Segment Routing Configuration Guide for Cisco 8000 Series Routers, IOS XR Release 7.5.x
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The Segment Routing architecture associates prefix-SIDs to an algorithm that defines how the path is computed. This feature
allows for user-defined algorithms where the IGP computes paths based on a combination of metric type and constraint. An operator
can assign custom SR prefix-SIDs to realize forwarding beyond link-cost-based SPF. As a result, this feature provides a traffic-engineered
path computed automatically by the IGP to any destination reachable by the IGP.
This release supports the following functionality:
Segment Routing Flexible Algorithm allows operators to customize IGP shortest path computation according to their own needs.
An operator can assign custom SR prefix-SIDs to realize forwarding beyond link-cost-based SPF. As a result, Flexible Algorithm
provides a traffic engineered path automatically computed by the IGP to any destination reachable by the IGP.
The SR architecture associates prefix-SIDs to an algorithm which defines how the path is computed. Flexible Algorithm allows
for user-defined algorithms where the IGP computes paths based on a user-defined combination of metric type and constraint.
This document describes the IS-IS and OSPF extensions to support Segment Routing Flexible Algorithm on an MPLS data-plane.
Prerequisites for Flexible Algorithm
Segment routing must be enabled on the router before the Flexible Algorithm functionality is activated.
Building Blocks of Segment Routing Flexible Algorithm
This section describes the building blocks that are required to support the SR Flexible Algorithm functionality in IS-IS and OSPF.
Flexible Algorithm Definition
Many possible constrains may be used to compute a path over a network. Some networks are deployed with multiple planes. A
simple form of constrain may be to use a particular plane. A more sophisticated form of constrain can include some extended
metric, like delay, as described in [RFC7810]. Even more advanced case could be to restrict the path and avoid links with
certain affinities. Combinations of these are also possible. To provide a maximum flexibility, the mapping between the algorithm
value and its meaning can be defined by the user. When all the routers in the domain have the common understanding what the
particular algorithm value represents, the computation for such algorithm is consistent and the traffic is not subject to
looping. Here, since the meaning of the algorithm is not defined by any standard, but is defined by the user, it is called
as Flexible Algorithm.
Flexible Algorithm Support Advertisement
An algorithm defines how the best path is computed by IGP. Routers advertise the support for the algorithm as a node capability.
Prefix-SIDs are also advertised with an algorithm value and are tightly coupled with the algorithm itself.
An algorithm is a one octet value. Values from 128 to 255 are reserved for user defined values and are used for Flexible Algorithm
representation.
Flexible Algorithm Definition Advertisement
To guarantee the loop free forwarding for paths computed for a particular Flexible Algorithm, all routers in the network must
share the same definition of the Flexible Algorithm. This is achieved by dedicated router(s) advertising the definition of
each Flexible Algorithm. Such advertisement is associated with the priority to make sure that all routers will agree on a
single and consistent definition for each Flexible Algorithm.
Definition of Flexible Algorithm includes:
Metric type
Affinity constraints
To enable the router to advertise the definition for the particular Flexible Algorithm, advertise-definition command is used. At least one router in the area, preferably two for redundancy, must advertise the Flexible Algorithm definition.
Without the valid definition being advertised, the Flexible Algorithm will not be functional.
Flexible Algorithm Prefix-SID Advertisement
To be able to forward traffic on a Flexible Algorithm specific path, all routers participating in the Flexible Algorithm will
install a MPLS labeled path for the Flexible Algorithm specific SID that is advertised for the prefix. Only prefixes for which
the Flexible Algorithm specific Prefix-SID is advertised is subject to Flexible Algorithm specific forwarding.
Calculation of Flexible Algorithm Path
A router may compute path for multiple Flexible Algorithms. A router must be configured to support particular Flexible Algorithm
before it can compute any path for such Flexible Algorithm. A router must have a valid definition of the Flexible Algorithm
before such Flexible Algorithm is used.
When computing the shortest path tree for particular Flexible Algorithm:
All nodes that do not advertise support for such Flexible Algorithm will be pruned from the topology.
If the Flexible Algorithm definition includes affinities that are excluded, then all links for which any of such affinities
are advertised will be pruned from the topology.
Router uses the metric that is part of the Flexible Algorithm definition. If the metric is not advertised for the particular
link, such link will be pruned from the topology.
IS-IS supports Loop Free Alternate (LFA) paths, TI-LFA backup paths, and Microloop Avoidance paths for particular Flexible
Algorithm. OSPF supports Loop Free Alternate (LFA) and TI-LFA backup paths for particular Flexible Algorithm. These paths
are computed using the same constraints as the calculation of the primary paths for such Flexible Algorithm. These paths use
Prefix-SIDs advertised specifically for such Flexible Algorithm in order to enforce a backup or microloop avoidance path.
Installation of Forwarding Entries for Flexible Algorithm Paths
Flexible Algorithm path to any prefix must be installed in the forwarding using the Prefix-SID that was advertised for such
Flexible Algorithm. If the Prefix-SID for Flexible Algorithm is not known, such Flexible Algorithm path is not installed in
forwarding for such prefix..
Only MPLS to MPLS entries are installed for a Flexible Algorithm path. No IP to IP or IP to MPLS entries are installed. These
follow the native IGP paths computed based on the default algorithm and regular IGP metrics.
Flexible Algorithm Prefix-SID Redistribution
Table 2. Feature History Table
Feature Name
Release Information
Feature Description
Flexible Algorithm Prefix-SID Redistribution for External Route Propagation
Release 7.5.2
You can now propagate flexible algorithm prefix-SIDs and their algorithm-specific metric between different IGP domains, such
as OSPF to IS-IS RIP to OSPF. With this functionality enabling interdomain traffic engineering, you can export flexible algorithm
labels from the OSPF domain to other domains and import the labels from other domains into OSPF.
The show ospf route flex-algo command has been modified to include additional attributes to indicate the external routes.
Previously, prefix redistribution from IS-IS to another IS-IS instance or protocol was limited to SR algorithm 0 (regular
SPF) prefix SIDs; SR algorithm 1 (Strict SPF) and SR algorithms 128-255 (Flexible Algorithm) prefix SIDs were not redistributed
along with the prefix. The Segment Routing IS-IS Flexible Algorithm Prefix SID Redistribution feature allows redistribution
of strict and flexible algorithms prefix SIDs from IS-IS to another IS-IS instance or protocols. This feature is enabled automatically
when you configure redistribution of IS-IS Routes with strict or Flexible Algorithm SIDs.
Prefix redistribution from OSPF to another AS was limited to SR algorithm 0 (regular SPF) prefix SIDs; SR algorithm 1 (Strict
SPF) and SR algorithms 128-255 (Flexible Algorithm) prefix SIDs were not redistributed along with the prefix. Starting from
Cisco IOS XR Release 7.5.2, the Flexible Algorithm Prefix-SID Redistribution for External Route Propagation feature allows redistribution of strict
and flexible algorithm prefixes SIDs from OSPF to another AS and also from another AS into OSPF.
Verification
This following show output displays the route-type as 'Extern' for the external routes.
Router#show ospf routes flex-algo 240 route-type external detail
Route Table of ospf-1 with router ID 192.168.0.2 (VRF default)
Algorithm 240
Route entry for 192.168.4.3/32, Metric 220, SID 536, Label 16536
Priority : Medium
Route type : Extern Type 1
Last updated : Apr 25 14:30:12.718
Flags: Inuse
Prefix Contrib Algo 240 SID 536
From 192.168.0.4 Route-type 5
Total Metric : 220 Base metric 20 FAPM 20
Contrib Flags : Inuse, Reachable
SID Flags : PHP off, Index, Global, Valid
Path: 10.1.1.3, from 192.168.0.4, via GigabitEthernet0/2/0/2
Out Label : 16536
Weight : 0
Area : 0
Path: 10.1.2.3, from 192.168.0.4, via GigabitEthernet0/2/0/3
Out Label : 16536
Weight : 0
Area : 0
Path: 10.2.1.5, from 192.168.0.4, via GigabitEthernet0/2/0/4
Out Label : 16536
Weight : 0
Area : 0
Route entry for 192.168.4.5/32, Metric 120, SID 556, Label 16556
Priority : Medium
Route type : Extern Type 1
Last updated : Apr 25 14:30:12.724
Flags: Inuse
Prefix Contrib Algo 240 SID 556
From 192.168.0.3 Route-type 5
Total Metric : 120 Base metric 1 FAPM 20
Contrib Flags : Inuse, Reachable
SID Flags : PHP off, Index, Global, Valid
Path: 10.1.1.3, from 192.168.0.3, via GigabitEthernet0/2/0/2
Out Label : 16556
Weight : 0
Area : 0
Path: 10.1.2.3, from 192.168.0.3, via GigabitEthernet0/2/0/3
Out Label : 16556
Weight : 0
Area : 0
The following show output displays label information for flexible algorithm and its corresponding metric as added in RIB:
RP/0/RP0/CPU0:ios# show route 192.168.0.2/32 detail
Wed Apr 6 16:24:46.021 IST
Routing entry for 192.168.0.2/32
Known via "ospf 1", distance 110, metric 2, labeled SR, type intra area
Installed Apr 6 15:51:57.973 for 00:32:48
Routing Descriptor Blocks
10.10.10.2, from 192.168.0.2, via GigabitEthernet0/2/0/0, Protected
Route metric is 2
Label: 0x3 (3)
Tunnel ID: None
Binding Label: None
Extended communities count: 0
Path id:1 Path ref count:0
NHID:0x1(Ref:1)
Backup path id:65
OSPF area: 1
10.11.11.2, from 192.168.0.2, via GigabitEthernet0/2/0/1, Backup (Local-LFA)
Route metric is 6
Label: 0x3 (3)
Tunnel ID: None
Binding Label: None
Extended communities count: 0
Path id:65 Path ref count:1
NHID:0x2(Ref:1)
OSPF area:
Route version is 0x12 (18)
Local Label: 0x3ee6 (16102)
Local Label Algo Set (ID, Label, Metric): (1, 16202, 0),(128, 17282, 2)
IP Precedence: Not Set
QoS Group ID: Not Set
Flow-tag: Not Set
Fwd-class: Not Set
Route Priority: RIB_PRIORITY_NON_RECURSIVE_MEDIUM (7) SVD Type RIB_SVD_TYPE_LOCAL
Download Priority 1, Download Version 38
No advertising protos.
Configuring Flexible Algorithm
Note
For information about the commands usage, see the Segment Routing Command Reference for Cisco 8000 Series Routers.
The following ISIS and OSPF configuration sub-mode is used to configure Flexible Algorithm:
flex-algoalgorithm number
algorithm number —value from 128 to 255
Commands under Flexible Algorithm Configuration Mode
The following commands are used to configure Flexible Algorithm definition under the flex-algo sub-mode:
metric-type delay
Note
By default the regular IGP metric is used. If delay metric is enabled, the advertised delay on the link is used as a metric
for Flexible Algorithm computation.
priority value—priority used during the Flexible Algorithm definition election.
The following command is used to enable advertisement of the Flexible Algorithm definition in IS-IS:
advertise-definition
Commands for Affinity Configuration
The following command is used for defining the affinity-map. Affinity-map associates the name with the particular bit positions
in the Extended Admin Group bitmask.
affinity-mapnamebit-positionbit number
name—name of the affinity-map.
bit number—bit position in the Extended Admin Group bitmask.
The following command is used to associate the affinity with an interface:
affinity flex-algoname 1, name 2, …
name—name of the affinity-map
Command for Prefix-SID Configuration
The following command is used to advertise prefix-SID for default and strict-SPF algorithm:
prefix-sid [strict-spf | algorithmalgorithm-number] [index | absolute] sid value
algorithm-number—Flexible Algorithm number
sid value—SID value
IS-IS Enhancements: max-metric and data plane updates
With the IOS XR Release 7.8.1, the new optional keyword anomaly is introduced to the interface submode of affinity flex-algo. This keyword option helps to advertise flex-algo affinity on PM anomaly. The following command is used to associate the
affinity with an interface:
router isisinstanceinterfacetype interface-path-idaffinity flex-algoanomaly name 1, name 2, …router ospfprocessareaareainterfacetype interface-path-idaffinity flex-algoanomaly name 1, name 2, …
name—name of the affinity-map
You can configure both normal and anomaly values. For the following example, the blue affinity is advertised. However, if a metric is received with the anomaly flag set, it will change to red:
Router# configure
Router(config)# router isis 1
Router(config-isis)#flex-algo 128
Router(config-isis-flex-algo)# interface GigabitEthernet0/0/0/2
Router(config-isis-flex-algo)# affinity flex-algo blue
Router(config-isis-flex-algo)# affinity flex-algo anomaly red
Configuring Flexible Algorithm with Exclude SRLG Constraint
Table 3. Feature History Table
Feature Name
Release Information
Feature Description
Flexible Algorithm to Exclude SRLGs for OSPF
Release 7.5.2
You can now configure the flexible algorithm to exclude any link belonging to the Shared Risk Link Groups (SRLGs) from the
path computation for OSPF. The ability to exclude the at-risk links ensures that the rest of the links in the network remain
unaffected.
This feature allows the Flexible Algorithm definition to specify Shared Risk Link Groups (SRLGs) that the operator wants to
exclude during the Flex-Algorithm path computation. A set of links that share a resource whose failure can affect all links
in the set constitute a SRLG. An SRLG provides an indication of which links in the network might be at risk from the same
failure.
This allows the setup of disjoint paths between two or more Flex Algos by leveraging deployed SRLG configurations. For example,
multiple Flex Algos could be defined by excluding all SRLGs except one. Each FA will prune the links belonging to the excluded
SRLGs from its topology on which it computes its paths.
This provides a new alternative to creating disjoint paths with FA, in addition to leveraging FA with link admin group (affinity)
constraints.
The Flexible Algorithm definition (FAD) can advertise SRLGs that you want to exclude during the Flexible Algorithm path computation.
The IS-IS Flexible Algorithm Exclude SRLG Sub-TLV (FAESRLG) is used to advertise the exclude rule that is used during the
Flexible Algorithm path calculation, as specified in IETF draft https://datatracker.ietf.org/doc/draft-ietf-lsr-flex-algo/
The Flexible Algorithm path computation checks if an “exclude SRLG” rule is part of the FAD. If an “exclude SRLG” rule exists,
it then checks if the link is part of an SRLG that is also part of the “exclude SRLG” rule. If the link is part of an excluded
SRLG, the link is pruned from the path computation.
The figure below shows a topology configured with the following flex algos:
Flex algo 128: metric IGP and exclude SRLG X constraint
Flex algo 129: metric IGP and exclude SRLG Y constraint
The horizontal links between nodes 3 and 4 and between 2 and 5 are part of SRLG group X. The diagonal links between nodes
3 and 5 and between 2 and 4 are part of SRLG group Y. As a result, traffic from node 1 to node 6's FA 128 prefix SID (16806)
avoids interfaces part of SRLG X. While traffic from node 1 to node 6's FA 129 prefix SID (16906) avoids interfaces part of
SRLG Y.
Note
See Constraints section in the Configure SR-TE Policies chapter for information about configuring SR policies with Flex-Algo constraints.
Figure 1. Flex Algo with Exclude SRLG Constraint
Configuration
Use the router isisinstanceaddress-family ipv4 unicast advertise application flex-algo link-attributes srlg command to enable the Flexible Algorithm ASLA-specific advertisement of SRLGs.
Use the router isisinstanceflex-algoalgosrlg exclude-anysrlg-name . . . srlg-name command to configure the SRLG constraint which is advertised in the Flexible Algorithm definition (FAD) if the FAD advertisement
is enabled under the flex-algo sub-mode. You can specify up to 32 SRLG names.
The SRLG configuration (value and port mapping) is performed under the global SRLG sub-mode. Refer to Configuring SRLG Node Protection for more information.
Example
The following example shows how to enable the Flexible Algorithm ASLA-specific advertisement of SRLGs and to exclude SRLG
groups from Flexible Algorithm path computation:
The following example shows how to enable the Flexible Algorithm ASLA-specific advertisement of SRLGs and to exclude SRLG
groups from Flexible Algorithm path computation for OSPF:
RP/0/RP0/CPU0:router(config)# srlg
RP/0/RP0/CPU0:router(config-srlg)# interface HunGigE0/0/0/0
RP/0/RP0/CPU0:router(config-srlg-if)# name groupX
RP/0/RP0/CPU0:router(config-srlg-if)# exit
RP/0/RP0/CPU0:router(config-srlg)# interface TenGigE0/0/0/1
RP/0/RP0/CPU0:router(config-srlg-if)# name groupX
RP/0/RP0/CPU0:router(config-srlg-if)# exit
RP/0/RP0/CPU0:router(config-srlg)# interface HunGigE0/0/1/0
RP/0/RP0/CPU0:router(config-srlg-if)# name groupY
RP/0/RP0/CPU0:router(config-srlg-if)# exit
RP/0/RP0/CPU0:router(config-srlg)# interface TenGigE0/0/1/1
RP/0/RP0/CPU0:router(config-srlg-if)# name groupY
RP/0/RP0/CPU0:router(config-srlg-if)# exit
RP/0/RP0/CPU0:router(config-srlg)# name groupX value 100
RP/0/RP0/CPU0:router(config-srlg)# name groupY value 200
RP/0/RP0/CPU0:router(config-srlg)# exit
RP/0/0/CPU0:r1(config)#router ospf 1
RP/0/0/CPU0:r1(config-ospf)#flex-algo 128
RP/0/0/CPU0:r1(config-ospf-flex-algo)#srlg exclude-any
RP/0/0/CPU0:r(config-ospf-flex-algo-srlg-exclude-any)#groupX
RP/0/0/CPU0:r(config-ospf-flex-algo-srlg-exclude-any)#groupY
RP/0/0/CPU0:r(config-ospf-flex-algo-srlg-exclude-any)#commit
Verification
The following example shows how to verify the number of SRLGs excluded for OSPF:
RP/0/RP0/CPU0:router# show ospf topology summary
Process ospf-1
Instance default
Router ID : 192.168.0.1
Number of Areas : 1
Number of Algos : 1
Max Path count : 16
Route count : 10
SR Global Block : 16000 - 23999
Area 0
Number of Nodes : 6
Algo 128
FAD Advertising Router : 192.168.0.1
FAD Area ID : 0
Algo Type : 0
Metric Type : 0
Number of Exlclude SRLGs : (2)
[1]: 100 [2]: 200
FAPM supported : No
Example: Configuring IS-IS Flexible Algorithm
router isis 1
affinity-map red bit-position 65
affinity-map blue bit-position 8
affinity-map green bit-position 201
flex-algo 128
advertise-definition
affinity exclude-any red
affinity include-any blue
!
flex-algo 129
affinity exclude-any green
!
!
address family ipv4 unicast
segment-routing mpls
!
interface Loopback0
address-family ipv4 unicast
prefix-sid algorithm 128 index 100
prefix-sid algorithm 129 index 101
!
!
interface GigabitEthernet0/0/0/0
affinity flex-algo red
!
interface GigabitEthernet0/0/0/1
affinity flex-algo blue red
!
interface GigabitEthernet0/0/0/2
affinity flex-algo blue
!
Example: Configuring OSPF Flexible Algorithm
router ospf 1
flex-algo 130
priority 200
affinity exclude-any
red
blue
!
metric-type delay
!
flex-algo 140
affinity include-all
green
!
affinity include-any
red
!
!
interface Loopback0
prefix-sid index 10
prefix-sid strict-spf index 40
prefix-sid algorithm 128 absolute 16128
prefix-sid algorithm 129 index 129
prefix-sid algorithm 200 index 20
prefix-sid algorithm 210 index 30
!
!
interface GigabitEthernet0/0/0/0
flex-algo affinity
color red
color blue
!
!
affinity-map
color red bit-position 10
color blue bit-position 11
!
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