Broadcast and NBMA Links
Verification of the link-protection property of a next hop in the
case of a broadcast link is more elaborate than for a point-to-point
link. This is because a broadcast link is represented as a pseudo-
node with zero-cost links connecting it to other nodes.
Atlas, et al. Expires January 7, 2008 [Page 10]
Internet-Draft draft-ietf-rtgwg-ipfrr-spec-base-07 July 2007
Because failure of an interface attached to a broadcast segment may
mean loss of connectivity of the whole segment, the condition
described for broadcast link protection is pessimistic and requires
that the alternate is loop-free with regard to the pseudo-node.
Consider the example in Figure 3.
+-----+ 15
| S |--------
+-----+ |
| 5 |
| |
| 0 |
/----\ 0 5 +-----+
| PN |-----| N |
\----/ +-----+
| 0 |
| | 8
| 5 |
+-----+ 5 +-----+
| E |----| D |
+-----+ +-----+
Figure 3: Loop-Free Alternate that is Link-Protecting
In Figure 3, N offers a loop-free alternate which is link-protecting.
If the primary next-hop uses a broadcast link, then an alternate
SHOULD be loop-free with respect to that link's pseudo-node to
provide link protection. This requirement is described in
Inequality 4 below.
D_opt(N, D) < D_opt(N, pseudo) + D_opt(pseudo, D)
Inequality 4: Loop-Free Link-Protecting Criterion for Broadcast Links
Because the shortest path from the pseudo-node goes through E, if a
loop-free alternate from a neighbor N is node-protecting, the
alternate will also be link-protecting unless the router S can only
reach the alternate neighbor N via the same pseudo-node. Because S
can direct the traffic away from the shortest path to use the
alternate N, traffic might pass through the same broadcast link as it
would when S sent the traffic to the primary E. Thus, an LFA from N
that is node-protecting is not automatically link-protecting.
To obtain link protection, it is necessary both that the path from
the selected alternate next-hop does not traverse the link of
interest and that the link used from S to reach that alternate next-
hop is not the link of interest. The latter can only occur with non-
point-to-point links. Therefore, if the primary next-hop is across a
Atlas, et al. Expires January 7, 2008 [Page 11]
Internet-Draft draft-ietf-rtgwg-ipfrr-spec-base-07 July 2007
broadcast or NBMA interface, it is necessary to consider link
protection during the alternate selection. To clarify, consider the
topology in Figure 3. For N to provide link-protection, it is first
necessary that N's shortest path to D does not traverse the pseudo-
node PN. Second, it is necessary that the alternate next-hop
selected by S does not traverse PN. In this example, S's shortest
path to N is via the pseudo-node. Thus, to obtain link-protection, S
must find a next-hop to N (the point-to-point link from S to N in
this example) that avoids the pseudo-node PN.
Similar consideration of the link from S to the selected alternate
next-hop as well as the path from the selected alternate next-hop is
also necessary for SRLG protection. S's shortest path to the
selected neighbor N may not be acceptable as an alternate next-hop to
provide SRLG protection, even if the path from N to D can provide
SRLG protection.
3.4. ECMP and Alternates
With equal-cost multi-path, a prefix may have multiple primary next-
hops that are used to forward traffic. When a particular primary
next-hop fails, alternate next-hops should be used to preserve the
traffic. These alternate next-hops may themselves also be primary
next-hops, but need not be. Other primary next-hops are not
guaranteed to provide protection against the failure scenarios of
concern.
20 L1 L3 3
[N]-----[ S ]--------[E3]
| | |
| 5 | L2 |
20 | | |
| --------- | 2
| 5 | | 5 |
| [E1] [E2]------|
| | |
| 10 | 10 |
|---[A] [B]
| |
2 |--[D]--| 2
Figure 4: ECMP where Primary Next-Hops Provide Limited Protection
In Figure 4 S has three primary next-hops to reach D; these are L2 to
E1, L2 to E2 and L3 to E3. The primary next-hop L2 to E1 can obtain
link and node protection from L3 to E3, which is one of the other
primary next-hops; L2 to E1 cannot obtain link protection from the
other primary next-hop L2 to E2. Similarly, the primary next-hop L2
Atlas, et al. Expires January 7, 2008 [Page 12]
Internet-Draft draft-ietf-rtgwg-ipfrr-spec-base-07 July 2007
to E2 can only get node protection from L2 to E1 and can only get
link protection from L3 to E3. The third primary next-hop L3 to E3
can obtain link and node protection from L2 to E1 and from L2 to E2.
It is possible for both the primary next-hop L2 to E2 and the primary
next-hop L2 to E1 to obtain an alternate next-hop that provides both
link and node protection by using L1.
Alternate next-hops are determined for each primary next-hop
separately. As with alternate selection in the non-ECMP case, these
alternate next-hops should maximize the coverage of the failure
cases.
3.5. Interactions with ISIS Overload, RFC 3137 and Costed Out Links
As described in [RFC3137], there are cases where it is desirable not
to have a router used as a transit node. For those cases, it is also
desirable not to have the router used on an alternate path.
For computing an alternate, a router MUST NOT use an alternate next-
hop that is along a link whose cost or reverse cost is LSInfinity
(for OSPF) or the maximum cost (for ISIS) or which has the overload
bit set (for ISIS). For a broadcast link, the reverse cost
associated with a potential alternate next-hop is the cost towards
the pseudo-node advertised by the next-hop router. For point-to-
point links, if a specific link from the next-hop router cannot be
associated with a particular link, then the reverse cost considered
is that of the minimum cost link from the next-hop router back to S.
In the case of OSPF, if all links from router S to a neighbor N_i
have a reverse cost of LSInfinity, then router S MUST NOT use N_i as
an alternate.
Similarly in the case of ISIS, if N_i has the overload bit set, then
S MUST NOT consider using N_i as an alternate.
This preserves the desired behavior of diverting traffic away from a
router which is following [RFC3137] and it also preserves the desired
behavior when an operator sets the cost of a link to LSInfinity for
maintenance which is not permitting traffic across that link unless
there is no other path.
If a link or router which is costed out was the only possible
alternate to protect traffic from a particular router S to a
particular destination, then there should be no alternate provided
for protection.
Atlas, et al. Expires January 7, 2008 [Page 13]
Internet-Draft draft-ietf-rtgwg-ipfrr-spec-base-07 July 2007
3.5.1. Interactions with ISIS Link Attributes
[I-D.ietf-isis-link-attr] describes several flags whose interactions
with LFAs needs to be defined. A router MAY specify the "local
protection available" flag as a result of having LFAs. A router
SHOULD NOT use an alternate next-hop that is along a link for which
the link has been advertised with the attribute "link excluded from
local protection path" or with the attribute "local maintenance
required".
3.6. Selection Procedure
A router supporting this specification SHOULD attempt to select at
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
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
the failure scenario of interest.
The alternate selection should maximize the coverage of the failure
cases.
When calculating alternate next-hops, the calculating router S
applies the following rules.
1. S SHOULD select a loop-free node-protecting alternate next-hop,
if one is available. If no loop-free node-protecting alternate
is available, then S MAY select a loop-free link-protecting
alternate.
2. If S has a choice between a loop-free link-protecting node-
protecting alternate and a loop-free node-protecting alternate
which is not link-protecting, S SHOULD select a loop-free node-
protecting alternate which is also link-protecting. This can
occur as explained in Section 3.3.
3. If S has multiple primary next-hops, then S SHOULD select as a
loop-free alternate either one of the other primary next-hops or
a loop-free node-protecting alternate if available. If no loop-
free node-protecting alternate is available and no other primary
next-hop can provide link-protection, then S SHOULD select a
loop-free link-protecting alternate.
4. Implementations SHOULD support a mode where other primary next-
hops satisfying the basic loop-free condition and providing at
least link or node protection are preferred over any non-primary
alternates. This mode is provided to allow the administrator to
preserve traffic patterns based on regular ECMP behavior.
Atlas, et al. Expires January 7, 2008 [Page 14]
Internet-Draft draft-ietf-rtgwg-ipfrr-spec-base-07 July 2007
5. Implementations considering SRLGs MAY use SRLG-protection to
determine that a node-protecting or link-protecting alternate is
not available for use.
Following the above rules maximizes the level of protection and use
of primary (ECMP) next-hops.
Each next-hop is associated with a set of non-mutually-exclusive
characteristics based on whether it is used as a primary next-hop to
a particular destination D, and the type of protection it can provide
relative to a specific primary next-hop E:
Primary Path - The next-hop is used by S as primary.
Loop-Free Node-Protecting Alternate - This next-hop satisfies
Inequality 1 and Inequality 3. The path avoids S, S's primary
neighbor E, and the link from S to E.
Loop-Free Link-Protecting Alternate - This next-hop satisfies
Inequality 1 but not Inequality 3. If the primary next-hop uses a
broadcast link, then this next-hop satisfies Inequality 4.
An alternate path may also provide none, some or complete SRLG
protection as well as node and link or link protection. For
instance, a link may belong to two SRLGs G1 and G2. The alternate
path might avoid other links in G1 but not G2, in which case the
alternate would only provide partial SRLG protection.
Below is an algorithm that can be used to calculate loop-free
alternate next-hops. The algorithm is given for informational
purposes and implementations are free to use any other algorithm as
long as it satisfies the rules described above.
The following procedure describes how to select an alternate next-
hop. The procedure is described to determine alternate next-hops to
use to reach each router in the topology. Prefixes that are
advertised by a single router can use the alternate next-hop computed
for the router to which they are attached. The same procedure can be
used to reach a prefix that is advertised by more than one router
when the logical topological transformation described in Section 6.1
is used.
S is the computing router and has candidate next-hops H_1 through
H_k. N_i and N_j are used to refer to neighbors of S. For a next-hop
to be a candidate, the next-hop must be associated with a bi-
directional link, as is determined by the IGP. For a particular
destination router D, let S have already computed D_opt(S, D), and
for each neighbor N_i, D_opt(N_i, D), D_opt(N_i, S), and D_opt(N_i,
Atlas, et al. Expires January 7, 2008 [Page 15]
Internet-Draft draft-ietf-rtgwg-ipfrr-spec-base-07 July 2007
N_j), the distance from N_i to each other neighbor N_j, and the set
of SRLGs traversed by the path D_opt(N_i, D). S should follow the
below procedure for every primary next-hop selected to reach D. This
set of primary next-hops is represented P_1 to P_p. This procedure
finds the alternate next-hop(s) for P_i.
First, initialize the alternate information for P_i as follows:
P_i.alt_next_hops = {}
P_i.alt_type = NONE
P_i.alt_link-protect = FALSE
P_i.alt_node-protect = FALSE
P_i.alt_srlg-protect = {}
For each candidate next-hop H_h,
1. Initialize variables as follows:
cand_type = NONE
cand_link-protect = FALSE
cand_node-protect = FALSE
cand_srlg-protect = {}
2. If H_h is P_i, skip it and continue to the next candidate next-
hop.
3. If H_h.link is administratively allowed to be used as an
alternate,
and the cost of H_h.link is less than the maximum,
and the reverse cost of H_h is less than the maximum,
and H_h.neighbor is not overloaded (for ISIS),
and H_h.link is bi-directional,
then H_h can be considered as an alternate. Otherwise, skip it
and continue to the next candidate next-hop.
4. If D_opt( H_h.neighbor, D) >= D_opt( H_h.neighbor, S) + D_opt(S,
D), then H_h is not loop-free. Skip it and continue to the next
candidate next-hop.
5. cand_type = LOOP-FREE.
6. If H_h is a primary next-hop, set cand_type to PRIMARY.
7. If H_h.link is not P_i.link, set cand_link-protect to TRUE.
Atlas, et al. Expires January 7, 2008 [Page 16]
Internet-Draft draft-ietf-rtgwg-ipfrr-spec-base-07 July 2007
8. If D_opt(H_h.neighbor, D) < D_opt(H_h.neighbor, P_i.neighbor) +
D_opt(P_i.neighbor, D), set cand_node-protect to TRUE.
9. If the router considers SRLGs, then set the cand_srlg-protect to
the set of SRLGs traversed on the path from S via P_i to
P_i.neighbor. Remove the set of SRLGs to which H_h belongs from
cand_srlg-protect. Remove from cand_srlg-protect the set of
SRLGs traversed on the path from H_h.neighbor to D. Now
cand_srlg-protect holds the set of SRLGs to which P_i belongs
and that are not traversed on the path from S via H_h to D.
10. If cand_type is PRIMARY, the router prefers other primary next-
hops for use as the alternate, and the P_i.alt_type is not
PRIMARY, goto Step 19.
11. If cand_node-protect is TRUE and P_i.alt_node-protect is FALSE,
goto Paragraph 19.
12. If cand_link-protect is TRUE and P_i.alt_link-protect is FALSE,
goto Step 19.
13. If cand_srlg-protect has a better set of SRLGs than
P_i.alt_srlg-protect, goto Step 19.
14. If cand_srlg-protect is different from P_i.alt_srlg-protect,
then select between H_h and P_i.alt_next_hops based upon
distance, IP addresses, or any router-local tie-breaker. If H_h
is preferred, then goto Step 19. Otherwise, skip H_h and
continue to the next candidate next-hop.
15. If D_opt(H_h.neighbor, D) < D_opt(P_i.neighbor, D) and
D_opt(P_i.alt_next_hops, D) >= D_opt(P_i.neighbor, D), then H_h
is a downstream alternate and P_i.alt_next_hops is simply an
LFA. Prefer H_h and goto Step 19.
16. Based upon the alternate types, the alternate distances, IP
addresses, or other tie-breakers, decide if H_h is preferred to
P_i.alt_next_hops. If so, goto Step 19.
17. Decide if P_i.alt_next_hops is preferred to H_h. If so, then
skip H_h and continue to the next candidate next-hop.
18. Add H_h into P_i.alt_next_hops. Set P_i.alt_type to the better
type of H_h.alt_type and P_i.alt_type. Continue to the next
candidate next-hop.
19. Replace the P_i alternate next-hop set with H_h as follows:
Atlas, et al. Expires January 7, 2008 [Page 17]
Internet-Draft draft-ietf-rtgwg-ipfrr-spec-base-07 July 2007
P_i.alt_next_hops = {H_h}
P_i.alt_type = cand_type
P_i.alt_link-protect = cand_link-protect
P_i.alt_node-protect = cand_node-protect
P_i.alt_srlg-protect = cand_srlg-protect
Continue to the next candidate next-hop.
4. Using an Alternate
If an alternate next-hop is available, the router redirects traffic
to the alternate next-hop in case of a primary next-hop failure as
follows.
When a next-hop failure is detected via a local interface failure or
other failure detection mechanisms (see [FRAMEWORK]), the router
SHOULD:
1. Remove the primary next-hop associated with the failure.
2. Install the loop-free alternate calculated for the failed next-
hop if it is not already installed (e.g. the alternate is also a
primary next-hop).
Note that the router MAY remove other next-hops if it believes (via
SRLG analysis) that they may have been affected by the same failure,
even if it is not visible at the time of failure detection.
The alternate next-hop MUST be used only for traffic types which are
routed according to the shortest path. Multicast traffic is
specifically out of scope for this specification.
4.1. Terminating Use of Alternate
A router MUST limit the amount of time an alternate next-hop is used
after the primary next-hop has become unavailable. This ensures that
the router will start using the new primary next-hops. It ensures
that all possible transient conditions are removed and the network
converges according to the deployed routing protocol.
A router that implements [I-D.ietf-rtgwg-microloop-analysis] SHOULD
follow the rules given there for terminating the use of an alternate.
A router that implements [I-D.francois-ordered-fib] SHOULD follow the
rules given there for terminating the use of an alternate.
It is desirable to avoid micro-forwarding loops involving S. An
Atlas, et al. Expires January 7, 2008 [Page 18]
Internet-Draft draft-ietf-rtgwg-ipfrr-spec-base-07 July 2007
example illustrating the problem is given in Figure 5. If the link
from S to E fails, S will use N1 as an alternate and S will compute
N2 as the new primary next-hop to reach D. If S starts using N2 as
soon as S can compute and install its new primary, it is probable
that N2 will not have yet installed its new primary next-hop. This
would cause traffic to loop and be dropped until N2 has installed the
new topology. This can be avoided by S delaying its installation and
leaving traffic on the alternate next-hop.
+-----+
| N2 |-------- |
+-----+ 1 | \|/
| |
| +-----+ @@> +-----+
| | S |---------| N1 |
10 | +-----+ 10 +-----+
| | |
| 1 | | |
| | \|/ 10 |
| +-----+ | |
| | E | | \|/
| +-----+ |
| | |
| 1 | | |
| | \|/ |
| +-----+ |
|----| D |--------------
+-----+
Figure 5: Example where Continued Use of Alternate is Desirable
This is an example of a case where the new primary is not a loop-free
alternate before the failure and therefore may have been forwarding
traffic through S. This will occur when the path via a previously
upstream node is shorter than the the path via a loop-free alternate
neighbor. In these cases, it is useful to give sufficient time to
ensure that the new primary neighbor and other nodes on the new
primary path have switched to the new route.
If the newly selected primary was loop-free before the failure, then
it is safe to switch to that new primary immediately; the new primary
wasn't dependent on the failure and therefore its path will not have
changed.
Given that there is an alternate providing appropriate protection and
while the assumption of a single failure holds, it is safe to delay
the installation of the new primaries; this will not create
forwarding loops because the alternate's path to the destination is
Atlas, et al. Expires January 7, 2008 [Page 19]
Internet-Draft draft-ietf-rtgwg-ipfrr-spec-base-07 July 2007
known to not go via S or the failed element and will therefore not be
affected by the failure.
An implementation SHOULD continue to use the alternate next-hops for
packet forwarding even after the new routing information is available
based on the new network topology. The use of the alternate next-
hops for packet forwarding SHOULD terminate:
a. if the new primary next-hop was loop-free prior to the topology
change, or
b. if a configured hold-down, which represents a worst-case bound on
the length of the network convergence transition, has expired, or
c. if notification of an unrelated topological change in the network
is received.
5. Requirements on LDP Mode
Since LDP [RFC3036] traffic will follow the path specified by the
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
to have the appropriate labels available for the alternate so that
the appropriate out-segments can be installed in the forwarding plane
before the failure occurs.
This means that a Label Switched Router (LSR) running LDP must
distribute its labels for the FECs it can provide to all its
neighbors, regardless of whether or not they are upstream.
Additionally, LDP must be acting in liberal label retention mode so
that the labels which correspond to neighbors that aren't currently
the primary neighbor are stored. Similarly, LDP should be in
downstream unsolicited mode, so that the labels for the FEC are
distributed other than along the SPT.
If these requirements are met, then LDP can use the loop-free
alternates without requiring any targeted sessions or signaling
extensions for this purpose.
6. Routing Aspects
6.1. Multi-Homed Prefixes
An SPF-like computation is run for each topology, which corresponds
to a particular OSPF area or ISIS level. The IGP needs to determine
loop-free alternates to multi-homed routes. Multi-homed routes occur
Atlas, et al. Expires January 7, 2008 [Page 20]
Internet-Draft draft-ietf-rtgwg-ipfrr-spec-base-07 July 2007
for routes obtained from outside the routing domain by multiple
routers, for subnets on links where the subnet is announced from
multiple ends of the link, and for routes advertised by multiple
routers to provide resiliency.
Figure 6 demonstrates such a topology. In this example, the shortest
path to reach the prefix p is via E. The prefix p will have the link
to E as its primary next-hop. If the alternate next-hop for the
prefix p is simply inherited from the router advertising it on the
shortest path to p, then the prefix p's alternate next-hop would be
the link to C. This would provide link protection, but not the node
protection that is possible via A.
5 +---+ 4 +---+ 5 +---+
------| S |------| A |-----| B |
| +---+ +---+ +---+
| | |
| 5 | 5 |
| | |
+---+ 5 +---+ 5 7 +---+
| C |---| E |------ p -------| F |
+---+ +---+ +---+
Figure 6: Multi-homed prefix
To determine the best protection possible, the prefix p can be
treated in the SPF computations as a node with uni-directional links
to it from those routers that have advertised the prefix. Such a
node need never have its links explored, as it has no out-going
links.
If there exist multiple multi-homed prefixes that share the same
connectivity and the difference in metrics to those routers, then a
single node can be used to represent the set. For instance, if in
Figure 6 there were another prefix X that was connected to E with a
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.
A router SHOULD compute the alternate next-hop for an IGP multi-homed
prefix by considering alternate paths via all routers that have
announced that prefix.
6.2. ISIS
The applicability and interactions of LFAs with multi-topology ISIS
[I-D.ietf-isis-wg-multi-topology] is out of scope for this
specification.
Atlas, et al. Expires January 7, 2008 [Page 21]
Internet-Draft draft-ietf-rtgwg-ipfrr-spec-base-07 July 2007
6.3. OSPF
OSPF introduces certain complications because it is possible for the
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.
There are several cases where issues such as this can occur. They
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
clarify, an example topology is given in Appendix A.
a. Virtual Links: These allow paths to leave the backbone area and
traverse the transit area. The path provided via the transit
area can exit via any ABR. The path taken is not the shortest
path determined by doing an SPF in the backbone area.
b. Alternate ABR[RFC3509]: When an ABR is not connected to the
backbone, it considers the inter-area summaries from multiple
areas. The ABR A may determine to use area 2 but that path could
traverse another alternate ABR B that determines to use area 1.
This can lead to scenarios similar to that illustrated in
Figure 7.
c. ASBR Summaries: An ASBR may itself be an ABR and can be announced
into multiple areas. This presents other ABRs with a decision as
to which area to use. This is the example illustrated in
Figure 7.
d. AS External Prefixes: A prefix may be advertised by multiple
ASBRs in different areas and/or with multiple forwarding
addresses that are in different areas, which are connected via at
least one common ABR. This presents such ABRs with a decision as
to which area to use to reach the prefix.
Loop-free alternates should not be used in an area where one of the
above issues affects that area.
6.3.1. OSPF External Routing
When a forwarding address is set in an OSPF AS-external LSA, all
routers in the network calculate their next-hops for the external
prefix by doing a lookup for the forwarding address in the routing
table, rather than using the next-hops calculated for the ASBR. In
this case, the alternate next-hops SHOULD be computed by selecting
among the alternate paths to the forwarding link(s) instead of among
alternate paths to the ASBR.
Atlas, et al. Expires January 7, 2008 [Page 22]
Internet-Draft draft-ietf-rtgwg-ipfrr-spec-base-07 July 2007
6.3.2. OSPF Multi-Topology
The applicabilty and interactions of LFAs with multi-topology OSPF
[I-D.ietf-ospf-mt] [I-D.ietf-ospf-mt-ospfv3] is out of scope for this
specification.
6.4. BGP Next-Hop Synchronization
Typically BGP prefixes are advertised with AS exit routers router-id
as the BGP next-hop, and AS exit routers are reached by means of IGP
routes. BGP resolves its advertised next-hop to the immediate next-
hop by potential recursive lookups in the routing database. IP Fast-
Reroute computes the alternate next-hops to all IGP destinations,
which include alternate next-hops to the AS exit router's router-id.
BGP simply inherits the alternate next-hop from IGP. The BGP
decision process is unaltered; BGP continues to use the IGP optimal
distance to find the nearest exit router. MBGP routes do not need to
copy the alternate next hops.
It is possible to provide ASBR protection if BGP selected a set of
BGP next-hops and allowed the IGP to determine the primary and
alternate next-hops as if the BGP route were a multi-homed prefix.
This is for future study.
6.5. Multicast Considerations
Multicast traffic is out of scope for this specification of IP Fast-
Reroute. The alternate next-hops SHOULD NOT be used for multicast
RPF checks.
7. Security Considerations
The mechanism described in this document does not modify any routing
protocol messages, and hence no new threats related to packet
modifications or replay attacks are introduced. Traffic to certain
destinations can be temporarily routed via next-hop routers that
would not be used with the same topology change if this mechanism
wasn't employed. However, these next-hop routers can be used anyway
when a different topological change occurs, and hence this can't be
viewed as a new security threat.
In LDP, the wider distribution of FEC label information is still to
neighbors with whom a trusted LDP session has been established. This
wider distribution and the recommendation of using liberal label
retention mode are believed to have no significant security impact.
Atlas, et al. Expires January 7, 2008 [Page 23]
Internet-Draft draft-ietf-rtgwg-ipfrr-spec-base-07 July 2007
8. IANA Considerations
This document requires no IANA considerations.
9. Acknowledgements
The authors would like to thank Joel Halpern, Mike Shand, Stewart
Bryant, and Stefano Previdi for their assistance and useful review.
10. References
10.1. Normative References
[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, December 1990.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[RFC2740] Coltun, R., Ferguson, D., and J. Moy, "OSPF for IPv6",
RFC 2740, December 1999.
[RFC2966] Li, T., Przygienda, T., and H. Smit, "Domain-wide Prefix
Distribution with Two-Level IS-IS", RFC 2966,
October 2000.
[RFC3036] Andersson, L., Doolan, P., Feldman, N., Fredette, A., and
B. Thomas, "LDP Specification", RFC 3036, January 2001.
10.2. Informative References
[I-D.francois-ordered-fib]
Francois, P., "Loop-free convergence using oFIB",
draft-francois-ordered-fib-02 (work in progress),
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-11 (work in
progress), October 2005.
[I-D.ietf-ospf-mt]
Atlas, et al. Expires January 7, 2008 [Page 24]
Internet-Draft draft-ietf-rtgwg-ipfrr-spec-base-07 July 2007
Psenak, P., "Multi-Topology (MT) Routing in OSPF",
draft-ietf-ospf-mt-09 (work in progress), June 2007.
[I-D.ietf-ospf-mt-ospfv3]
Mirtorabi, S. and A. Roy, "Multi-topology routing in
OSPFv3 (MT-OSPFv3)", draft-ietf-ospf-mt-ospfv3-03 (work in
progress), July 2007.
[I-D.ietf-ospf-ospfv3-update]
Ferguson, D., "OSPF for IPv6",
draft-ietf-ospf-ospfv3-update-16 (work in progress),
May 2007.
[I-D.ietf-rtgwg-ipfrr-framework]
Shand, M. and S. Bryant, "IP Fast Reroute Framework",
draft-ietf-rtgwg-ipfrr-framework-07 (work in progress),
July 2007.
[I-D.ietf-rtgwg-microloop-analysis]
Zinin, A., "Analysis and Minimization of Microloops in
Link-state Routing Protocols",
draft-ietf-rtgwg-microloop-analysis-01 (work in progress),
October 2005.
[RFC3137] Retana, A., Nguyen, L., White, R., Zinin, A., and D.
McPherson, "OSPF Stub Router Advertisement", RFC 3137,
June 2001.
[RFC3509] Zinin, A., Lindem, A., and D. Yeung, "Alternative
Implementations of OSPF Area Border Routers", RFC 3509,
April 2003.
[RFC4203] Kompella, K. and Y. Rekhter, "OSPF Extensions in Support
of Generalized Multi-Protocol Label Switching (GMPLS)",
RFC 4203, October 2005.
[RFC4205] Kompella, K. and Y. Rekhter, "Intermediate System to
Intermediate System (IS-IS) Extensions in Support of
Generalized Multi-Protocol Label Switching (GMPLS)",
RFC 4205, October 2005.
Appendix A. OSPF Example Where LFA Based on Local Area Topology is
Insufficient
This appendix provides an example scenario where the local area
topology does not suffice to determine that an LFA is available. As
described in Section 6.3, one problem scenario is for ASBR summaries
Atlas, et al. Expires January 7, 2008 [Page 25]
Internet-Draft draft-ietf-rtgwg-ipfrr-spec-base-07 July 2007
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.
The following Figure 7 illustrates this case.
5
[ F ]-----------[ C ]
| |
| | 5
20 | 5 | 1
| [ N ]-----[ A ]*****[ F ]
| | # *
| 40 | # 50 * 2
| | 5 # 2 *
| [ S ]-----[ B ]*****[ G ]
| | *
| 5 | * 15
| | *
| [ E ] [ H ]
| | *
| 5 | * 10**
| | *
|---[ X ]-----[ASBR]
5
---- Link in Area 1
**** Link in Area 2
#### Link in Backbone Area 0
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
1 and area 2. A and B are both ABRs that are also connected to the
backbone area. S determines that N can provide a loop-free alternate
to reach the ASBR. N's path goes via A. A also sees an intra-area
route to ASBR via Area 2; the cost of the path in area 2 is 30, which
is less than 35, the cost of the path in area 1. Therefore, A uses
the path from area 2 and directs traffic to F. The path from F in
area 2 goes to B. B is also an ABR and learns the ASBR from both
areas 1 and area 2; B's path via area 1 is shorter (cost 20) than B's
path via area 2 (cost 25). Therefore, B uses the path from area 1
that connects to S.
Atlas, et al. Expires January 7, 2008 [Page 26]
Internet-Draft draft-ietf-rtgwg-ipfrr-spec-base-07 July 2007
Authors' Addresses
Alia K. Atlas (editor)
Google, Inc.
One Broadway, 7th Floor
Cambridge, MA 02142
USA
Email: akatlas@google.com
Alex Zinin (editor)
Alcatel
701 E Middlefield Rd.
Mountain View, CA 94043
USA
Email: alex.zinin@alcatel.com
Raveendra Torvi
FutureWei Technologies Inc.
1700 Alma Dr. Suite 100
Plano, TX 75075
USA
Email: traveendra@huawei.com
Gagan Choudhury
AT&T
200 Laurel Avenue, Room D5-3C21
Middletown, NJ 07748
USA
Phone: +1 732 420-3721
Email: gchoudhury@att.com
Christian Martin
iPath Technologies
Email: chris@ipath.net
Atlas, et al. Expires January 7, 2008 [Page 27]
Internet-Draft draft-ietf-rtgwg-ipfrr-spec-base-07 July 2007
Brent Imhoff
Juniper Networks
1194 North Mathilda
Sunnyvale, CA 94089
USA
Phone: +1 314 378 2571
Email: bimhoff@planetspork.com
Don Fedyk
Nortel Networks
600 Technology Park
Billerica, MA 01821
USA
Phone: +1 978 288 3041
Email: dwfedyk@nortelnetworks.com
Atlas, et al. Expires January 7, 2008 [Page 28]
Internet-Draft draft-ietf-rtgwg-ipfrr-spec-base-07 July 2007
Full Copyright Statement
Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights 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; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Acknowledgment
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
Atlas, et al. Expires January 7, 2008 [Page 29]
