Routing Area Working Group S. Litkowski
Internet-Draft B. Decraene
Intended status: Standards Track Orange
Expires: September 5, 2015 C. Filsfils
K. Raza
Cisco Systems
M. Horneffer
Deutsche Telekom
P. Sarkar
Juniper Networks
March 4, 2015
Operational management of Loop Free Alternates
draft-ietf-rtgwg-lfa-manageability-08
Abstract
Loop Free Alternates (LFA), as defined in RFC 5286 is an IP Fast
ReRoute (IP FRR) mechanism enabling traffic protection for IP traffic
(and MPLS LDP traffic by extension). Following first deployment
experiences, this document provides operational feedback on LFA,
highlights some limitations, and proposes a set of refinements to
address those limitations. It also proposes required management
specifications.
This proposal is also applicable to remote LFA solution.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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This Internet-Draft will expire on September 5, 2015.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Operational issues with default LFA tie breakers . . . . . . 4
3.1. Case 1: PE router protecting failures within core network 4
3.2. Case 2: PE router choosen to protect core failures while
P router LFA exists . . . . . . . . . . . . . . . . . . . 5
3.3. Case 3: suboptimal P router alternate choice . . . . . . 6
3.4. Case 4: IS-IS overload bit on LFA computing node . . . . 7
4. Need for coverage monitoring . . . . . . . . . . . . . . . . 8
5. Need for LFA activation granularity . . . . . . . . . . . . . 9
6. Configuration requirements . . . . . . . . . . . . . . . . . 9
6.1. LFA enabling/disabling scope . . . . . . . . . . . . . . 9
6.2. Policy based LFA selection . . . . . . . . . . . . . . . 10
6.2.1. Connected vs remote alternates . . . . . . . . . . . 11
6.2.2. Mandatory criteria . . . . . . . . . . . . . . . . . 11
6.2.3. Enhanced criteria . . . . . . . . . . . . . . . . . . 12
6.2.4. Retrieving alternate path attributes . . . . . . . . 12
6.2.5. ECMP LFAs . . . . . . . . . . . . . . . . . . . . . . 14
6.2.6. SRLG . . . . . . . . . . . . . . . . . . . . . . . . 15
6.2.7. Link coloring . . . . . . . . . . . . . . . . . . . . 16
6.2.8. Bandwidth . . . . . . . . . . . . . . . . . . . . . . 17
6.2.9. Alternate preference/Node coloring . . . . . . . . . 18
7. Operational aspects . . . . . . . . . . . . . . . . . . . . . 19
7.1. IS-IS overload bit on LFA computing node . . . . . . . . 19
7.2. Manual triggering of FRR . . . . . . . . . . . . . . . . 20
7.3. Required local information . . . . . . . . . . . . . . . 21
7.4. Coverage monitoring . . . . . . . . . . . . . . . . . . . 21
7.5. LFA and network planning . . . . . . . . . . . . . . . . 22
8. Security Considerations . . . . . . . . . . . . . . . . . . . 22
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9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 22
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 23
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
12.1. Normative References . . . . . . . . . . . . . . . . . . 23
12.2. Informative References . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
1. Definitions
o Per-prefix LFA : LFA computation, and best alternate evaluation is
done for each destination prefix. As opposed to "Per-next hop"
simplification also proposed in [RFC5286] Section 3.8.
o PE router : Provider Edge router. These routers are connecting
customers
o P router : Provider router. These routers are core routers,
without customer connections. They provide transit between PE
routers and they form the core network.
o Core network : subset of the network composed by P routers and
links between them.
o Core link : network link part of the core network i.e. a P router
to P router link.
o Link-protecting LFA : alternate providing protection against link
failure.
o Node-protecting LFA : alternate providing protection against node
failure.
o Connected alternate : alternate adjacent (at IGP level) to the
point of local repair (i.e. an IGP neighbor).
o Remote alternate : alternate which is does not share an IGP
adjacency with the point of local repair.
2. Introduction
Following the first deployments of Loop Free Alternates (LFA), this
document provides feedback to the community about the management of
LFA.
Section 3 provides real uses cases illustrating some limitations
and suboptimal behavior.
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Section 5 proposes requirements for activation granularity and
policy based selection of the alternate.
Section 6 express requirements for the operational management of
LFA.
3. Operational issues with default LFA tie breakers
[RFC5286] introduces the notion of tie breakers when selecting the
LFA among multiple candidate alternate next-hops. When multiple LFA
exist, RFC 5286 has favored the selection of the LFA providing the
best coverage of the failure cases. While this is indeed a goal,
this is one among multiple and in some deployment this lead to the
selection of a suboptimal LFA. The following sections details real
use cases of such limitations.
Note that the use case of LFA computation per destination (per-prefix
LFA) is assumed throughout this analysis. We also assume in the
network figures that all IP prefixes are advertised with zero cost.
3.1. Case 1: PE router protecting failures within core network
P1 --------- P2 ---------- P3 --------- P4
| 1 100 1 |
| |
| 100 | 100
| |
| 1 100 1 | 1 5k
P5 --------- P6 ---------- P7 --------- P8 --- P9 -- PE1
| | | | | |
5k| |5k 5k| |5k | 5k | 5k
| | | | | |
| +-- PE4 --+ | +---- PE2 ----+
| | |
+---- PE5 ----+ | 5k
|
PE3
Figure 1
Px routers are P routers using n*10G links. PEs are connected using
links with lower bandwidth.
In figure 1, let us consider the traffic flowing from PE1 to PE4.
The nominal path is P9-P8-P7-P6-PE4. Let us consider the failure of
link P7-P8. For P8, P4 is not an LFA and the only available LFA is
PE2.
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When the core link P8-P7 fails, P8 switches all traffic destined to
PE4/PE5 towards the node PE2. Hence a PE node and PE links are used
to protect the failure of a core link. Typically, PE links have less
capacity than core links and congestion may occur on PE2 links. Note
that although PE2 was not directly affected by the failure, its links
become congested and its traffic will suffer from the congestion.
In summary, in case of P8-P7 link failure, the impact on customer
traffic is:
o From PE2 point of view :
* without LFA: no impact
* with LFA: traffic is partially dropped (but possibly
prioritized by a QoS mechanism). It must be highlighted that
in such situation, traffic not affected by the failure may be
affected by the congestion.
o From P8 point of view:
* without LFA: traffic is totally dropped until convergence
occurs.
* with LFA: traffic is partially dropped (but possibly
prioritized by a QoS mechanism).
Besides the congestion aspects of using an Edge router as an
alternate to protect a core failure, a service provider may consider
this as a bad routing design and would like to prevent it.
3.2. Case 2: PE router choosen to protect core failures while P router
LFA exists
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P1 --------- P2 ------------ P3 -------- P4
| 1 100 | 1 |
| | |
| 100 | 30 | 30
| | |
| 1 50 50 | 10 | 1 5k
P5 --------- P6 --- P10 ---- P7 -------- P8 --- P9 -- PE1
| | | | \ |
5k| |5k 5k| |5k \ 5k | 5k
| | | | \ |
| +-- PE4 --+ | +---- PE2 ----+
| | |
+---- PE5 ----+ | 5k
|
PE3
Figure 2
Px routers are P routers meshed with n*10G links. PEs are meshed
using links with lower bandwidth.
In the figure 2, let us consider the traffic coming from PE1 to PE4.
Nominal path is P9-P8-P7-P10-P6-PE4. Let us consider the failure of
the link P7-P8. For P8, P4 is a link-protecting LFA and PE2 is a
node-protecting LFA. PE2 is chosen as best LFA due to its better
protection type. Just like in case 1, this may lead to congestion on
PE2 links upon LFA activation.
3.3. Case 3: suboptimal P router alternate choice
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+--- PE3 --+
/ \
1000 / \ 1000
/ \
+----- P1 ---------------- P2 ----+
| | 500 | |
| 10 | | | 10
| | | |
R5 | 10 | 10 R7
| | | |
| 10 | | | 10
| | 500 | |
+---- P3 ---------------- P4 -----+
\ /
1000 \ / 1000
\ /
+--- PE1 ---+
Figure 3
Px routers are P routers. P1-P2 and P3-P4 links are 1G links. All
others inter Px links are 10G links.
In the figure above, let us consider the failure of link P1-P3. For
destination PE3, P3 has two possible alternates:
o P4, which is node-protecting
o P5, which is link-protecting
P4 is chosen as best LFA due to its better protection type. However,
it may not be desirable to use P4 for bandwidth capacity reason. A
service provider may prefer to use high bandwidth links as prefered
LFA. In this example, prefering shortest path over protection type
may achieve the expected behavior, but in cases where metric are not
reflecting bandwidth, it would not work and some other criteria would
need to be involved when selecting the best LFA.
3.4. Case 4: IS-IS overload bit on LFA computing node
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P1 P2
| \ / |
50 | 50 \/ 50 | 50
| /\ |
PE1-+ +-- PE2
\ /
45 \ / 45
-PE3-+
(OL set)
Figure 4
In the figure above, PE3 has its overload bit set (permanently, for
design reason) and wants to protect traffic using LFA for destination
PE2.
On PE3, the loop-free condition is not satisfied : 100 !< 45 + 45.
PE1 is thus not considered as an LFA. However thanks to the overload
bit set on PE3, we know that PE1 is loop-free so PE1 is an LFA to
reach PE2.
In case of overload condition set on a node, LFA behavior must be
clarified.
4. Need for coverage monitoring
As per [RFC6571], LFA coverage highly depends on the used network
topology. Even if remote LFA ([I-D.ietf-rtgwg-remote-lfa]) extends
significantly the coverage of the basic LFA specification, there is
still some cases where protection would not be available. As network
topologies are constantly evolving (network extension, capacity
addings, latency optimization ...), the protection coverage may
change. Fast reroute functionality may be critical for some services
supported by the network, a service provider must constantly know
what protection coverage is currently available on the network.
Moreover, predicting the protection coverage in case of network
topology change is mandatory.
Today network simulation tool associated with whatif scenarios
functionality are often used by service providers for the overall
network design (capacity, path optimization ...). Section 7.5,
Section 7.4 and Section 7.3 of this document propose to add LFA
informations into such tool and within routers, so a service provider
may be able :
o to evaluate protection coverage after a topology change.
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o to adjust the topology change to cover the primary need (e.g.
latency optimization or bandwidth increase) as well as LFA
protection.
o monitor constantly the LFA coverage in the live network and being
alerted.
Implementers SHOULD document their LFA selection algorithms (default
and tuning options) in order to leave possibility for 3rd party
modules to model these policy-LFA expressions.
5. Need for LFA activation granularity
As all FRR mechanism, LFA installs backup paths in Forwarding
Information Base (FIB). Depending of the hardware used by a service
provider, FIB resource may be critical. Activating LFA, by default,
on all available components (IGP topologies, interface, address
families ...) may lead to waste of FIB resource as generally in a
network only few destinations should be protected (e.g. loopback
addresses supporting MPLS services) compared to the amount of
destinations in RIB.
Moreover a service provider may implement multiple different FRR
mechanism in its networks for different usages (MRT, TE FRR). In
this scenario, an implementation MAY permit to compute alternates for
a specific destination even if the destination is already protected
by another mechanism. This will bring redundancy and let the ability
for the operator to select the best option for FRR using a policy
langage.
Section 6 of this document propose some implementation guidelines.
6. Configuration requirements
Controlling best alternate and LFA activation granularity is a
requirement for Service Providers. This section defines
configuration requirements for LFA.
6.1. LFA enabling/disabling scope
The granularity of LFA activation should be controlled (as alternate
next hop consume memory in forwarding plane).
An implementation of LFA SHOULD allow its activation with the
following criteria:
o Per routing context: VRF, virtual/logical router, global routing
table, ...
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o Per interface
o Per protocol instance, topology, area
o Per prefixes: prefix protection SHOULD have a better priority
compared to interface protection. This means that if a specific
prefix must be protected due to a configuration request, LFA must
be computed and installed for this prefix even if the primary
outgoing interface is not configured for protection.
An implementation of LFA MAY allow its activation with the following
criteria:
o Per address-family: ipv4 unicast, ipv6 unicast
o Per MPLS control plane: for MPLS control planes that inherit
routing decision from the IGP routing protocol, MPLS dataplane may
be protected by LFA. The implementation may allow operator to
control this inheritance of protection from the IP prefix to the
MPLS label bound to this prefix. The protection inheritance will
concern : IP to MPLS, MPLS to MPLS, and MPLS to IP entries. As
example, LDP and segment-routing extensions for ISIS and OSPF are
control plane eligible to this inheritance of protection.
6.2. Policy based LFA selection
When multiple alternates exist, LFA selection algorithm is based on
tie breakers. Current tie breakers do not provide sufficient control
on how the best alternate is chosen. This document proposes an
enhanced tie breaker allowing service providers to manage all
specific cases:
1. An implementation of LFA SHOULD support policy-based decision for
determining the best LFA.
2. Policy based decision SHOULD be based on multiple criterions,
with each criteria having a level of preference.
3. If the defined policy does not permit to determine a unique best
LFA, an implementation SHOULD pick only one based on its own
decision, as a default behavior. An implementation SHOULD also
support election of multiple LFAs, for loadbalancing purposes.
4. Policy SHOULD be applicable to a protected interface or to a
specific set of destinations. In case of application on the
protected interface, all destinations primarily routed on this
interface SHOULD use the interface policy.
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5. It is an implementation choice to reevaluate policy dynamically
or not (in case of policy change). If a dynamic approach is
chosen, the implementation SHOULD recompute the best LFAs and
reinstall them in FIB, without service disruption. If a non-
dynamic approach is chosen, the policy would be taken into
account upon the next IGP event. In this case, the
implementation SHOULD support a command to manually force the
recomputation/reinstallation of LFAs.
6.2.1. Connected vs remote alternates
In addition to connected LFAs, tunnels (e.g. IP, LDP, RSVP-TE or
Segment Routing) to distant routers may be used to complement LFA
coverage (tunnel tail used as virtual neighbor). When a router has
multiple alternate candidates for a specific destination, it may have
connected alternates and remote alternates (reachable via a tunnel).
Connected alternates may not always provide an optimal routing path
and it may be preferable to select a remote alternate over a
connected alternate. Some usage of tunnels to extend LFA ([RFC5286])
coverage is described in either [I-D.ietf-rtgwg-remote-lfa] or
[I-D.francois-segment-routing-ti-lfa]. These documents present some
use cases of LDP tunnels ([I-D.ietf-rtgwg-remote-lfa]) or Segment
Routing tunnels ([I-D.francois-segment-routing-ti-lfa]). This
document considers any type of tunneling techniques to reach remote
alternates (IP, GRE, LDP, RSVP-TE, L2TP, Segment Routing ...) and
does not restrict the remote alternates to the usage presented in the
referenced document.
In figure 1, there is no P router alternate for P8 to reach PE4 or
PE5 , so P8 is using PE2 as alternate, which may generate congestion
when FRR is activated. Instead, we could have a remote alternate for
P8 to protect traffic to PE4 and PE5. For example, a tunnel from P8
to P3 (following shortest path) can be setup and P8 would be able to
use P3 as remote alternate to protect traffic to PE4 and PE5. In
this scenario, traffic will not use a PE link during FRR activation.
When selecting the best alternate, the selection algorithm MUST
consider all available alternates (connected or tunnel). Especially,
computation of PQ set ([I-D.ietf-rtgwg-remote-lfa]) SHOULD be
performed before best alternate selection.
6.2.2. Mandatory criteria
An implementation of LFA MUST support the following criteria:
o Non candidate link: A link marked as "non candidate" will never be
used as LFA.
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o A primary next hop being protected by another primary next hop of
the same prefix (ECMP case).
o Type of protection provided by the alternate: link protection,
node protection. In case of node protection preference, an
implementation SHOULD support fall back to link protection if node
protection is not available.
o Shortest path: lowest IGP metric used to reach the destination.
o SRLG (as defined in [RFC5286] Section 3, see also Section 6.2.6
for more details).
6.2.3. Enhanced criteria
An implementation of LFA SHOULD support the following enhanced
criteria:
o Downstreamness of an alternate : preference of a downstream path
over a non downstream path SHOULD be configurable.
o Link coloring with : include, exclude and preference based system
(see Section 6.2.7).
o Link Bandwidth (see Section 6.2.8).
o Alternate preference/Node coloring (see Section 6.2.9).
6.2.4. Retrieving alternate path attributes
The policy to select the best alternate evaluate multiple criterions
(e.g. metric, SRLG, link colors ...) which first need to be computed
for each alternate.. In order to compare the different alternate
path, a router must retrieve the attributes of each alternate path.
The alternate path is composed of two distinct parts : PLR to
alternate and alternate to destination.
6.2.4.1. Connected alternate
For alternate path using a connected alternate :
o attributes from PLR to alternate path are retrieved from the
interface connected to the alternate.
o attributes from alternate to destination path are retrieved from
SPF rooted at the alternate. As the alternate is a connected
alternate, the SPF has already been computed to find the
alternate, so there is no need of additional computation.
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6.2.4.2. Remote alternate
For alternate path using a remote alternate (tunnel) :
o attributes from the PLR to alternate path are retrieved using the
PLR's primary SPF if P space is used or using the neighbor's SPF
if extended P space is used, combined with the attributes of the
link(s) to reach that neighbor. In both cases, no additional SPF
is required.
o attributes from alternate to destination path may be retrieved
from SPF rooted at the remote alternate. An additional forward
SPF is required for each remote alternate as indicated in
[I-D.ietf-rtgwg-rlfa-node-protection] section 3.2.. . In some
remote alternate scenarios, like
[I-D.francois-segment-routing-ti-lfa], alternate to destination
path attributes may be obtained using a different technique.
The number of remote alternates may be very high. In case of remote
LFA, simulations of real-world network topologies, reveal that order
of hundreths of PQ ...
To handle this situation, it is needed to limit the number of remote
alternates to be evaluated to a finite number before collecting
alternate path attributes and running the policy evaluation. [I-
D.ietf-rtgwg-rlfa-node-protection] Section 2.3.3 provides a way to
reduce the number of PQ to be evaluated.
Some other remote alternate techniques using static or dynamic
tunnels may not require this pruning.
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Link Remote Remote
alternate alternate alternate
------------- ------------------ -------------
Alternates | LFA | | rLFA (PQs) | | Static/ |
| | | | | Dynamic |
sources | | | | | tunnels |
------------- ------------------ -------------
| | |
| | |
| -------------------------- |
| | Prune some alternates | |
| | (sorting strategy) | |
| -------------------------- |
| | |
| | |
------------------------------------------------
| Collect alternate attributes |
------------------------------------------------
|
|
-------------------------
| Evaluate policy |
-------------------------
|
|
Best alternates
6.2.5. ECMP LFAs
10
PE2 - PE3
| |
50 | 5 | 50
P1----P2
\\ //
50 \\ // 50
PE1
Figure 5
Links between P1 and PE1 are L1 and L2, links between P2 and PE1 are
L3 and L4
In the figure above, primary path from PE1 to PE2 is through P1 using
ECMP on two parallel links L1 and L2. In case of standard ECMP
behavior, if L1 is failing, postconvergence next hop would become L2
and there would be no longer ECMP. If LFA is activated, as stated in
[RFC5286] Section 3.4., "alternate next-hops may themselves also be
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primary next-hops, but need not be" and "alternate next-hops should
maximize the coverage of the failure cases". In this scenario there
is no alternate providing node protection, LFA will so prefer L2 as
alternate to protect L1 which makes sense compared to postconvergence
behavior.
Considering a different scenario using figure 5, where L1 and L2 are
configured as a layer 3 bundle using a local feature, as well as L3/
L4 being a second layer 3 bundle. Layer 3 bundles are configured as
if a link in the bundle is failing, the traffic must be rerouted out
of the bundle. Layer 3 bundles are generally introduced to increase
bandwidth between nodes. In nominal situation, ECMP is still
available from PE1 to PE2, but if L1 is failing, postconvergence next
hop would become ECMP on L3 and L4. In this case, LFA behavior
SHOULD be adapted in order to reflect the bandwidth requirement.
We would expect the following FIB entry on PE1 :
On PE1 : PE2 +--> ECMP -> L1
| |
| +----> L2
|
+--> LFA(ECMP) -> L3
|
+---------> L4
If L1 or L2 is failing, traffic must be switched on the LFA ECMP
bundle rather than using the other primary next hop.
As mentioned in [RFC5286] Section 3.4., protecting a link within an
ECMP by another primary next hop is not a MUST. Moreover, we already
presented in this document, that maximizing the coverage of the
failure case may not be the right approach and policy based choice of
alternate may be preferred.
An implementation SHOULD permit to prefer to protect a primary next
hop by another primary next hop. An implementation SHOULD permit to
prefer to protect a primary next hop by a NON primary next hop. An
implementation SHOULD permit to use an ECMP bundle as a LFA.
6.2.6. SRLG
[RFC5286] Section 3. proposes to reuse GMPLS IGP extensions to encode
SRLGs ([RFC4205] and [RFC4203]). The section is also describing the
algorithm to compute SRLG protection.
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When SRLG protection is computed, and implementation SHOULD permit to
:
o Exclude alternates violating SRLG.
o Maintain a preference system between alternates based on number of
SRLG violations : more violations = less preference.
When applying SRLG criteria, the SRLG violation check SHOULD be
performed on source to alternate as well as alternate to destination
paths based on the SRLG set of the primary path. In the case of
remote LFA, PQ to destination path attributes would be retrieved from
SPT rooted at PQ.
6.2.7. Link coloring
Link coloring is a powerful system to control the choice of
alternates. Protecting interfaces are tagged with colors. Protected
interfaces are configured to include some colors with a preference
level, and exclude others.
Link color information SHOULD be signalled in the IGP. How
signalling is done is out of scope of the document but it may be
useful to reuse existing admin-groups from traffic-engineering
extensions.
PE2
| +---- P4
| /
PE1 ---- P1 --------- P2
| 10Gb
1Gb |
|
P3
Figure 5
Example : P1 router is connected to three P routers and two PEs.
P1 is configured to protect the P1-P4 link. We assume that given the
topology, all neighbors are candidate LFA. We would like to enforce
a policy in the network where only a core router may protect against
the failure of a core link, and where high capacity links are
prefered.
In this example, we can use the proposed link coloring by:
o Marking PEs links with color RED
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o Marking 10Gb CORE link with color BLUE
o Marking 1Gb CORE link with color YELLOW
o Configured the protected interface P1->P4 with :
* Include BLUE, preference 200
* Include YELLOW, preference 100
* Exclude RED
Using this, PE links will never be used to protect against P1-P4 link
failure and 10Gb link will be be preferred.
The main advantage of this solution is that it can easily be
duplicated on other interfaces and other nodes without change. A
Service Provider has only to define the color system (associate color
with a significance), as it is done already for TE affinities or BGP
communities.
An implementation of link coloring:
o SHOULD support multiple include and exclude colors on a single
protected interface.
o SHOULD provide a level of preference between included colors.
o SHOULD support multiple colors configuration on a single
protecting interface.
6.2.8. Bandwidth
As mentionned in previous sections, not taking into account bandwidth
of an alternate could lead to congestion during FRR activation. We
propose to base the bandwidth criteria on the link speed information
for the following reason :
o if a router S has a set of X destinations primarly forwarded to N,
using per prefix LFA may lead to have a subset of X protected by a
neighbor N1, another subset by N2, another subset by Nx ...
o S is not aware about traffic flows to each destination and is not
able to evaluate how much traffic will be sent to N1,N2, ... Nx in
case of FRR activation.
Based on this, it is not useful to gather available bandwidth on
alternate paths, as the router does not know how much bandwidth it
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requires for protection. The proposed link speed approach provides a
good approximation with a small cost as information is easily
available.
The bandwidth criteria of the policy framework SHOULD work in two
ways :
o PRUNE : exclude a LFA if link speed to reach it is lower than the
link speed of the primary next hop interface.
o PREFER : prefer a LFA based on his bandwidth to reach it compared
to the link speed of the primary next hop interface.
6.2.9. Alternate preference/Node coloring
Rather than tagging interface on each node (using link color) to
identify alternate node type (as example), it would be helpful if
routers could be identified in the IGP. This would permit a grouped
processing on multiple nodes. As an implementation need to exclude
some specific alternates (see Section 6.2.3), an implementation :
o SHOULD be able to give a preference to specific alternate.
o SHOULD be able to give a preference to a group of alternate.
o SHOULD be able to exclude a group of alternate.
A specific alternate may be identified by its interface, IP address
or router ID and group of alternates may be identified by a marker
(tag) (for example, in case of IS-IS protocol
[I-D.ietf-isis-node-admin-tag] can be used). Using a tag is referred
as Node coloring in comparison to link coloring option presented in
Section 6.2.7.
Consider the following network:
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PE3
|
|
PE2
| +---- P4
| /
PE1 ---- P1 -------- P2
| 10Gb
1Gb |
|
P3
Figure 6
In the example above, each node is configured with a specific tag
flooded through the IGP.
o PE1,PE3: 200 (non candidate).
o PE2: 100 (edge/core).
o P1,P2,P3: 50 (core).
A simple policy could be configured on P1 to choose the best
alternate for P1->P4 based on router function/role as follows :
o criteria 1 -> alternate preference: exclude tag 100 and 200.
o criteria 2 -> bandwidth.
7. Operational aspects
7.1. IS-IS overload bit on LFA computing node
In [RFC5286], Section 3.5, the setting of the overload bit condition
in LFA computation is only taken into account for the case where a
neighbor has the overload bit set.
In addition to RFC 5286 inequality 1 Loop-Free Criterion
(Distance_opt(N, D) < Distance_opt(N, S) + Distance_opt(S, D)), the
IS-IS overload bit of the LFA calculating neighbor (S) SHOULD be
taken into account. Indeed, if it has the overload bit set, no
neighbor will loop back to traffic to itself.
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7.2. Manual triggering of FRR
Service providers often perform manual link shutdown (using router
CLI) to perform some network changes/tests. A manual link shutdown
may be done at multiple level : physical interface, logical
interface, IGP interface, BFD session ... Especially testing or
troubleshooting FRR requires to perform the manual shutdown on the
remote end of the link as generally a local shutdown would not
trigger FRR.
To enhance such situation, an implementation SHOULD support
triggering/activating LFA Fast Reroute for a given link when a manual
shutdown is done on a component that currently supports FRR
activation.
An implementation MAY also support FRR activation for a specific
interface or a specific prefix on a primary next-hop interface and
revert without any action on any running component of the node (links
or protocols). In this use case, the FRR activation time need to be
controlled by a timer in case the operator forgot to revert traffic
on primary path. When the timer expires, the traffic is
automatically reverted to the primary path. This will make easier
tests of fast-reroute path and then revert back to the primary path
without causing a global network convergence.
For example :
o if an implementation supports FRR activation upon BFD session down
event, this implementation SHOULD support FRR activation when a
manual shutdown is done on the BFD session. But if an
implementation does not support FRR activation on BFD session
down, there is no need for this implementation to support FRR
activation on manual shutdown of BFD session.
o if an implementation supports FRR activation on physical link down
event (e.g. Rx laser Off detection, or error threshold raised
...), this implementation SHOULD support FRR activation when a
manual shutdown at physical interface is done. But if an
implementation does not support FRR activation on physical link
down event, there is no need for this implementation to support
FRR activation on manual physical link shutdown.
o A CLI command may permit to switch from primary path to FRR path
for testing FRR path for a specific. There is no impact on
controlplane, only dataplane of the local node could be changed.
A similar command may permit to switch back traffic from FRR path
to primary path.
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7.3. Required local information
LFA introduction requires some enhancement in standard routing
information provided by implementations. Moreover, due to the non
100% coverage, coverage informations is also required.
Hence an implementation :
o MUST be able to display, for every prefixes, the primary next hop
as well as the alternate next hop information.
o MUST provide coverage information per activation domain of LFA
(area, level, topology, instance, virtual router, address family
...).
o MUST provide number of protected prefixes as well as non protected
prefixes globally.
o SHOULD provide number of protected prefixes as well as non
protected prefixes per link.
o MAY provide number of protected prefixes as well as non protected
prefixes per priority if implementation supports prefix-priority
insertion in RIB/FIB.
o SHOULD provide a reason for choosing an alternate (policy and
criteria) and for excluding an alternate.
o SHOULD provide the list of non protected prefixes and the reason
why they are not protected (no protection required or no alternate
available).
7.4. Coverage monitoring
It is pretty easy to evaluate the coverage of a network in a nominal
situation, but topology changes may change the coverage. In some
situations, the network may no longer be able to provide the required
level of protection. Hence, it becomes very important for service
providers to get alerted about changes of coverage.
An implementation SHOULD :
o provide an alert system if total coverage (for a node) is below a
defined threshold or comes back to a normal situation.
o provide an alert system if coverage of a specific link is below a
defined threshold or comes back to a normal situation.
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An implementation MAY :
o provide an alert system if a specific destination is not protected
anymore or when protection comes back up for this destination
Although the procedures for providing alerts are beyond the scope of
this document, we recommend that implementations consider standard
and well used mechanisms like syslog or SNMP traps.
7.5. LFA and network planning
The operator may choose to run simulations in order to ensure full
coverage of a certain type for the whole network or a given subset of
the network. This is particularly likely if he operates the network
in the sense of the third backbone profiles described in [RFC6571],
that is, he seeks to design and engineer the network topology in a
way that a certain coverage is always achieved. Obviously a complete
and exact simulation of the IP FRR coverage can only be achieved, if
the behavior is deterministic and if the algorithm used is available
to the simulation tool. Thus, an implementation SHOULD:
o Behave deterministic in its selection LFA process. I.e. in the
same topology and with the same policy configuration, the
implementation MUST always choose the same alternate for a given
prefix.
o Document its behavior. The implementation SHOULD provide enough
documentation of its behavior that allows an implementer of a
simulation tool, to foresee the exact choice of the LFA
implementation for every prefix in a given topology. This SHOULD
take into account all possible policy configuration options. One
possible way to document this behavior is to disclose the
algorithm used to choose alternates.
8. Security Considerations
This document does not introduce any change in security consideration
compared to [RFC5286].
9. Contributors
Significant contributions were made by Pierre Francois, Hannes
Gredler, Chris Bowers, Jeff Tantsura, Uma Chunduri and Mustapha
Aissaoui which the authors would like to acknowledge.
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10. Acknowledgements
11. IANA Considerations
This document has no action for IANA.
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[RFC5286] Atlas, A. and A. Zinin, "Basic Specification for IP Fast
Reroute: Loop-Free Alternates", RFC 5286, September 2008.
[RFC5307] Kompella, K. and Y. Rekhter, "IS-IS Extensions in Support
of Generalized Multi-Protocol Label Switching (GMPLS)",
RFC 5307, October 2008.
[RFC6571] Filsfils, C., Francois, P., Shand, M., Decraene, B.,
Uttaro, J., Leymann, N., and M. Horneffer, "Loop-Free
Alternate (LFA) Applicability in Service Provider (SP)
Networks", RFC 6571, June 2012.
12.2. Informative References
[I-D.francois-segment-routing-ti-lfa]
Francois, P., Filsfils, C., Bashandy, A., and B. Decraene,
"Topology Independent Fast Reroute using Segment Routing",
draft-francois-segment-routing-ti-lfa-00 (work in
progress), November 2013.
[I-D.ietf-isis-node-admin-tag]
Sarkar, P., Gredler, H., Hegde, S., Litkowski, S.,
Decraene, B., Li, Z., Aries, E., Rodriguez, R., and H.
Raghuveer, "Advertising Per-node Admin Tags in IS-IS",
draft-ietf-isis-node-admin-tag-00 (work in progress),
December 2014.
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[I-D.ietf-rtgwg-remote-lfa]
Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
So, "Remote Loop-Free Alternate (LFA) Fast Re-Route
(FRR)", draft-ietf-rtgwg-remote-lfa-11 (work in progress),
January 2015.
[I-D.ietf-rtgwg-rlfa-node-protection]
Sarkar, P., Gredler, H., Hegde, S., Bowers, C., Litkowski,
S., and H. Raghuveer, "Remote-LFA Node Protection and
Manageability", draft-ietf-rtgwg-rlfa-node-protection-01
(work in progress), December 2014.
Authors' Addresses
Stephane Litkowski
Orange
Email: stephane.litkowski@orange.com
Bruno Decraene
Orange
Email: bruno.decraene@orange.com
Clarence Filsfils
Cisco Systems
Email: cfilsfil@cisco.com
Kamran Raza
Cisco Systems
Email: skraza@cisco.com
Martin Horneffer
Deutsche Telekom
Email: Martin.Horneffer@telekom.de
Pushpasis Sarkar
Juniper Networks
Email: psarkar@juniper.net
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