Loop-Free Alternate (LFA) Applicability in Service Provider (SP) Networks
RFC 6571
| Document | Type | RFC - Informational (June 2012; No errata) | |
|---|---|---|---|
| Authors | Clarence Filsfils , Pierre Francois , Mike Shand , Bruno Decraene , Jim Uttaro , Nicolai Leymann , Martin Horneffer | ||
| Last updated | 2020-07-29 | ||
| Replaces | draft-filsfils-rtgwg-lfa-applicability | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text html pdf htmlized bibtex | ||
| Reviews | |||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Alvaro Retana | ||
| Shepherd write-up | Show (last changed 2011-09-27) | ||
| IESG | IESG state | RFC 6571 (Informational) | |
| Action Holders |
(None)
|
||
| Consensus Boilerplate | Unknown | ||
| Telechat date | |||
| Responsible AD | Stewart Bryant | ||
| IESG note | Alvaro Retana (alvaro.retana@hp.com) is the document shepherd. | ||
| Send notices to | (None) | ||
Internet Engineering Task Force (IETF) C. Filsfils, Ed.
Request for Comments: 6571 Cisco Systems
Category: Informational P. Francois, Ed.
ISSN: 2070-1721 Institute IMDEA Networks
M. Shand
B. Decraene
France Telecom
J. Uttaro
AT&T
N. Leymann
M. Horneffer
Deutsche Telekom
June 2012
Loop-Free Alternate (LFA) Applicability
in Service Provider (SP) Networks
Abstract
In this document, we analyze the applicability of the Loop-Free
Alternate (LFA) method of providing IP fast reroute in both the core
and access parts of Service Provider networks. We consider both the
link and node failure cases, and provide guidance on the
applicability of LFAs to different network topologies, with special
emphasis on the access parts of the network.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6571.
Filsfils, et al. Informational [Page 1]
RFC 6571 LFA Applicability in SP Networks June 2012
Copyright Notice
Copyright (c) 2012 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.
Table of Contents
1. Introduction ....................................................3
2. Terminology .....................................................4
3. Access Network ..................................................6
3.1. Triangle ...................................................8
3.1.1. E1C1 Failure ........................................8
3.1.2. C1E1 Failure ........................................9
3.1.3. uLoop ...............................................9
3.1.4. Conclusion .........................................10
3.2. Full Mesh .................................................10
3.2.1. E1A1 Failure .......................................10
3.2.2. A1E1 Failure .......................................11
3.2.3. A1C1 Failure .......................................11
3.2.4. C1A1 Failure .......................................12
3.2.5. uLoop ..............................................12
3.2.6. Conclusion .........................................12
3.3. Square ....................................................13
3.3.1. E1A1 Failure .......................................13
3.3.2. A1E1 Failure .......................................14
3.3.3. A1C1 Failure .......................................15
3.3.4. C1A1 Failure .......................................15
3.3.5. Conclusion .........................................17
3.3.6. A Square Might Become a Full Mesh ..................17
3.3.7. A Full Mesh Might Be More Economical Than a
Square .............................................17
3.4. Extended U ................................................18
3.4.1. E1A1 Failure .......................................19
3.4.2. A1E1 Failure .......................................20
3.4.3. A1C1 Failure .......................................20
3.4.4. C1A1 Failure .......................................21
3.4.5. Conclusion .........................................21
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3.5. Dual-Plane Core and Its Impact on the Access LFA
Analysis ..................................................21
3.6. Two-Tiered IGP Metric Allocation ..........................22
3.7. uLoop Analysis ............................................22
3.8. Summary ...................................................23
4. Core Network ...................................................24
4.1. Simulation Framework ......................................25
4.2. Data Set ..................................................26
4.3. Simulation Results ........................................26
5. Core and Access Protection Schemes Are Independent .............27
6. Simplicity and Other LFA Benefits ..............................27
7. Capacity Planning with LFA in Mind .............................28
7.1. Coverage Estimation - Default Topology ....................28
7.2. Coverage Estimation in Relation to Traffic ................29
7.3. Coverage Verification for a Given Set of Demands ..........29
7.4. Modeling - What-If Scenarios - Coverage Impact ............29
7.5. Modeling - What-If Scenarios - Load Impact ................30
7.6. Discussion on Metric Recommendations ......................31
8. Security Considerations ........................................32
9. Conclusions ....................................................32
10. Acknowledgments ...............................................32
11. References ....................................................33
11.1. Normative References .....................................33
11.2. Informative References ...................................33
1. Introduction
In this document, we analyze the applicability of the Loop-Free
Alternate (LFA) [RFC5714] [RFC5286] method of providing IP fast
reroute (IPFRR) in both the core and access parts of Service Provider
(SP) networks. We consider both the link and node failure cases, and
provide guidance on the applicability of LFAs to different network
topologies, with special emphasis on the access parts of the network.
We first introduce the terminology used in this document in
Section 2. In Section 3, we describe typical access network designs,
and we analyze them for LFA applicability. In Section 4, we describe
a simulation framework for the study of LFA applicability in SP core
networks, and present results based on various SP networks. We then
emphasize the independence between protection schemes used in the
core and at the access level of the network. Finally, we discuss the
key benefits of the LFA method, which stem from its simplicity, and
we draw some conclusions.
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2. Terminology
We use IS-IS [RFC1195] [IS-IS] as a reference. It is assumed that
normal routing (i.e., when traffic is not being fast-rerouted around
a failure) occurs along the shortest path. The analysis is equally
applicable to OSPF [RFC2328] [RFC5340].
A per-prefix LFA for a destination D at a node S is a pre-computed
backup IGP next hop for that destination. This backup IGP next hop
can be link-protecting or node-protecting. In this document, we
assume that all links to be protected with LFAs are point-to-point.
Link-protecting: A neighbor N is a link-protecting per-prefix LFA for
S's route to D if equation eq1 is satisfied. This is in line with
the definition of an LFA in [RFC5714].
eq1: ND < NS + SD
where XY refers to the IGP distance from X to Y
Equation eq1
Node-protecting: A neighbor N is a node-protecting LFA for S's route
to D with initial IGP next hop F if N is a link-protecting LFA for D
and equation eq2 is satisfied. This is in line with the definition
of a Loop-Free Node-Protecting Alternate (also known as a node-
protecting LFA) in [RFC5714].
eq2: ND < NF + FD
Equation eq2
De facto node-protecting LFA: This is a link-protecting LFA that
turns out to be node-protecting. This occurs in cases illustrated by
the following examples:
o The LFA candidate that is picked by S actually satisfies Equation
eq2, but S did not verify that property. The show command issued
by the operator would not indicate this LFA as "node-protecting",
while in practice (de facto), it is.
o A cascading effect of multiple LFAs can also provide de facto node
protection. Equation eq2 is not satisfied, but the combined
activation of LFAs by some other neighbors of the failing node F
provides (de facto) node protection. In other words, it puts the
data plane in a state such that packets forwarded by S ultimately
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reach a neighbor of F that has a node-protecting LFA. Note that
in this case, S cannot indicate the node-protecting behavior of
the repair without running additional computations.
Per-link LFA: A per-link LFA for the link SF is one pre-computed
backup IGP next hop for all of the destinations reached through SF.
This is a neighbor of the repairing node that is a per-prefix LFA for
all of the destinations that the repairing node reaches through SF.
Note that such a per-link LFA exists if S has a per-prefix LFA for
destination F.
D
/ \
10 / \ 10
/ \
G H----------.
| | |
1 | 1 | |
| | |
B C | 10
| |\ |
| | \ |
| | \ 6 |
| | \ |
7 | 10 | E F
| | / /
| | / 6 / 5
| | / /
| |/ /
A-------S-----/
7
Figure 1: Example 1
In Figure 1, considering the protection of link SC, we can see that
A, E, and F are per-prefix LFAs for destination D, as none of them
use S to reach D.
For destination D, A and F are node-protecting LFAs, as they do not
reach D through node C, while E is not node-protecting for S, as it
reaches D through C.
If S does not compute and select node-protecting LFAs, there is a
chance that S picks the non-node-protecting LFA E, although A and F
were node-protecting LFAs. If S enforces the selection of node-
protecting LFAs, then in the case of the single failure of link SC,
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S will first activate its LFA, deviate traffic addressed to D along
S-A-B-G-D and/or S-F-H-D, and then converge to its post-convergence
optimal path S-E-C-H-D.
A reaches C via S; thus, A is not a per-link LFA for link SC. E
reaches C through link EC; thus, E is a per-link LFA for link SC.
This per-link LFA does not provide de facto node protection. Upon
failure of node C, S would fast-reroute D-destined packets to its
per-link LFA (= E). E would itself detect the failure of EC; hence,
it would activate its own per-link LFA (= S). Traffic addressed to D
would be trapped in a loop; hence, there is no de facto node
protection behavior.
If there were a link between E and F that E would pick as its LFA for
destination D, then E would provide de facto node protection for S,
as upon the activation of its LFA, S would deviate traffic addressed
to D towards E. In turn, E deviates that traffic to F, which does
not reach D through C.
F is a per-link LFA for link SC, as F reaches C via H. This per-link
LFA is de facto node-protecting for destination D, as F reaches D via
F-H-D.
Micro-Loop (uLoop): the occurrence of a transient forwarding loop
during a routing transition (as defined in [RFC5715]).
In Figure 1, the loss of link SE cannot create any uLoop, because of
the following:
1. The link is only used to reach destination E.
2. S is the sole node changing its path to E upon link SE failure.
3. S's shortest path to E after the failure goes via C.
4. C's best path to E (before and after link SC failure) is via CE.
On the other hand, upon failure of link AB, a micro-loop may form for
traffic destined to B. Indeed, if A updates its Forwarding
Information Base (FIB) before S, A will reroute B-destined traffic
towards S, while S is still forwarding this traffic to A.
3. Access Network
The access part of the network often represents the majority of the
nodes and links. It is organized in several tens or more of regions
interconnected by the core network. Very often, the core acts as an
IS-IS level-2 domain (OSPF area 0), while each access region is
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confined in an IS-IS level-1 domain (OSPF non-0 area). Very often,
the network topology within each access region is derived from a
unique template common across the whole access network. Within an
access region itself, the network is made of several aggregation
regions, each following the same interconnection topologies.
For these reasons, in the next sections, we base the analysis of the
LFA applicability in a single access region, with the following
assumptions:
o Two routers (C1 and C2) provide connectivity between the access
region and the rest of the network. If a link connects these two
routers in the region area, then it has a symmetric IGP metric c.
o We analyze a single aggregation region within the access region.
Two aggregation routers (A1 and A2) interconnect the aggregation
region to the two routers C1 and C2 for the analyzed access
region. If a link connects A1 to A2, then it has a symmetric IGP
metric a. If a link connects a router A to a router C, then for
the sake of generality we will call d the metric for the directed
link CA and u the metric for the directed link AC.
o We analyze two edge routers, E1 and E2, in the access region.
Each is dual-homed directly either to C1 and C2 (Section 3.1) or
to A1 and A2 (Sections 3.2, 3.3, and 3.4). The directed link
metric between Cx/Ax and Ey is d and u in the opposite direction.
o We assume a multi-level IGP domain. The analyzed access region
forms a level-1 (L1) domain. The core is the level-2 (L2) domain.
We assume that the link between C1 and C2, if it exists, is
configured as L1L2. We assume that the loopbacks of the C routers
are part of the L2 topology. L1 routers learn about them as
propagated routes (L2=>L1 with the Down bit set). We remind the
reader that if an L1L2 router learns about X/x as an L1 path P1,
an L2 path P2, and an L1L2 path P12, then it will prefer path P1.
If path P1 is lost, then it will prefer path P2.
o We assume that all of the C, A, and E routers may be connected to
customers; hence, we analyze LFA coverage for the loopbacks of
each type of node.
o We assume that no useful traffic is directed to router-to-router
subnets; hence, we do not analyze LFA applicability for such
subnets.
o A prefix P models an important IGP destination that is not present
in the local access region. The IGP metric from C1 to P is x, and
the metric from C2 to P is x + e.
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o We analyze LFA coverage against all link and node failures within
the access region.
o WxYz refers to the link from Wx to Yz.
o We assume that c < d + u and a < d + u (a commonly agreed-upon
design rule).
o In the square access design (Section 3.3), we assume that c < a (a
commonly agreed-upon design rule).
o We analyze the most frequent topologies found in an access region.
o We first analyze per-prefix LFA applicability and then per-link.
o The topologies are symmetric with respect to a vertical axis;
hence, we only detail the logic for the link and node failures of
the left half of the topology.
3.1. Triangle
We describe the LFA applicability for the failures of C1E1, E1, and
C1 (Figure 2).
P
/ \
x/ \x+e
/ \
C1--c--C2
|\ /|
| \ / |
d/u| \ |d/u
| / \ |
|/ \|
E1 E2
Figure 2: Triangle
3.1.1. E1C1 Failure
3.1.1.1. Per-Prefix LFA
Three destinations are impacted by this link failure: C1, E2, and P.
The LFA for destination C1 is C2, because eq1: c < d + u. Node
protection for route C1 is not applicable. (If C1 goes down, traffic
destined to C1 is lost anyway.)
Filsfils, et al. Informational [Page 8]
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The LFA to E2 is via C2, because eq1: d < d + u + d. It is node-
protecting, because eq2: d < c + d.
The LFA to P is via C2, because c < d + u. It is node-protecting if
eq2: x + e < x + c, i.e., if e < c. This relationship between e and
c is an important aspect of the analysis, which is discussed in
detail in Sections 3.5 and 3.6.
Conclusion: All important intra-PoP (Point of Presence) routes with
primary interface E1C1 benefit from LFA link and node protection.
All important inter-PoP routes with primary interface E1C1 benefit
from LFA link protection, and also from node protection if e < c.
3.1.1.2. Per-Link LFA
We have a per-prefix LFA to C1; hence, we have a per-link LFA for
link E1C1. All impacted destinations are protected against link
failure. In the case of C1 node failure, the traffic to C1 is lost
(by definition), the traffic to E2 is de facto protected against node
failure, and the traffic to P is de facto protected when e < c.
3.1.2. C1E1 Failure
3.1.2.1. Per-Prefix LFA
C1 only has one primary route via C1E1: the route to E1
(because c < d + u).
C1's LFA to E1 is via C2, because eq1: d < c + d.
Node protection upon E1's failure is not applicable, as the only
impacted traffic is sinked at E1 and hence is lost anyway.
Conclusion: All important routes with primary interface C1E1 benefit
from LFA link protection. Node protection is not applicable.
3.1.2.2. Per-Link LFA
We have a per-prefix LFA to E1; hence, we have a per-link LFA for
link C1E1. De facto node protection is not applicable.
3.1.3. uLoop
The IGP convergence cannot create any uLoop. See Section 3.7.
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3.1.4. Conclusion
All important intra-PoP routes benefit from LFA link and node
protection or de facto node protection. All important inter-PoP
routes benefit from LFA link protection. De facto node protection is
ensured if e < c. (This is particularly the case for dual-plane core
or two-tiered IGP metric design; see Sections 3.5 and 3.6.)
The IGP convergence does not cause any uLoop.
Per-link LFAs and per-prefix LFAs provide the same protection
benefits.
3.2. Full Mesh
We describe the LFA applicability for the failures of C1A1, A1E1, E1,
A1, and C1 (Figure 3).
P
/ \
x/ \x+e
/ \
C1--c--C2
|\ /|
| \ / |
d/u | \ | d/u
| / \ |
|/ \|
A1--a--A2
|\ /|
| \ / |
d/u| \ |d/u
| / \ |
|/ \|
E1 E2
Figure 3: Full Mesh
3.2.1. E1A1 Failure
3.2.1.1. Per-Prefix LFA
Four destinations are impacted by this link failure: A1, C1, E2,
and P.
The LFA for A1 is A2: eq1: a < d + u. Node protection for route A1
is not applicable. (If A1 goes down, traffic to A1 is lost anyway.)
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The LFA for C1 is A2: eq1: u < d + u + u. Node protection for route
C1 is guaranteed: eq2: u < a + u.
The LFA to E2 is via A2: eq1: d < d + u + d. Node protection is
guaranteed: eq2: d < a + d.
The LFA to P is via A2: eq1: u + x < d + u + u + x. Node protection
is guaranteed: eq2: u + x < a + u + x.
Conclusion: All important intra-PoP and inter-PoP routes with primary
interface E1A1 benefit from LFA link and node protection.
3.2.1.2. Per-Link LFA
We have a per-prefix LFA to A1; hence, we have a per-link LFA for
link E1A1. All impacted destinations are protected against link
failure. De facto node protection is provided for all destinations
(except to A1, which is not applicable).
3.2.2. A1E1 Failure
3.2.2.1. Per-Prefix LFA
A1 only has one primary route via A1E1: the route to E1
(because a < d + u).
A1's LFA to E1 is via A2: eq1: d < a + d.
Node protection upon E1's failure is not applicable, as the only
impacted traffic is sinked at E1 and hence is lost anyway.
Conclusion: All important routes with primary interface A1E1 benefit
from LFA link protection. Node protection is not applicable.
3.2.2.2. Per-Link LFA
We have a per-prefix LFA to E1; hence, we have a per-link LFA for
link C1E1. De facto node protection is not applicable.
3.2.3. A1C1 Failure
3.2.3.1. Per-Prefix LFA
Two destinations are impacted by this link failure: C1 and P.
The LFA for C1 is C2, because eq1: c < d + u. Node protection for
route C1 is not applicable. (If C1 goes down, traffic to C1 is lost
anyway.)
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The LFA for P is via C2, because c < d + u. It is de facto protected
against node failure if eq2: x + e < x + c.
Conclusion: All important intra-PoP routes with primary interface
A1C1 benefit from LFA link protection. (Node protection is not
applicable.) All important inter-PoP routes with primary interface
E1C1 benefit from LFA link protection (and from de facto node
protection if e < c).
3.2.3.2. Per-Link LFA
We have a per-prefix LFA to C1; hence, we have a per-link LFA for
link A1C1. All impacted destinations are protected against link
failure. In the case of C1 node failure, the traffic to C1 is lost
(by definition), and the traffic to P is de facto node protected
if e < c.
3.2.4. C1A1 Failure
3.2.4.1. Per-Prefix LFA
C1 has three routes via C1A1: A1, E1, and E2. E2 behaves like E1 and
hence is not analyzed further.
C1's LFA to A1 is via C2, because eq1: d < c + d. Node protection
upon A1's failure is not applicable, as the traffic to A1 is lost
anyway.
C1's LFA to E1 is via A2: eq1: d < u + d + d. Node protection upon
A1's failure is guaranteed, because eq2: d < a + d.
Conclusion: All important routes with primary interface C1A1 benefit
from LFA link protection. Node protection is guaranteed where
applicable.
3.2.4.2. Per-Link LFA
We have a per-prefix LFA to A1; hence, we have a per-link LFA for
link C1E1. De facto node protection is available.
3.2.5. uLoop
The IGP convergence cannot create any uLoop. See Section 3.7.
3.2.6. Conclusion
All important intra-PoP routes benefit from LFA link and node
protection.
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All important inter-PoP routes benefit from LFA link protection.
They benefit from node protection upon failure of A nodes. They
benefit from node protections upon failure of C nodes if e < c.
(This is particularly the case for dual-plane core or two-tiered IGP
metric design; see Sections 3.5 and 3.6.)
The IGP convergence does not cause any uLoop.
Per-link LFAs and per-prefix LFAs provide the same protection
benefits.
3.3. Square
We describe the LFA applicability for the failures of C1A1, A1E1, E1,
A1, and C1 (Figure 4).
P
/ \
x/ \x+e
/ \
C1--c--C2
|\ | \
| \ | +-------+
d/u | \ | \
| +-|-----+ \
| | \ \
A1--a--A2 A3--a--A4
|\ /| | /
| \ / | | /
d/u| \ |d/u | /
| / \ | | /
|/ \| |/
E1 E2 E3
Figure 4: Square
3.3.1. E1A1 Failure
3.3.1.1. Per-Prefix LFA
E1 has six routes via E1A1: A1, C1, P, E2, A3, and E3.
E1's LFA route to A1 is via A2, because eq1: a < d + u. Node
protection for traffic to A1 upon A1 node failure is not applicable.
E1's LFA route to A3 is via A2, because eq1: u + c + d < d + u +
u + d. This LFA is guaranteed to be node-protecting, because
eq2: u + c + d < a + u + d.
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E1's LFA route to C1 is via A2, because eq1: u + c < d + u + u. This
LFA is guaranteed to be node-protecting, because eq2: u + c < a + u.
E1's primary route to E2 is via ECMP(E1A1, E1A2) (Equal-Cost
Multi-Path). The LFA for the first ECMP path (via A1) is the second
ECMP path (via A2). This LFA is guaranteed to be node-protecting,
because eq2: d < a + d.
E1's primary route to E3 is via ECMP(E1A1, E1A2). The LFA for the
first ECMP path (via A1) is the second ECMP path (via A2). This LFA
is guaranteed to be node-protecting, because eq2: u + d + d < a + u +
d + d.
If e = 0: E1's primary route to P is via ECMP(E1A1, E1A2). The LFA
for the first ECMP path (via A1) is the second ECMP path (via A2).
This LFA is guaranteed to be node-protecting, because eq2: u + x + 0
< a + u + x.
If e <> 0: E1's primary route to P is via E1A1. Its LFA is via A2,
because eq1: u + c + x < d + u + u + x. This LFA is guaranteed to be
node-protecting, because eq2: u + c + x < a + u + x.
Conclusion: All important intra-PoP and inter-PoP routes with primary
interface E1A1 benefit from LFA link protection and node protection.
3.3.1.2. Per-Link LFA
We have a per-prefix LFA for A1; hence, we have a per-link LFA for
link E1A1. All important intra-PoP and inter-PoP routes with primary
interface E1A1 benefit from LFA per-link protection and de facto node
protection.
3.3.2. A1E1 Failure
3.3.2.1. Per-Prefix LFA
A1 only has one primary route via A1E1: the route to E1.
A1's LFA for route E1 is the path via A2, because eq1: d < a + d.
Node protection is not applicable.
Conclusion: All important routes with primary interface A1E1 benefit
from LFA link protection. Node protection is not applicable.
3.3.2.2. Per-Link LFA
All important routes with primary interface A1E1 benefit from LFA
link protection. De facto node protection is not applicable.
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3.3.3. A1C1 Failure
3.3.3.1. Per-Prefix LFA
Four destinations are impacted when A1C1 fails: C1, A3, E3, and P.
A1's LFA to C1 is via A2, because eq1: u + c < a + u. Node
protection is not applicable for traffic to C1 when C1 fails.
A1's LFA to A3 is via A2, because eq1: u + c + d < a + u + d. It is
de facto node-protecting, as a < u + c + d (as we assumed
a < u + d). Indeed, for destination A3, A2 forwards traffic to C2,
and C2 has a node-protecting LFA -- A4 -- for the failure of link
C2C1, as a < u + c + d. Hence, the cascading application of LFAs by
A1 and C2 during the failure of C1 provides de facto node protection.
A1's LFA to E3 is via A2, because eq1: u + d + d < a + u + d + d. It
is node-protecting, because eq2: u + d + d < u + c + d + d.
A1's primary route to P is via C1 (even if e = 0, u + x < u + c + x).
The LFA is via A2, because eq1: u + c + x < a + u + x (case where
c <= e) and eq1: u + x + e < a + u + x (case where c >= e). This LFA
is node-protecting (from the viewpoint of A1 computing eq2) if
eq2: u + x + e < u + c + x. This inequality is true if e < c.
Conclusion: All important intra-PoP routes with primary interface
A1C1 benefit from LFA link protection and node protection. Note that
A3 benefits from de facto node protection. All important inter-PoP
routes with primary interface A1C1 benefit from LFA link protection.
They also benefit from node protection if e < c.
3.3.3.2. Per-Link LFA
All important intra-PoP routes with primary interface A1C1 benefit
from LFA link protection and de facto node protection. All important
inter-PoP routes with primary interface A1C1 benefit from LFA link
protection. They also benefit from de facto node protection if
e < c.
3.3.4. C1A1 Failure
3.3.4.1. Per-Prefix LFA
Three destinations are impacted by C1A1 link failure: A1, E1, and E2.
E2's analysis is the same as E1 and hence is omitted.
C1 has no LFA for A1. Indeed, its neighbors (C2 and A3) have a
shortest path to A1 via C1. This is due to the assumption (c < a).
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C1's LFA for E1 is via C2, because eq1: d + d < c + d + d. It
provides node protection, because eq2: d + d < d + a + d.
Conclusion: All important intra-PoP routes with primary interface
A1C1, except A1, benefit from LFA link protection and node
protection.
3.3.4.2. Per-Link LFA
C1 does not have a per-prefix LFA for destination A1; hence, there is
no per-link LFA for link C1A1.
3.3.4.3. Assumptions on the Values of c and a
The commonly agreed-upon design rule (c < a) is especially beneficial
for a deployment using per-link LFA: it provides a per-link LFA for
the most important direction (A1C1). Indeed, there are many more
destinations reachable over A1C1 than over C1A1. As the IGP
convergence duration is proportional to the number of routes to
update, there is a better benefit in leveraging LFA FRR for link A1C1
than for link C1A1.
Note as well that the consequence of this assumption is much more
important for per-link LFA than for per-prefix LFA.
For per-prefix LFAs, in the case of link C1A1 failure, we do have a
per-prefix LFA for E1, E2, and any node subtended below A1 and A2.
Typically, most of the traffic traversing link C1A1 is directed to
these E nodes; hence, the lack of per-prefix LFAs for the destination
A1 might be insignificant. This is a good example of the coverage
benefit of per-prefix LFAs over per-link LFAs.
In the remainder of this section, we analyze the consequence of not
having c < a.
It definitely has a negative impact upon per-link LFAs.
With c > a, C1A1 has a per-link LFA, while A1C1 has no per-link LFA.
The number of destinations impacted by A1C1 failure is much larger
than the direction C1A1; hence, the protection is provided for the
wrong direction.
For per-prefix LFAs, the availability of an LFA depends on the
topology and needs to be assessed individually for each per-prefix
LFA. Some backbone topologies will lead to very good protection
coverage, while some others might provide very poor coverage.
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More specifically, upon A1C1 failure, the coverage of a remote
destination P depends on whether e < a. In such a case, A2 is de
facto node-protecting per-prefix LFA for P.
Such a study likely requires a planning tool, as each remote
destination P would have a different e value (exception: all of the
edge devices of other aggregation pairs within the same region, as
for these e = 0 by definition, e.g., E3.)
Finally, note that c = a is the worst choice. In this case, C1 has
no per-prefix LFA for A1 (and vice versa); hence, there is no
per-link LFA for C1A1 and A1C1.
3.3.5. Conclusion
All important intra-PoP routes benefit from LFA link and node
protection with one exception: C1 has no per-prefix LFA to A1.
All important inter-PoP routes benefit from LFA link protection.
They benefit from node protection if e < c.
Per-link LFA provides the same protection coverage as per-prefix LFA,
with two exceptions: first, C1A1 has no per-link LFA at all. Second,
when per-prefix LFA provides node protection (eq2 is satisfied),
per-link LFA provides effective de facto node protection.
3.3.6. A Square Might Become a Full Mesh
If the vertical links of the square are made of parallel links (at
the IP topology or below), then one should consider splitting these
"vertical links" into "vertical and crossed links". The topology
becomes "full mesh". One should also ensure that the two resulting
sets of links (vertical and crossed) do not share any Shared Risk
Link Group (SRLG).
A typical scenario in which this is prevented would be when the A1C1
bandwidth may be within a building while the A1C2 is between
buildings. Hence, while from a router-port viewpoint the operation
is cost-neutral, from a cost-of-bandwidth viewpoint it is not.
3.3.7. A Full Mesh Might Be More Economical Than a Square
In a full mesh, the vertical and crossed links play the dominant
role, as they support most of the primary and backup paths. The
capacity of the horizontal links can be dimensioned on the basis of
traffic destined to a single C node or a single A node, and to a
single E node.
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3.4. Extended U
For the Extended U topology, we define the following terminology:
C1L1: the node "C1" as seen in topology L1.
C1L2: the node "C1" as seen in topology L2.
C1LO: the loopback of C1. This loopback is in L2.
C2LO: the loopback of C2. This loopback is in L2.
We remind the reader that C1 and C2 are L1L2 routers and that their
loopbacks are in L2 only.
P
/ \
x/ \x+e
/ \
C1<...>C2
|\ | \
| \ | +-------+
d/u | \ | \
| +-|-----+ \
| | \ \
A1--a--A2 A3--a--A4
|\ /| | /
| \ / | | /
d/u| \ |d/u | /
| / \ | | /
|/ \| |/
E1 E2 E3
Figure 5: Extended U
There is no L1 link between C1 and C2. There might be an L2 link
between C1 and C2. This is not relevant, as this is not seen from
the viewpoint of the L1 topology, which is the focus of our analysis.
It is guaranteed that there is a path from C1LO to C2LO within the L2
topology (except if the L2 topology partitions, which is very
unlikely and hence not analyzed here). We call "c" its path cost.
Once again, we assume that c < a.
We exploit this property to create a tunnel T between C1LO and C2LO.
Once again, as the source and destination addresses are the loopbacks
of C1 and C2 and these loopbacks are in L2 only, it is guaranteed
that the tunnel does not transit via the L1 domain.
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IS-IS does not run over the tunnel; hence, the tunnel is not used for
any primary paths within the L1 or L2 topology.
Within level-1, we configure C1 (C2) with a level-1 LFA extended
neighbor "C2 via tunnel T" ("C1 via tunnel T").
A router supporting such an extension learns that it has one
additional potential neighbor in topology level-1 when checking for
LFAs.
The L1 topology learns about C1LO as an L2=>L1 route with the Down
bit set, propagated by C1L1 and C2L1. The metric advertised by C2L1
is bigger than the metric advertised by C1L1 by "c".
The L1 topology learns about P as an L2=>L1 route with the Down bit
set, propagated by C1L1 and C2L1. The metric advertised by C2L1 is
bigger than the metric advertised by C1L1 by "e". This implies that
e <= c.
3.4.1. E1A1 Failure
3.4.1.1. Per-Prefix LFA
Five destinations are impacted by E1A1 link failure: A1, C1LO, E2,
E3, and P.
The LFA for A1 is via A2, because eq1: a < d + u. Node protection
for traffic to A1 upon A1 node failure is not applicable.
The LFA for E2 is via A2, because eq1: d < d + u + d. Node
protection is guaranteed, because eq2: d < a + d.
The LFA for E3 is via A2, because eq1: u + d + d < d + u + d + d.
Node protection is guaranteed, because eq2: u + d + d
< a + u + d + d.
The LFA for C1LO is via A2, because eq1: u + c < d + u + u. Node
protection is guaranteed, because eq2: u + c < a + u.
If e = 0: E1's primary route to P is via ECMP(E1A1, E1A2). The LFA
for the first ECMP path (via A1) is the second ECMP path (via A2).
Node protection is possible, because eq2: u + x < a + u + x.
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If e <> 0: E1's primary route to P is via E1A1. Its LFA is via A2,
because eq1: a + c + x < d + u + u + x. Node protection is
guaranteed, because eq2: u + x + e < a + u + x <=> e < a. This is
true, because e <= c and c < a.
Conclusion: Same as that for the square topology.
3.4.1.2. Per-Link LFA
Same as the square topology.
3.4.2. A1E1 Failure
3.4.2.1. Per-Prefix LFA
Same as the square topology.
3.4.2.2. Per-Link LFA
Same as the square topology.
3.4.3. A1C1 Failure
3.4.3.1. Per-Prefix LFA
Three destinations are impacted when A1C1 fails: C1, E3, and P.
A1's LFA to C1LO is via A2, because eq1: u + c < a + u. Node
protection is not applicable for traffic to C1 when C1 fails.
A1's LFA to E3 is via A2, because eq1: u + d + d < d + u + u + d + d.
Node protection is guaranteed, because eq2: u + d + d < a + u +
d + d.
A1's primary route to P is via C1 (even if e = 0, u + x < a + u + x).
The LFA is via A2, because eq1: u + x + e < a + u + x <=> e < a
(which is true; see above). Node protection is guaranteed, because
eq2: u + x + e < a + u + x.
Conclusion: Same as that for the square topology.
3.4.3.2. Per-Link LFA
Same as the square topology.
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3.4.4. C1A1 Failure
3.4.4.1. Per-Prefix LFA
Three destinations are impacted by C1A1 link failure: A1, E1, and E2.
E2's analysis is the same as E1 and hence is omitted.
C1L1 has an LFA for A1 via the extended neighbor C2L1 reachable via
tunnel T. Indeed, eq1 is true: d + a < d + a + u + d. From the
viewpoint of C1L1, C2L1's path to C1L1 is C2L1-A2-A1-C1L1. Remember
that the tunnel is not seen by IS-IS for computing primary paths!
Node protection is not applicable for traffic to A1 when A1 fails.
C1L1's LFA for E1 is via extended neighbor C2L1 (over tunnel T),
because eq1: d + d < d + a + u + d + d. Node protection is
guaranteed, because eq2: d + d < d + a + d.
3.4.4.2. Per-Link LFA
C1 has a per-prefix LFA for destination A1; hence, there is a
per-link LFA for the link C1A1. Node resistance is applicable for
traffic to E1 (and E2).
3.4.5. Conclusion
The Extended U topology is as good as the square topology.
It does not require any crossed links between the A and C nodes
within an aggregation region. It does not need an L1 link between
the C routers in an access region. Note that a link between the C
routers might exist in the L2 topology.
3.5. Dual-Plane Core and Its Impact on the Access LFA Analysis
A dual-plane core is defined as follows:
o Each access region k is connected to the core by two C routers
(C(1,k) and C(2,k)).
o C(1,k) is part of plane-1 of the dual-plane core.
o C(2,k) is part of plane-2 of the dual-plane core.
o C(1,k) has a link to C(2, l) iff k = l.
o {C(1,k) has a link to C(1, l)} iff {C(2,k) has a link to C(2, l)}.
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In a dual-plane core design, e = 0; hence, the LFA node-protection
coverage is improved in all of the analyzed topologies.
3.6. Two-Tiered IGP Metric Allocation
A two-tiered IGP metric allocation scheme is defined as follows:
o All of the link metrics used in the L2 domain are part of
range R1.
o All of the link metrics used in an L1 domain are part of range R2.
o Range R1 << range R2 such that the difference e = C2P - C1P is
smaller than any link metric within an access region.
Assuming such an IGP metric allocation, the following properties are
guaranteed: c < a, e < c, and e < a.
3.7. uLoop Analysis
In this section, we analyze a case where the routing transition
following the failure of a link may have some uLoop potential for one
destination. Then, we show that all of the other cases do not have
uLoop potential.
In the square design, upon the failure of link C1A1, traffic
addressed to A1 can undergo a transient forwarding loop as C1
reroutes traffic to C2, which initially reaches A1 through C1, as
c < a. This loop will actually occur when C1 updates its FIB for
destination A1 before C2.
It can be shown that all of the other routing transitions following a
link failure in the analyzed topologies do not have uLoop potential.
Indeed, in each case, for all destinations affected by the failure,
the rerouting nodes deviate their traffic directly to adjacent nodes
whose paths towards these destinations do not change. As a
consequence, all of these routing transitions cannot undergo
transient forwarding loops.
For example, in the square topology, the failure of directed link
A1C1 does not lead to any uLoop. The destinations reached over that
directed link are C1 and P. A1's and E1's shortest paths to these
destinations after the convergence go via A2. A2's path to C1 and P
is not using A1C1 before the failure; hence, no uLoop may occur.
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3.8. Summary
In this section, we summarize the applicability of LFAs detailed in
the previous sections. For link protection, we use "Full" to refer
to the applicability of LFAs for each destination, reached via any
link of the topology. For node protection, we use "Yes" to refer to
the fact that node protection is achieved for a given node.
1. Intra-Area Destinations
Link Protection
+ Triangle: Full
+ Full Mesh: Full
+ Square: Full, except C1 has no LFA for dest A1
+ Extended U: Full
Node Protection
+ Triangle: Yes
+ Full Mesh: Yes
+ Square: Yes
+ Extended U: Yes
2. Inter-Area Destinations
Link Protection
+ Triangle: Full
+ Full Mesh: Full
+ Square: Full
+ Extended U: Full
Node Protection
+ Triangle: Yes, if e < c
+ Full Mesh: Yes for A failure, if e < c for C failure
+ Square: Yes for A failure, if e < c for C failure
+ Extended U: Yes, if e <= c and c < a
3. uLoops
* Triangle: None
* Full Mesh: None
* Square: None, except traffic to A1 when C1A1 fails
* Extended U: None, if a > e
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4. Per-Link LFA vs. Per-Prefix LFA
* Triangle: Same
* Full Mesh: Same
* Square: Same, except C1A1 has no per-link LFA. In practice,
this means that per-prefix LFAs will be used. (Hence, C1 has
no LFA for dest = E1 and dest = A1.)
* Extended U: Same
4. Core Network
In the backbone, the optimization of the network design to achieve
the maximum LFA protection is less straightforward than in the case
of the access/aggregation network.
The main optimization objectives for backbone topology design are
cost, latency, and bandwidth, constrained by the availability of
fiber. Optimizing the design for local IP restoration is more likely
to be considered as a non-primary objective. For example, the way
the fiber is laid out and the resulting cost to change it lead to
ring topologies in some backbone networks.
Also, the capacity-planning process is already complex in the
backbone. The process needs to make sure that the traffic matrix
(demand) is supported by the underlying network (capacity) under all
possible variations of the underlying network (what-if scenario
related to one-SRLG failure). Classically, "supported" means that no
congestion is experienced and that the demands are routed along the
appropriate latency paths. Selecting the LFA method as a
deterministic FRR solution for the backbone would require enhancement
of the capacity-planning process to add a third constraint: Each
variation of the underlying network should lead to sufficient LFA
coverage. (We detail this aspect in Section 7.)
On the other hand, the access network is based on many replications
of a small number of well-known (well-engineered) topologies. The
LFA coverage is deterministic and is independent of additions/
insertions of a new edge device, a new aggregation sub-region, or a
new access region.
In practice, we believe that there are three profiles for the
backbone applicability of the LFA method:
In the first profile, the designer plans all of the network
resilience on IGP convergence. In such a case, the LFA method is
a free bonus. If an LFA is available, then the loss of
connectivity is likely reduced by a factor of 10 (50 msec vs.
500 msec); otherwise, the loss of connectivity depends on IGP
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convergence, which is the initial target anyway. The LFA method
should be very successful here, as it provides a significant
improvement without any additional cost.
In the second profile, the designer seeks a very high and
deterministic FRR coverage, and he either does not want or cannot
engineer the topology. The LFA method should not be considered in
this case. MPLS Traffic Engineering (TE) FRR would perform much
better in this environment. Explicit routing ensures that a
backup path exists, whatever the underlying topology.
In the third profile, the designer seeks a very high and
deterministic FRR coverage, and he does engineer the topology.
The LFA method is appealing in this scenario, as it can provide a
very simple way to obtain protection. Furthermore, in practice,
the requirement for FRR coverage might be limited to a certain
part of the network (e.g., a given sub-topology) and/or is likely
limited to a subset of the demands within the traffic matrix. In
such a case, if the relevant part of the network natively provides
a high degree of LFA protection for demands of interest, it might
actually be straightforward to improve the topology and achieve
the level of protection required for the sub-topology and the
demands that matter. Once again, the practical problem needs to
be considered (which sub-topology, and which real demands need
50 msec), as it is often simpler than the theoretical generic one.
For the reasons explained previously, the backbone applicability
should be analyzed on a case-by-case basis, and it is difficult to
derive generic rules.
In order to help the reader to assess the LFA applicability in his
own case, we provide some simulation results based on 11 real
backbone topologies in the next section.
4.1. Simulation Framework
In order to perform an analysis of LFA applicability in the core, we
usually receive the complete IS-IS/OSPF linkstate database taken on a
core router. We parse it to obtain the topology. During this
process, we eliminate all nodes connected to the topology with a
single link and all prefixes except a single "node address" per
router. We compute the availability of per-prefix LFAs to all of
these node addresses, which we hereafter call "destinations". We
treat each link in each direction.
For each (directed) link, we compute whether we have a per-prefix LFA
to the next hop. If so, we have a per-link LFA for the link.
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The per-link-LFA coverage for a topology T is the fraction of the
number of links with a per-link LFA divided by the total number of
links.
For each link, we compute the number of destinations whose primary
path involves the analyzed link. For each such destination, we
compute whether a per-prefix LFA exists.
The per-prefix LFA coverage for a topology T is the following
fraction:
(the sum across all links of the number of destinations with a
primary path over the link and a per-prefix LFA)
divided by
(the sum across all links of the number of destinations with a
primary path over the link)
4.2. Data Set
Our data set is based on 11 SP core topologies with different
geographical scopes: worldwide, national, and regional. The number
of nodes ranges from 600 to 16. The average link-to-node ratio is
2.3, with a minimum of 1.2 and maximum of 6.
4.3. Simulation Results
+----------+--------------+----------------+
| Topology | Per-Link LFA | Per-Prefix LFA |
+----------+--------------+----------------+
| T1 | 45% | 76% |
| T2 | 49% | 98% |
| T3 | 88% | 99% |
| T4 | 68% | 84% |
| T5 | 75% | 94% |
| T6 | 87% | 98% |
| T7 | 16% | 67% |
| T8 | 87% | 99% |
| T9 | 67% | 79% |
| T10 | 98% | 99% |
| T11 | 59% | 77% |
| Average | 67% | 89% |
| Median | 68% | 94% |
+----------+--------------+----------------+
Table 1: Core LFA Coverages
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In Table 1, we observe a wide variation in terms of LFA coverage
across topologies: from 67% to 99% for the per-prefix LFA coverage,
and from 16% to 98% for the per-link LFA coverage. Several
topologies have been optimized for LFAs (T3, 6, 8, and 10). This
illustrates the need for case-by-case analysis when considering LFAs
for core networks.
It should be noted that, contrary to the access/aggregation
topologies, per-prefix LFA outperforms per-link LFA in the backbone.
5. Core and Access Protection Schemes Are Independent
Specifically, a design might use LFA FRR in the access and MPLS TE
FRR in the core.
The LFA method provides great benefits for the access network, due to
its excellent access coverage and its simplicity.
MPLS TE FRR's topology independence might prove beneficial in the
core when the LFA FRR coverage is judged too small and/or the
designer feels unable to optimize the topology to improve the LFA
coverage.
6. Simplicity and Other LFA Benefits
The LFA solution provides significant benefits that mainly stem from
its simplicity.
Behavior of LFAs is an automated process that makes fast restoration
an intrinsic part of the IGP, with no additional configuration burden
in the IGP or any other protocol.
Thanks to this integration, the use of multiple areas in the IGP does
not make fast restoration more complex to achieve than in a single
area IGP design.
There is no requirement for network-wide upgrade, as LFAs do not
require any protocol change and hence can be deployed router by
router.
With LFAs, the backup paths are pre-computed and installed in the
data plane in advance of the failure. Assuming a fast enough FIB
update time compared to the total number of (important) destinations,
a "<50-msec repair" requirement becomes achievable. With a prefix-
independent implementation, LFAs have a fixed repair time, as the
repair time depends on the failure detection time and the time
required to activate the behavior of an LFA, which does not scale
with the number of destinations to be fast-rerouted.
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Link and node protection are provided together and without any
operational differences. (As a comparison, MPLS TE FRR link and node
protections require different types of backup tunnels and different
grades of operational complexity.)
Also, compared to MPLS TE FRR, an important simplicity aspect of the
LFA solution is that it does not require the introduction of yet
another virtual layer of topology. Maintaining a virtual topology of
explicit MPLS TE tunnels clearly increases the complexity of the
network. MPLS TE tunnels would have to be represented in a network
management system in order to be monitored and managed. In large
networks, this may significantly contribute to the number of network
entities polled by the network management system and monitored by
operational staff. An LFA, on the other hand, only has to be
monitored for its operational status once per router, and it needs to
be considered in the network-planning process. If the latter is done
based on offline simulations for failure cases anyway, the
incremental cost of supporting LFAs for a defined set of demands may
be relatively low.
The per-prefix mode of LFAs allows for simpler and more efficient
capacity planning. As the backup path of each destination is
optimized individually, the load to be fast-rerouted can be spread on
a set of shortest repair paths (as opposed to a single backup
tunnel). This leads to a simpler and more efficient capacity-
planning process that takes congestion during protection into
account.
7. Capacity Planning with LFA in Mind
We briefly describe the functionality a designer should expect from a
capacity-planning tool that supports LFAs, and the related capacity-
planning process.
7.1. Coverage Estimation - Default Topology
Per-Link LFA Coverage Estimation: The tool would color each
unidirectional link in, depending on whether or not per-link LFAs are
available.
Per-Prefix LFA Coverage Estimation: The tool would color each
unidirectional link with a colored gradient, based on the percent of
destinations that have a per-prefix LFA.
In addition to the visual GUI reporting, the tool should provide
detailed tables that list, on a per-interface basis, the percentage
of LFAs, the number of prefixes with LFAs, the number of prefixes
without LFAs, and a list of those prefixes without LFAs.
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Furthermore, the tool should list and provide percentages for the
traffic matrix demands with less than 100% source-to-destination LFA
coverage, as well as average coverage (number of links on which a
demand has an LFA/number of links traversed by this demand) for every
demand (using a threshold).
The user should be able to alter the color scheme to show whether
these LFAs are guaranteed node-protecting or de facto node-
protecting, or only link-protecting.
This functionality provides the same level of information as we
described in Sections 4.1 to 4.3.
7.2. Coverage Estimation in Relation to Traffic
Instead of reporting the coverage as a ratio of the number of
destinations with a backup, one might prefer a ratio of the amount of
traffic on a link that benefits from protection.
This is likely much more relevant, as not all destinations are equal,
and it is much more important to have an LFA for a destination
attracting lots of traffic rather than an unpopular destination.
7.3. Coverage Verification for a Given Set of Demands
Depending on the requirements on the network, it might be more
relevant to verify the complete LFA coverage of a given sub-topology,
or a given set of demands, rather than to calculate the relative
coverage of the overall traffic. This is most likely true for the
third engineering profile described in Section 4.
In that case, the tool should be able to separately report the LFA
coverage on a given set of demands and highlight each part of the
network that does not support 100% coverage for any of those demands.
7.4. Modeling - What-If Scenarios - Coverage Impact
The tool should be able to compute the coverage for all of the
possible topologies that result from a set of expected failures
(i.e., one-SRLG failure).
Filtering the key information from the huge amount of generated data
should be a key property of the tool.
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For example, the user could set a threshold (at least 80% per-prefix
LFA coverage in all one-SRLG what-if scenarios), and the tool would
report only the cases where this condition is not met, hopefully with
some assistance on how to remedy the problem (IGP metric
optimization).
As an application example, a designer who is not able to ensure that
c < a could leverage such a tool to assess the per-prefix LFA
coverage for square aggregation topologies grafted to the backbone of
his network. The tool would analyze the per-prefix LFA availability
for each remote destination and would help optimize the backbone
topology to increase the LFA protection coverage for failures within
the square aggregation topologies.
7.5. Modeling - What-If Scenarios - Load Impact
The tool should be able to compute the link load for all routing
states that result from a set of expected failures (i.e., one-SRLG
failure).
The routing states that should be supported are 1) network-wide
converged state before the failure, 2) state in which all of the LFAs
protecting the failure are active, and 3) network-wide converged
state after the failure.
Filtering the key information from the huge amount of generated data
should be a key property of the tool.
For example, the user could set a threshold (at most 100% link load
in all one-SRLG what-if scenarios), and the tool would report only
the cases where this condition is violated, hopefully with some
assistance on how to remedy the problem (IGP metric optimization).
The tool should be able to do this for the aggregate load, and on a
per-class-of-service basis as well.
Note: In cases where the traffic matrix is unknown, an
intermediate solution consists of identifying the destinations
that would attract traffic (i.e., Provider Edge (PE) routers), and
those that would not (i.e., Provider (P) routers). One could
achieve this by creating a traffic matrix with equal demands
between the sources/destinations that would attract traffic (PE to
PE). This will be more relevant than considering all demands
between all prefixes (e.g., when there is no customer traffic from
P to P).
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7.6. Discussion on Metric Recommendations
While LFA FRR has many benefits (Section 6), LFA FRR's applicability
depends on topology.
The purpose of this document is to show how to introduce a level of
control over this topology parameter.
On the one hand, we wanted to show that by adopting a small set of
IGP metric constraints and a repetition of well-behaved patterns, the
designer could deterministically guarantee maximum link and node
protection for the vast majority of the network (the access/
aggregation). By doing so, he would obtain an extremely simple
resiliency solution.
On the other hand, we also wanted to show that it might not be so bad
to not apply (all of) these constraints.
Indeed, we explained in Section 3.3.4.3 that the per-prefix LFA
coverage in a square where c >= a might still be very good, depending
on the backbone topology.
We showed in Section 4.3 that the median per-prefix LFA coverage for
11 SP backbone topologies still provides 94% coverage. (Most of
these topologies were built without any idea of LFA!)
Furthermore, we showed that any topology may be analyzed with an LFA-
aware capacity-planning tool. This would readily assess the coverage
of per-prefix LFAs and would assist the designer in fine-tuning it to
obtain the level of protection he seeks.
While this document highlights LFA applicability and benefits for SP
networks, it also notes that LFAs are not meant to replace MPLS
TE FRR.
With a very LFA-unfriendly topology, a designer seeking guaranteed
<50-msec protection might be better off leveraging the explicit-
routed backup capability of MPLS TE FRR to provide 100% protection
while ensuring no congestion along the backup paths during
protection.
But when LFAs provide 100% link and node protection without any
uLoop, then clearly the LFA method seems a technology to consider to
drastically simplify the operation of a large-scale network.
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8. Security Considerations
The security considerations applicable to LFAs are described in
[RFC5286]. This document does not introduce any new security
considerations.
9. Conclusions
The LFA method is an important protection alternative for IP/MPLS
networks.
Its simplicity benefit is significant, in terms of automation and
integration with the default IGP behavior and the absence of any
requirement for network-wide upgrade. The technology does not
require any protocol change and hence can be deployed router by
router.
At first sight, these significant simplicity benefits are negated by
the topological dependency of its applicability.
The purpose of this document is to highlight that very frequent
access and aggregation topologies benefit from excellent link and
node LFA coverage.
A second objective consists of describing the three different
profiles of LFA applicability for the IP/MPLS core networks and
illustrating them with simulation results based on real SP core
topologies.
10. Acknowledgments
We would like to thank Alvaro Retana and especially Stewart Bryant
for their valuable comments on this work.
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11. References
11.1. Normative References
[RFC5286] Atlas, A., Ed., and A. Zinin, Ed., "Basic Specification
for IP Fast Reroute: Loop-Free Alternates", RFC 5286,
September 2008.
11.2. Informative References
[RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework",
RFC 5714, January 2010.
[RFC5715] Shand, M. and S. Bryant, "A Framework for Loop-Free
Convergence", RFC 5715, January 2010.
[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, December 1990.
[IS-IS] ISO/IEC 10589:2002, Second Edition, "Intermediate System
to Intermediate System Intra-Domain Routeing Exchange
Protocol for use in Conjunction with the Protocol for
Providing the Connectionless-mode Network Service
(ISO 8473)", 2002.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, July 2008.
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Authors' Addresses
Clarence Filsfils (editor)
Cisco Systems
Brussels 1000
BE
EMail: cf@cisco.com
Pierre Francois (editor)
Institute IMDEA Networks
Avda. del Mar Mediterraneo, 22
Leganese 28918
ES
EMail: pierre.francois@imdea.org
Mike Shand
EMail: imc.shand@googlemail.com
Bruno Decraene
France Telecom
38-40 rue du General Leclerc
92794 Issy Moulineaux cedex 9
FR
EMail: bruno.decraene@orange.com
James Uttaro
AT&T
200 S. Laurel Avenue
Middletown, NJ 07748
US
EMail: uttaro@att.com
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Nicolai Leymann
Deutsche Telekom
Winterfeldtstrasse 21
10781, Berlin
DE
EMail: N.Leymann@telekom.de
Martin Horneffer
Deutsche Telekom
Hammer Str. 216-226
48153, Muenster
DE
EMail: Martin.Horneffer@telekom.de
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