Enabling Segment Routing Flexible Algorithm

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 constraints may be used to compute a path over a network. Some networks are deployed with multiple planes. A simple form of constraint may be to use a particular plane. A more sophisticated form of constraint 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 a Flexible Algorithm.

Flexible Algorithm Membership

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

  • Exclude SRLG constraint

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 Link Attribute Advertisement

Various link attributes may be used during the Flexible Algorithm path calculation. For example, include or exclude rules based on link affinities can be part of the Flexible Algorithm definition, as defined in IETF draft draft-ietf-lsr-flex-algo.

Link attribute advertisements used during Flexible Algorithm calculation must use the Application-Specific Link Attribute (ASLA) advertisements, as defined in RFC8919 (IS-IS) and RFC8920 (OSPF). In the case of IS-IS, if the L-Flag is set in the ASLA advertisement, then legacy advertisements (IS-IS Extended Reachability TLV) are used instead.

The mandatory use of ASLA advertisements applies to the following link attributes:

  • Minimum Unidirectional Link Delay

  • TE Default Metric

  • Administrative Group

  • Extended Administrative Group

  • Shared Risk Link Group

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

Table 1. Feature History Table

Feature Name

Release Information

Feature Description

OSPF: Microloop Avoidance for Flexible Algorithm

Release 7.4.1

This feature extends the current OSPF Flexible Algorithm functionality to support Microloop Avoidance.
Table 2. Feature History Table

Feature Name

Release Information

Feature Description

OSPF: Microloop Avoidance for Flexible Algorithm

Release 7.3.2

This feature extends the current OSPF Flexible Algorithm functionality to support Microloop Avoidance.

OSPF: TI-LFA for Flexible Algorithm

Release 7.3.1

This feature extends the current OSPF Flexible Algorithm functionality to support TI-LFA.

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 Flexible Algorithm is used.

The router uses the following rules to prune links from the topology during the Flexible Algorithm computation:

  • All nodes that don't advertise support for Flexible Algorithm are pruned from the topology.

  • Affinities:

    • Check if any exclude affinity rule is part of the Flexible Algorithm Definition. If such exclude rule exists, check if any color that is part of the exclude rule is also set on the link. If such a color is set, the link must be pruned from the computation.

    • Check if any include-any affinity rule is part of the Flexible Algorithm Definition. If such include-any rule exists, check if any color that is part of the include-any rule is also set on the link. If no such color is set, the link must be pruned from the computation.

    • Check if any include-all affinity rule is part of the Flexible Algorithm Definition. If such include-all rule exists, check if all colors that are part of the include-all rule are also set on the link. If all such colors are not set on the link, the link must be pruned from the computation


    Note
    See Flexible Algorithm Affinity Constraint.

  • If the Flexible Algorithm definition includes an "exclude SRLG" rule, then all links that are part of such SRLG are pruned from the topology.


    Note
    See Flexible Algorithm with Exclude SRLG Constraint.

  • Router uses the metric that is part of the Flexible Algorithm definition. If the metric isn't advertised for the particular link, the link is pruned from the topology.

Loop Free Alternate (LFA) paths, TI-LFA backup paths, and Microloop Avoidance paths for particular Flexible Algorithm 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.

Configuring Microloop Avoidance for Flexible Algorithm

By default, Microloop Avoidance per Flexible Algorithm instance follows Microloop Avoidance configuration for algo-0. For information about configuring Microloop Avoidance, see Configure Segment Routing Microloop Avoidance.

You can disable Microloop Avoidance for Flexible Algorithm using the following commands:

router isis instance flex-algo algo microloop avoidance disable

router ospf process flex-algo algo microloop avoidance disable

Configuring LFA / TI-LFA for Flexible Algorithm

By default, LFA/TI-LFA per Flexible Algorithm instance follows LFA/TI-LFA configuration for algo-0. For information about configuring TI-LFA, see Configure Topology-Independent Loop-Free Alternate (TI-LFA).

You can disable TI-LFA for Flexible Algorithm using the following commands:

router isis instance flex-algo algo fast-reroute disable

router ospf process flex-algo algo fast-reroute disable

Flexible Algorithm Affinity Constraint

Table 3. Feature History Table

Feature Name

Release Information

Feature Description

IS-IS: Flexible Algorithm Reverse Affinity

Release 7.9.1

This feature enhances the IS-IS Flexible Algorithm link admin group (affinity) constraint to include link colors on links in the reverse direction toward the calculating router.

The ability to apply affinity constraints in the reverse direction provides additional control for IS-IS Flexible Algorithm path computation.

This feature intoduces the reverse keyword to the router isis instance flex-algo algo affinity command.

You can apply a color or name to links or interfaces by assigning affinity bit-maps to them. You can then specify an affinity (or relationship) between a Flexible Algorithm path and link colors in the forwarding direction. Flexible Algorithm computes a path that includes or excludes links that have specific colors, or combinations of colors.

  • Affinity “blue” is assigned to interface on node A; exclude affinity "blue": Link A-B is pruned from path calculation

In Cisco IOS XR release 7.9.1, for IS-IS Flexible Algorithm, you can also specify a reverse affinity between a Flexible Algorithm path and link colors (in the direction toward the computing router). Flexible Algorithm computes a path that includes or excludes links in the reverse direction that have specific colors, or combinations of colors.

For example, on a point-to-point link between endpoints A and B and for the traffic flowing in the direction from A to B, the input errors can only be detected at node B. You may measure the rate of such input errors and set certain 'color' on a link locally on node B when the input error rate crosses a certain threshold.

  • Affinity “blue” is assigned to interface on node B; exclude affinity "blue": Link A-B is not pruned from path calculation

With IS-IS Flexible Algorithm Reverse Affinity, when Flex-Algorithm calculation processes link A to B, it may look at the 'colors' of the link in the reverse direction (link B to A). This enables you to exclude this link from the Flex-Algorithm topology.

  • Affinity “blue” is assigned to interface on node B; exclude reverse-affinity "blue": Link A-B is pruned from path calculation

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 IPG paths computed based on the default algorithm and regular IGP metrics.

Flexible Algorithm Prefix-SID Redistribution

Table 4. 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.

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.

Configuration Example

The following example shows how to configure redistribute and flexible algorithm to enable external routes.


RP/0/RP0/CPU0:ios(config)#router ospf 1
RP/0/RP0/CPU0:ios(config-ospf)#segment-routing mpls
RP/0/RP0/CPU0:ios(config-ospf)#segment-routing forwarding mpls
RP/0/RP0/CPU0:ios(config-ospf)#redistribute isis 2 route-policy loopback-type
RP/0/RP0/CPU0:ios(config-ospf)#flex-algo 240
RP/0/RP0/CPU0:ios(config-ospf-flex-algo)#metric-type delay
RP/0/RP0/CPU0:ios(config-ospf-flex-algo)#prefix-metric
RP/0/RP0/CPU0:ios(config-ospf-flex-algo)#advertise-definition

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.

Flexible Algorithm Prefix Metric

Table 5. Feature History Table

Feature Name

Release Information

Feature Description

Prefix Metric support for OSPF Flexible Algorithm

 Release 7.5.1

This feature extends the current OSPF Flexible Algorithm functionality to introduce a Flexible Algorithm-specific prefix-metric in the OSPF prefix advertisement. The prefix-metric provides a way to compute the best end-to-end Flexible Algorithm optimized paths across multiple areas or domains.

A limitation of the existing Flexible Algorithm functionality in IS-IS and OSPF is the inability to compute the best path to a prefix in a remote area or remote IGP domain. Prefixes are advertised between IS-IS areas, OSPF processes, or between protocol domains, but the existing prefix metric does not reflect any of the constraints used for Flexible Algorithm path. Although the best Flexible Algorithm path can be computed to the inter-area or redistributed prefix inside the area, the path may not represent the overall best path through multiple areas or IGP domains.

The Flexible Algorithm Prefix Metric feature introduces a Flexible Algorithm-specific prefix-metric in the IS-IS and OSPF prefix advertisement. The prefix-metric provides a way to compute the best end-to-end Flexible Algorithm optimized paths across multiple areas or domains.


Note
The Flexible Algorithm definition must be consistent between domains or areas. Refer to section 8 and section 9 in IETF draft https://datatracker.ietf.org/doc/draft-ietf-lsr-flex-algo/.

Configuring Flexible Algorithm

Table 6. Feature History Table

Feature Name

Release Information

Feature Description

IS-IS Enhancements: max-metric and data plane updates

Release 7.8.1

The new anomaly optional keyword is introduced to affinity flex-algo command. This keyword helps to advertise the flex-algo affinity when the performance measurement signals a link anomaly, such as an excessive delay on a link. You could use the anomaly option to exclude the link from flex-algo path computations.

affinity flex-algo
Table 7. Feature History Table

Feature Name

Release Information

Feature Description

TE Metric Support for IS-IS Flex Algo

Release 7.4.1

Flexible Algorithm allows for user-defined algorithms where the IGP computes paths based on a user-defined combination of metric type (path optimization objective) and constraint.

This feature adds support for TE metric as a metric type for IS-IS Flexible Algorithm. This allows the TE metric, along with IGP and delay metrics, to be used when running shortest path computations.

The following IS-IS and OSPF configuration sub-mode is used to configure Flexible Algorithm: 


router isis instance flex-algo algo

router ospf process flex-algo algo

algo—value from 128 to 255

Configuring Flexible Algorithm Definitions

The following commands are used to configure Flexible Algorithm definition under the flex-algo sub-mode:

  • router isis instance flex-algo algo metric-type {delay | te}

router ospf process flex-algo algo metric-type {delay | te-metric}

    Note
    By default the IGP metric is used. If delay or TE metric is enabled, the advertised delay or TE metric on the link is used as a metric for Flexible Algorithm computation.

    Note
    See Flexible Algorithm Link Attribute Advertisement Behavior for TE metric behaviors. 
  • router isis instance flex-algo algo affinity [reverse] { include-any | include-all | exclude-any}  name1, name2, …

router ospf process flex-algo algo affinity { include-any | include-all | exclude-any}  name1, name2, …

    name—name of the affinity map


    Note
    See Flexible Algorithm Affinity Constraint for information about affinity constraint behaviors. 
  • router isis instance flex-algo algo priority priority value

router ospf process flex-algo algo priority priority value

    priority value—priority used during the Flexible Algorithm definition election.

The following command is used to to include the Flexible Algorithm prefix metric in the advertised Flexible Algorithm definition in IS-IS and OSPF :

router isis instance flex-algo algo prefix-metric

router ospf process flex-algo algo prefix-metric

The following command is used to enable advertisement of the Flexible Algorithm definition in IS-IS:

router isis instance flex-algo algo advertise-definition

Configuring Affinity

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. 

router isis instance flex-algo algo affinity-map name bit-position bit number

router ospf process flex-algo algo affinity-map name bit-position bit number

  • name—name of the affinity-map.

  • bit number—bit position in the Extended Admin Group bitmask.

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 isis instance interface type interface-path-id affinity flex-algo anomaly name 1, name 2, … 

router ospf process area area interface type interface-path-id affinity flex-algo anomaly 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 Prefix-SID Advertisement

The following command is used to advertise prefix-SID for default and strict-SPF algorithm:

router isis instance interface type interface-path-id address-family {ipv4 | ipv6} [unicast] prefix-sid [strict-spf | algorithm algorithm-number]  [index | absolute] sid value

router ospf process area area interface Loopback interface-instance prefix-sid [strict-spf | algorithm algorithm-number]  [index | absolute] sid value

  • algorithm-number—Flexible Algorithm number

  • sid value—SID value

Flexible Algorithm Link Attribute Advertisement Behavior

Table 8. Feature History Table

Feature Name

Release Information

Feature Description

Advertisement of Link Attributes for IS-IS Flexible Algorithm

Release 7.4.1

Link attribute advertisements used during Flexible Algorithm path calculation must use the Application-Specific Link Attribute (ASLA) advertisements, as defined in IETF draft draft-ietf-lsr-flex-algo.

This feature introduces support for ASLA advertisements during IS-IS Flexible Algorithm path calculation.

The following tables explain the behaviors for advertising (transmitting) and processing (receiving) Flexible Algorithm link attributes.

Table 9. OSPF

Link Attribute

Transmit

Receive

Link Delay Metric

IOS XR OSPF Flex Algo implementation advertises the link delay metric value using the OSPF ASLA sub-TLV with the F-bit set.

IOS XR OSPF only uses the link delay metric advertised in the ASLA sub-TLV for Flex Algo.

ASLA sub-TLV is supported with non-zero-length or with zero-length Application Identifier Bit Masks.

Link TE Metric

IOS XR OSPF Flex Algo implementation advertises the link TE metric value using the OSPF ASLA sub-TLV with the F-bit set.

The link TE metric values advertised are configured under SR-TE.

IOS XR OSPF only uses the TE metric advertised in the ASLA sub-TLV for Flex Algo.

ASLA sub-TLV is supported with non-zero-length or with zero-length Application Identifier Bit Masks.

Link Admin Group/Extended Admin Group

IOS XR OSPF Flex Algo implementation advertises the link admin group value using both link admin group (AG) and link extended admin group (EAG) encoding using the OSPF ASLA sub-TLV with the F-bit set.

The link admin group values advertised can be configured directly under the IGP and are therefore FA-specific. Otherwise, they will be derived from the link admin group values configured under SR-TE.

IOS XR OSPF only uses the AG/EAG (either one or both) advertised in the ASLA sub-TLV for Flex Algo.

ASLA sub-TLV is supported with non-zero-length or with zero-length Application Identifier Bit Masks.
Table 10. IS-IS
Link Attribute Transmit Receive

Link Delay Metric

IOS XR IS-IS Flex Algo implementation advertises the link delay metric value using both the IS-IS Extended Reachability TLV and the IS-IS ASLA.

By default, IOS XR IS-IS Flex Algo implementation prefers the link delay metric value received in the IS-IS ASLA. Otherwise, it will use link delay metric value received in the IS-IS Extended Reachability TLV.

ASLA sub-TLV is supported with non-zero-length or with zero-length Application Identifier Bit Masks.

If the incoming ASLA includes the L-Flag, implementation derives the link delay metric value from the IS-IS Extended Reachability TLV.

You can configure the IOS XR IS-IS Flex Algo implementation to strictly use the link delay metric value received in the IS-IS ASLA. See Strict IS-IS ASLA Link Attribute.

Link TE Metric

IOS XR IS-IS Flex Algo implementation advertises the link TE metric value using the IS-IS ASLA.

The link TE metric values advertised can be configured directly under the IGP and are therefore FA-specific. Otherwise, they will be derived from the link TE metric values configured under SR-TE.

See Flexible Algorithm-Specific TE Metric.

IOS XR IS-IS Flex Algo implementation processes the link TE metric value received in the IS-IS ASLA.

ASLA sub-TLV is supported with non-zero-length or with zero-length Application Identifier Bit Masks.

If incoming ASLA includes the L-Flag, implementation derives the link TE metric value from the IS-IS Extended Reachability TLV.

Link Admin Group/Extended Admin Group

IOS XR IS-IS Flex Algo implementation advertises the affinity value as both the link admin group (AG) TLV and the link extended admin group (EAG) TLV using the IS-IS ASLA when its value falls within the first 32 bits. Otherwise, the affinity value is advertised only as link EAG TLV using the IS-IS ASLA.

The admin group values advertised are configured directly under the IGP and are therefore FA-specific.

IOS XR IS-IS Flex Algo implementation processes the affinity value received as either the link admin group TLV or link extended admin group TLV in the IS-IS ASLA.

ASLA sub-TLV is supported with non-zero-length or with zero-length Application Identifier Bit Masks.

If incoming ASLA includes the L-Flag, implementation derives the affinity value from the IS-IS Extended Reachability TLV.

Link SRLG

 IOS XR IS-IS LFA implementation advertises the link SRLG value in the IS-IS ASLA.

IOS XR IS-IS LFA implementation processes the link SRLG value received in the IS-IS ASLA.

If incoming ASLA includes the L-Flag, implementation derives the link SRLG value from the IS-IS Extended Reachability TLV.

Strict IS-IS ASLA Link Attribute

Use the following command to configure the IOS XR IS-IS Flex Algo implementation to strictly use the link delay metric value received in the IS-IS ASLA: 


router isis instance-id receive application flex-algo delay app-only

Flexible Algorithm-Specific TE Metric

Use the following command to configure the Flexible Algorithm-specific TE metric value under IS-IS, where metric_value is from 1 to 16777214:

  • router isis instance interface type interface-path-id address-family { ipv4 | ipv6} [unicast] te-metric flex-algo metric_value [level {1 | 2}]

The following example shows how to configure the IS-IS Flexible Algorithm-specific TE metric value to 50:


Router(config)# router isis 1
Router(config-isis)# interface HundredGigE 0/0/0/2
Router(config-isis-if)# address-family ipv4 unicast
Router(config-isis-if-af)# te-metric flex-algo 50

Use the following command to configure the Flexible Algorithm-specific TE metric value under OSPF, where metric_value is from 1 to 2147483647:

  • router ospf process-name area area interface type interface-path-id te-metric flex-algo metric_value

The following example shows how to configure the OSPF Flexible Algorithm-specific TE metric value to 50:


Router(config)# router ospf 1
Router(config-ospf)# area 0
Router(config-ospf-ar)# interface HundredGigE 0/0/0/2 
Router(config-ospf-ar-if# te-metric flex-algo 50

Flexible Algorithm with Exclude SRLG Constraint

Table 11. 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.

IS-IS Flexible Algorithm: Exclude-SRLG Constraint

Release 7.5.1

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. The ability to exclude the at-risk links ensures that the rest of the links in the network remain unaffected.

This allows the setup of disjoint paths between two or more Flex Algos by leveraging deployed SRLG configurations.

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 isis instance address-family ipv4 unicast advertise application flex-algo link-attributes srlg command to enable the Flexible Algorithm ASLA-specific advertisement of SRLGs.

Use the router isis instance flex-algo algo srlg exclude-any srlg-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 MPLS Traffic Engineering Shared Risk Link Groups 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: 

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/RP0/CPU0:router(config)# router isis 1
RP/0/RP0/CPU0:router(config-isis)# address-family ipv4 unicast
RP/0/RP0/CPU0:router(config-isis-af)# advertise application flex-algo link-attributes srlg
RP/0/RP0/CPU0:router(config-isis-af)# exit
RP/0/RP0/CPU0:router(config-isis)# flex-algo 128
RP/0/RP0/CPU0:router(config-isis-flex-algo)# advertise-definition
RP/0/RP0/CPU0:router(config-isis-flex-algo)# srlg exclude-any groupX
RP/0/RP0/CPU0:router(config-isis-flex-algo)# exit
RP/0/RP0/CPU0:router(config-isis)# flex-algo 129
RP/0/RP0/CPU0:router(config-isis-flex-algo)# advertise-definition
RP/0/RP0/CPU0:router(config-isis-flex-algo)# srlg exclude-any groupY
RP/0/RP0/CPU0:router(config-isis-flex-algo)# commit
RP/0/RP0/CPU0:router(config-isis-flex-algo)# exit
RP/0/RP0/CPU0:router(config-isis)#

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

Maximum Paths Per IS-IS Flexible Algorithm

Table 12. Feature History Table

Feature Name

Release

Description

IS-IS: Maximum Paths Per Flexible Algorithm

Release 7.8.1

This feature introduces a new subcommand under flex-algo command. This feature allows for maximum number of Equal-Cost Multi-path (ECMP) to be set for individual Flex Algorithms

A new subcommand under flex-algo is introduced.

The flex-algo command now includes the address-family <ipv4/ipv6> unicast subcommand, and the maximum-paths <maximum-paths> subcommand.


Note
For information on IS-IS Algo0 Maximum Paths, refer to the "Implementing IS-IS" chapter in the Routing Configuration Guide for Cisco NCS 5500 Series Routers.

The new subcommands allow for maximum number of Equal-Cost Multi-path (ECMP) to be set for individual algorithms. The value that is configured on a per-flex-algo per address-family basis overrides any value that is configured under the IS-IS global address-family submode.

Usage Guidelines and Limitations

  • The maximum-paths configuration is not part of the Flexible Algorithm Definitions (FAD). If the advertised definition is configured for the flexible algorithm, the maximum-paths will not be propagated by the IS-IS.

  • The maximum-paths per algorithm takes precedence over maximum-paths per address-family.

  • The maximum paths effective for each algorithm are as follows:

    • For Flex-Algo 128:

      • IPv4: 5

      • IPv6: 3

    • For Flex-Algos 129 through 255:

      • IPv4: 12

      • IPv6:8

Configuration Example - Max Path

This example shows how you can set the maximum paths per-Flex-Algo:

Router(config)# router isis 10
Router(config-isis)# flex-algo 128
Router(config-isis-flex-algo)# metric-type te
Router(config-isis-flex-algo)# address-family ipv4 unicast
Router(config-isis-flex-algo-af)# maximum-paths 5
Router(config-isis-flex-algo)# exit
Router(config-isis-flex-algo-af)# address-family ipv6 unicast
Router(config-isis-flex-algo-af)# maximum-paths 3
Router(config-isis-flex-algo-af)# exit

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
 !

Example: Traffic Steering to Flexible Algorithm Paths

BGP Routes on PE – Color Based Steering

SR-TE On Demand Next-Hop (ODN) feature can be used to steer the BGP traffic towards the Flexible Algorithm paths.

The following example configuration shows how to setup BGP steering local policy, assuming two router: R1 (2.2.2.2) and R2 (4.4.4.4), in the topology.

Configuration on router R1:

vrf Test
address-family ipv4 unicast
  import route-target
   1:150
  !
  export route-policy SET_COLOR_RED_HI_BW
  export route-target
   1:150
  !
!
!
interface Loopback0
ipv4 address 2.2.2.2 255.255.255.255
!
interface Loopback150
vrf Test
ipv4 address 2.2.2.222 255.255.255.255
!
interface TenGigE0/1/0/3/0
description exr1 to cxr1
ipv4 address 10.0.20.2 255.255.255.0
!
extcommunity-set opaque color129-red-igp
  129
end-set
!
route-policy PASS
  pass
end-policy
!
route-policy SET_COLOR_RED_HI_BW
  set extcommunity color color129-red-igp
  pass
end-policy
!
router isis 1
is-type level-2-only
net 49.0001.0000.0000.0002.00
log adjacency changes
affinity-map RED bit-position 28
flex-algo 128
  priority 228
!
address-family ipv4 unicast
  metric-style wide
  advertise link attributes
  router-id 2.2.2.2
  segment-routing mpls
!
interface Loopback0
  address-family ipv4 unicast
   prefix-sid index 2
   prefix-sid algorithm 128 index 282
  !
!
interface TenGigE0/1/0/3/0
  point-to-point
  address-family ipv4 unicast
  !
!
!
router bgp 65000
bgp router-id 2.2.2.2
address-family ipv4 unicast
!
address-family vpnv4 unicast
  retain route-target all
!
neighbor-group RR-services-group
  remote-as 65000
  update-source Loopback0
  address-family ipv4 unicast
  !
  address-family vpnv4 unicast
  !
!
neighbor 4.4.4.4
  use neighbor-group RR-services-group
!
vrf Test
  rd auto
  address-family ipv4 unicast
   redistribute connected
  !
segment-routing
traffic-eng
  logging
   policy status
  !
  segment-list sl-cxr1
   index 10 mpls label 16294
  !
  policy pol-foo
   color 129 end-point ipv4 4.4.4.4
   candidate-paths
    preference 100
     explicit segment-list sl-cxr1
     !
    !
   !
  !
!
!

Configuration on router R2:

vrf Test
address-family ipv4 unicast
  import route-target
   1:150
  !
  export route-policy SET_COLOR_RED_HI_BW
  export route-target
   1:150
  !
!
!
interface TenGigE0/1/0/1
description cxr1 to exr1
ipv4 address 10.0.20.1 255.255.255.0
!
extcommunity-set opaque color129-red-igp
  129
end-set
!
route-policy PASS
  pass
end-policy
!
route-policy SET_COLOR_RED_HI_BW
  set extcommunity color color129-red-igp
  pass
end-policy
!
router isis 1
is-type level-2-only
net 49.0001.0000.0000.0004.00
log adjacency changes
affinity-map RED bit-position 28
affinity-map BLUE bit-position 29
affinity-map GREEN bit-position 30
flex-algo 128
  priority 228
!
flex-algo 129
  priority 229
!
flex-algo 130
  priority 230
!
address-family ipv4 unicast
  metric-style wide
  advertise link attributes
  router-id 4.4.4.4
  segment-routing mpls
!
interface Loopback0
  address-family ipv4 unicast
   prefix-sid index 4
   prefix-sid algorithm 128 index 284
   prefix-sid algorithm 129 index 294
   prefix-sid algorithm 130 index 304
  !
!
interface GigabitEthernet0/0/0/0
  point-to-point
  address-family ipv4 unicast
  !
!
interface TenGigE0/1/0/1
  point-to-point
  address-family ipv4 unicast
  !
!
router bgp 65000
bgp router-id 4.4.4.4
address-family ipv4 unicast
!
address-family vpnv4 unicast
!
neighbor-group RR-services-group
  remote-as 65000
  update-source Loopback0
  address-family ipv4 unicast
  !
  address-family vpnv4 unicast
  !
!
neighbor 10.1.1.1
  use neighbor-group RR-services-group
!
neighbor 2.2.2.2
  use neighbor-group RR-services-group
!
vrf Test
  rd auto
  address-family ipv4 unicast
   redistribute connected
  !
  neighbor 25.1.1.2
   remote-as 4
   address-family ipv4 unicast
    route-policy PASS in
    route-policy PASS out
   !
  !
!
!
segment-routing
!
end