Configure Segment Routing for OSPF Protocol

Open Shortest Path First (OSPF) is an Interior Gateway Protocol (IGP) developed by the OSPF working group of the Internet Engineering Task Force (IETF). Designed expressly for IP networks, OSPF supports IP subnetting and tagging of externally derived routing information. OSPF also allows packet authentication and uses IP multicast when sending and receiving packets.

This module provides the configuration information to enable segment routing for OSPF.


Note

For additional information on implementing OSPF on your , see the Implementing OSPF module in the .

Enabling Segment Routing for OSPF Protocol

Segment routing on the OSPF control plane supports the following:

  • OSPFv2 control plane

  • Multi-area

  • IPv4 prefix SIDs for host prefixes on loopback interfaces

  • Adjacency SIDs for adjacencies

  • MPLS penultimate hop popping (PHP) and explicit-null signaling

This section describes how to enable segment routing MPLS and MPLS forwarding in OSPF. Segment routing can be configured at the instance, area, or interface level.

Before you begin

Your network must support the MPLS Cisco IOS XR software feature before you enable segment routing for OSPF on your router.


Note

You must enter the commands in the following task list on every OSPF router in the traffic-engineered portion of your network.

Procedure

  Command or Action Purpose

Step 1

configure

Example:


RP/0/RP0/CPU0:router# configure

Enters mode.

Step 2

router ospf process-name

Example:


RP/0/RP0/CPU0:router(config)# router ospf 1

Enables OSPF routing for the specified routing process and places the router in router configuration mode.

Step 3

segment-routing mpls

Example:

RP/0/RP0/CPU0:router(config-ospf)# segment-routing mpls

Enables segment routing using the MPLS data plane on the routing process and all areas and interfaces in the routing process.

Enables segment routing fowarding on all interfaces in the routing process and installs the SIDs received by OSPF in the forwarding table.

Step 4

segment-routing sr-prefer

Example:

RP/0/RP0/CPU0:router(config-ospf)# segment-routing sr-prefer

Sets the preference of segment routing (SR) labels over label distribution protocol (LDP) labels.

Step 5

area area

Example:

RP/0/RP0/CPU0:router(config-ospf)# area 0

Enters area configuration mode.

Step 6

segment-routing mpls

Example:

RP/0/RP0/CPU0:router(config-ospf-ar)# segment-routing mpls

(Optional) Enables segment routing using the MPLS data plane on the area and all interfaces in the area. Enables segment routing fowarding on all interfaces in the area and installs the SIDs received by OSPF in the forwarding table.

Step 7

exit

Example:


RP/0/RP0/CPU0:router(config-ospf-ar)# exit
RP/0/RP0/CPU0:router(config-ospf)# exit

Step 8

Use the commit or end command.

commit —Saves the configuration changes and remains within the configuration session.

end —Prompts user to take one of these actions:

  • Yes — Saves configuration changes and exits the configuration session.

  • No —Exits the configuration session without committing the configuration changes.

  • Cancel —Remains in the configuration session, without committing the configuration changes.

What to do next

Configure the prefix SID.

Configuring a Prefix-SID on the OSPF-Enabled Loopback Interface

A prefix segment identifier (SID) is associated with an IP prefix. The prefix SID is manually configured from the segment routing global block (SRGB) range of labels. A prefix SID is configured under the loopback interface with the loopback address of the node as the prefix. The prefix segment steers the traffic along the shortest path to its destination.

A prefix SID can be a node SID or an Anycast SID. A node SID is a type of prefix SID that identifies a specific node. An Anycast SID is a type of prefix SID that identifies a set of nodes, and is configured with n-flag clear. The set of nodes (Anycast group) is configured to advertise a shared prefix address and prefix SID. Anycast routing enables the steering of traffic toward multiple advertising nodes. Packets addressed to an Anycast address are forwarded to the topologically nearest nodes.

The prefix SID is globally unique within the segment routing domain.

This task describes how to configure prefix segment identifier (SID) index or absolute value on the OSPF-enabled Loopback interface.

Before you begin

Ensure that segment routing is enabled on an instance, area, or interface.

Procedure

  Command or Action Purpose

Step 1

configure

Example:


RP/0/RP0/CPU0:router# configure

Enters mode.

Step 2

router ospf process-name

Example:


RP/0/RP0/CPU0:router(config)# router ospf 1

Enables OSPF routing for the specified routing process, and places the router in router configuration mode.

Step 3

area value

Example:


RP/0/RP0/CPU0:router(config-ospf)# area 0

Enters area configuration mode.

Step 4

interface Loopback interface-instance

Example:

RP/0/RP0/CPU0:router(config-ospf-ar)# interface loopback 0

Specifies the loopback interface and instance.

Step 5

prefix-sid{index SID-index | absolute SID-value } [n-flag-clear] [explicit-null]

Example:


RP/0/RP0/CPU0:router(config-ospf-ar)# prefix-sid index 1001

RP/0/RP0/CPU0:router(config-ospf-ar)# prefix-sid absolute 17001

Configures the prefix-SID index or absolute value for the interface.

Specify index SID-index for each node to create a prefix SID based on the lower boundary of the SRGB + the index.

Specify absolute SID-value for each node to create a specific prefix SID within the SRGB.

By default, the n-flag is set on the prefix-SID, indicating that it is a node SID. For specific prefix-SID (for example, Anycast prefix-SID), enter the n-flag-clear keyword. OSPF does not set the N flag in the prefix-SID sub Type Length Value (TLV).

To disable penultimate-hop-popping (PHP) and add an explicit-Null label, enter the explicit-null keyword. OSPF sets the E flag in the prefix-SID sub TLV.

Step 6

Use the commit or end command.

commit —Saves the configuration changes and remains within the configuration session.

end —Prompts user to take one of these actions:

  • Yes — Saves configuration changes and exits the configuration session.

  • No —Exits the configuration session without committing the configuration changes.

  • Cancel —Remains in the configuration session, without committing the configuration changes.

Verify the prefix-SID configuration:

RP/0/RP0/CPU0:router# show ospf database opaque-area 7.0.0.1 self-originate
 OSPF Router with ID (10.0.0.1) (Process ID 1)
                Type-10 Opaque Link Area Link States (Area 0)
<...>
    Extended Prefix TLV: Length: 20
      Route-type: 1
      AF        : 0
      Flags     : 0x40
      Prefix    : 10.0.0.1/32

      SID sub-TLV: Length: 8
        Flags     : 0x0
        MTID      : 0
        Algo      : 0
        SID Index : 1001

Conditional Prefix Advertisement

In some situations, it’s beneficial to make the OSPF prefix advertisement conditional. For example, an Area Border Router (ABR) or Autonomous System Boundary Router (ASBR) that has lost its connection to one of the areas or autonomous systems (AS) might keep advertising a prefix. If an ABR or ASBR advertises the Segment Routing (SR) SID with this prefix, the label stack of the traffic routed toward the disconnected area or AS might use this SID, which would result in dropped traffic at the ABR or ASBR.

ABRs or ASBRs are often deployed in pairs for redundancy and advertise a shared Anycast prefix SID. Conditional Prefix Advertisement allows an ABR or an ASBR to advertise its Anycast SID only when connected to a specific area or domain. If an ABR or ASBR becomes disconnected from the particular area or AS, it stops advertising the address for a specified interface (for example, Loopback).

Configure the conditional prefix advertisement under a specific interface. The prefix advertisement on this interface is associated with the route-policy that tracks the presence of a set of prefixes (prefix-set) in the Routing Information Base (RIB).

For faster convergence, the route-policy used for conditional prefix advertisement uses the new event-based rib-has-route async condition to notify OSPF of the following situations:

  • When the last prefix from the prefix-set is removed from the RIB.

  • When the first prefix from the prefix-set is added to the RIB.

Configuration

To use the conditional prefix advertisement in OSPF, create a prefix-set to be tracked. Then create a route policy that uses the prefix-set. 

Router(config)# prefix-set prefix-set-name 
Router(config-pfx)# prefix-address-1/length[, prefix-address-2/length,,, prefix-address-16/length]
Router(config-pfx)# end-set 

Router(config)# route-policy rpl-name
Router(config-rpl)# if rib-has-route async prefix-set-name then
Router(config-rpl-if)# pass
Router(config-rpl-if)# endif
Router(config-rpl)# end-policy

To advertise the loopback address in OSPF conditionally, use the advertise prefix route-policy command under OSPF interface address-family configuration sub-mode. 

Router(config)# router ospf 1
Router(config-ospf)# area 0
Router(config-ospf-ar)# interface Loopback0
Router(config-ospf-ar-if)# advertise prefix route-policy rpl-name
Router(config-ospf-ar-if)# commit

Example

Router(config)# prefix-set domain_2 
Router(config-pfx)# 2.3.3.3/32, 2.4.4.4/32
Router(config-pfx)# end-set 
Router(config)# route-policy track_domain_2
Router(config-rpl)# if rib-has-route async domain_2 then
Router(config-rpl-if)# pass
Router(config-rpl-if)# endif
Router(config-rpl)# end-policy 
Router(config)# router ospf 1
Router(config-ospf)# area 0
Router(config-ospf-ar)# interface Loopback0
Router(config-ospf-ar-if)# advertise prefix route-policy track_domain-2
Router(config-ospf-ar-if)# commit

Running Configuration

prefix-set domain_2
  2.3.3.3/32,
  2.4.4.4/32
end-set
!
route-policy track_domain_2
  if rib-has-route async domain_2 then
    pass
  endif
end-policy
!
router ospf 1
 area 0
  interface Loopback0
   advertise prefix route-policy track_domain_2
  !
 !
!

Segment Routing ECMP-FEC Optimization

ECMP-FECs are used for any ECMP programming on the system, such as MPLS LSP ECMP, VPN multipath, and EVPN multi-homing.

The SR ECMP-FEC optimization solution minimizes ECMP-FEC resource consumption during underlay programming for an SR-MPLS network. This feature supports sharing the same ECMP-FEC, regular FEC, and Egress Encapsulation DB (EEDB) entries for all IPv4 and IPv6 Segment Routing prefixes with the same set of next hops. ECMP-FEC optimization is triggered when all the out_labels associated with the ECMP paths for a given prefix have the same value. If this rule is not met, then the prefix is programmed with a dedicated ECMP-FEC. Other prefixes that meet the rule are candidates for optimization.

Segment Routing Label Edge Router (LER) ECMP-FEC Optimization enables ECMP-FEC optimization originally developed for Label Switched Router (LSR) nodes (MPLS P) to be enabled on LER (Layer 3 MPLS PE) routers.

For usage guidelines, limitations, and configuration options, see Segment Routing ECMP-FEC Optimization.

Multi-area Loopback Interface Support for OSPF

Table 1. Feature History Table

Feature Name

Release Information

Feature Description

Multi-area Loopback Interface for OSPF

Release 24.2.11

This feature now extends support on the Cisco NCS 540 Series routers running on Cisco IOS XR7.
Multi-area Loopback Interface for OSPF

Release 24.2.1

Introduced in this release on the following Cisco NCS 540 router variants running on Cisco IOS XR:

  • N540-ACC-SYS

  • N540X-ACC-SYS

  • N540-24Z8Q2C-SYS

You can save IP addresses and resources, prevent the use of multiple node SIDs for labels associated with loopback interfaces, and save time configuring multiple loopback interfaces for an Area Border Router (ABR) in a network. These improvements are possible as you can now configure a single loopback interface for multiple areas. With this feature, an ABR can use a single loopback interface for all areas it connects to, eliminating the need for separate loopback interfaces for each area.

Previously, each loopback interface was linked to only one area.

The feature introduces these changes:

CLI:

The multi-area-interface command is extended to support loopback interfaces.

Loopback Interface Optimization for ABRs

With this feature, you can reuse a single loopback interface for multiple areas, with no specific limit on the number of areas that can be configured for an ABR. This means that an ABR requires only one loopback interface for all areas it connects to instead of requiring a separate loopback interface for each area.

In earlier releases, configuring separate loopback interfaces for each area was necessary, resulting in configuring as many loopback interfaces as areas on the ABR.

Key Benefits of Multi-area Loopback Interface

The key benefits of the feature are:

  • The feature offers an efficient networking solution, eliminating the need to configure multiple loopback interfaces.

  • The feature improves scaling by enabling a 1:n ratio of loopback interfaces to multiple areas.

Usage Guidelines and Limitations for Multi-area Loopback

The following usage guidelines and limitations apply:

  • The feature supports only IPv4 addresses.

  • Associate the loopback interface to an area of an ABR, and then reuse it as a multi-area interface in other areas.

  • The feature is limited to a single OSPF instance and does not extend to multiple OSPF instances.

  • The feature is limited to a single VRF and does not extend to multiple VRFs.

Configurations

Configure the loopback interface

Perform the following task to configure the loopback interface IPv4 or IPv6 address, which is the primary loopback address that you want to reuse for all areas: 

Router(config)#interface Loopback 0
Router(config-if)#ipv4 address 10.10.10.1/8
Router(config-if)#commit

Running Configuration

!
interface Loopback0
 ipv4 address 10.10.10.1 255.0.0.0
!

Associate the loopback interface to an area

Perform the following task to associate the primary loopback interface to area 1: 

Router(config)#router ospf 100
Router(config-ospf)#router-id 10.10.10.1
Router(config-ospf)#area 1
Router(config-ospf-ar)#interface Loopback 0
Router(config-ospf-ar-if)#commit

Running Configuration

!
router ospf 100
 router-id 10.10.10.1
 area 1
  interface Loopback 0
  !
 !

Reuse the loopback interface

Perform the following task to reuse the primary loopback interface in another OSPF area:

Router(config)#router ospf 100
Router(config-ospf)#router-id 10.10.10.1
Router(config-ospf)#area 0
Router(config-ospf-ar)#multi-area-interface Loopback 0
Router(config-ospf-ar-mif)#commit
Running Configuration
!
router ospf 100
 router-id 10.10.10.1
 area 0
  multi-area-interface Loopback 0
  !
 !

Verification

The following example shows how the loopback 0 interface is identified by area 1 in an ABR:

Router#show ospf routes 10.10.10.1/8 backup-path detail 
Thu Mar 21 18:54:55.729 UTC
 
OSPF Route entry for 10.10.10.1/8
  Route type:  Intra-area
  Last updated: Mar 11 02:43:17.876 
  Metric: 1
  SPF priority: 8,  SPF version: 38
  RIB version: 0,  Source: Unknown
  Label Info: Default 16004 SSPF 18004 Type SR
       10.10.10.1, directly connected, via Loopback0
        LSA: 1/10.10.10.1/10.10.10.1, Area: 1

The following example shows that summary LSA is not advertised by ABR in area 0 after you enable the loopback interface in area 1: 

Router#show ospf database summary 10.10.10.1
Thu Mar 21 18:30:57.294 UTC
 
 
            OSPF Router with ID (10.10.10.1) (Process ID 1)
 
                Summary Net Link States (Area 0)
 
  LS age: 1760
  Options: (No TOS-capability, DC)
  LS Type: Summary Links (Network)
  Link State ID: 10.10.10.1 (Summary Network Number)
  Advertising Router: 10.0.0.1
  LS Seq Number: 8000006e
  Checksum: 0x5550
  Length: 28
  Network Mask: /32
        TOS: 0  Metric: 2

The following example indicates the router link-state advertisement (LSA) in area 0:

Router#show ospf database opaque-area ext-prefix 10.10.10.1/8
Thu Mar 21 18:57:40.914 UTC
 
 
            OSPF Router with ID (10.10.10.1) (Process ID 1)
 
                Type-10 Opaque Link Area Link States (Area 0)
 
  LS age: 345
  Options: (No TOS-capability, DC)
  LS Type: Opaque Area Link
  Link State ID: 7.0.0.3
  Opaque Type: 7
  Opaque ID: 3
  Advertising Router: 10.10.10.1
  LS Seq Number: 80000001
  Checksum: 0xa3d6
  Length: 56
 
    Extended Prefix TLV: Length: 8
      Route-type: 1
      AF        : 0
      Flags     : 0x40
      Prefix    : 10.10.10.1/32
 
      SID sub-TLV: Length: 8
        Flags     : 0x0
        MTID      : 0
        Algo      : 0
        SID Index : 4
 
      SID sub-TLV: Length: 8
        Flags     : 0x0
        MTID      : 0
        Algo      : 1
        SID Index : 2004

Display Neighbor Router Capabilities in OSPF Networks

Table 2. Feature History Table

Feature Name

Release Information

Feature Description

Display Neighbor Router Capabilities in OSPF Networks

Release 24.2.11

This feature now extends support on the Cisco NCS 540 Series routers running on Cisco IOS XR7.

Display Neighbor Router Capabilities in OSPF Networks

Release 24.2.1

Introduced in this release on the following Cisco NCS 540 router variants running on Cisco IOS XR:

  • N540-ACC-SYS

  • N540X-ACC-SYS

  • N540-24Z8Q2C-SYS

You can now improve the user experience, understand the features and functionalities supported by neighboring routers, reduce operational work hours and human errors by gaining better visibility into the routing capabilities of directly connected neighboring routers in an OSPF network. These improvements are now possible when you enable Segment Routing on a neighboring connected router.

Previously, there was no straightforward method to display the segment routing and other capabilities of neighboring routers in an OSPF network.

The feature introduces these changes:

CLI:

YANG Data Models:

  • Cisco-IOS-XR-ipv4-ospf-oper.yang

See (GitHub, Yang Data Models Navigator)

The feature enables you to access detailed information of a neighboring router capability from the Router Information Link State Advertisement (RI LSA). This includes details such as segment routing feature, graceful restart and helper operation modes, hostname, and traffic engineering capabilities.

For more information about graceful restart and helper operation modes, see Implementing OSPF chapter in the Routing Configuration Guide.

Key Benefits of Displaying Neighbor Router Capabilities

  • The feature ensures optimal path selection for faster packet forwarding.

  • Reduces operational complexity, and enables you to make informed decisions for network upgrades.

  • Perform compatibility checks with neighboring routers.

Usage Guidelines and Limitations to Display Neighbor Router Capabilities

The following usage guidelines and limitations apply:

  • The RI LSA includes the capabilities of a router, only when Segment Routing capability is enabled for that router. This means that if Segment Routing is not enabled, the router does not advertise any capability in the RI LSA.

  • The feature is area-oriented, meaning that it is specific to an OSPF area. This suggests that the capabilities advertised by a neighboring router are relevant and visible only within the same OSPF area.

  • You must enable traffic engineering at the OSPF or at the area level.

  • You must configure graceful restart and helper capabilities at the OSPF level.

Verify the Neighbor Router Capabilities

You can use the show ospf neighbor capabilities command to display the traffic engineering support, hostname, and graceful restart and helper capabilities of neighboring connected routers, which are enabled for Segment Routing. 

Router#show ospf neighbor capabilities        
Mon Apr 29 08:27:07.289 UTC

Capabilities advertised in RI LSA
  TE - Traffic Engineering
  GR - Graceful Restart
  GH - Graceful Restart Helper

  NA - Capabilities TLV Not Advertised

Neighbors for OSPF 1

Area 1
Neighbor ID            Address              Interface       SR-Enabled    Capabilities
192.168.199.137        209.165.200.225/27       Gi100/0/0/0     Yes           TE, GR, GH
    HostName: Device_3                                                        
192.168.48.1           209.165.200.230/27        Gi100/0/0/4     No            NA
192.168.48.200         209.165.200.235/27       Gi10/0/0/0      Yes           TE, GR, GH
    HostName: Device_0                                                        

Total neighbor count: 3

In this scenario, Area 1 consists of three connected routers. Routers 192.168.199.137 and 192.168.48.200 support Segment Routing, so their RI LSA includes the TE, GH, and GR capabilities. When the TE and GR are not configured, only the GH capability is advertised by default. Router 192.168.48.1, which does not support Segment Routing, does not include any capabilities in its RI LSA.

When you use the show ospf neighbor neighbor-id command to view the summary information of a specific neighboring router, the summary information includes the capabilities of that neighbor router if Segment Routing is enabled for it.

The following example shows the capabilities of neighbor 192.168.199.130 listed in the summary information: 

Router#show ospf neighbor 192.168.199.130
Mon Nov 6 19:13:08.861 IST
* Indicates MADJ interface
# Indicates Neighbor awaiting BFD session up

Neighbors for OSPF 1

 Neighbor 192.168.199.130, interface address 172.31.80.37
    In the area 0 via interface GigabitEthernet0/2/0/4
    Neighbor priority is 1, State is FULL, 6 state changes
    DR is 0.0.0.0 BDR is 0.0.0.0
    Options is 0x52
    LLS Options is 0x1 (LR)
    Dead timer due in 00:00:32
    Neighbor is up for 00:08:50
    Number of DBD retrans during last exchange 0
    Index 3/6, retransmission queue length 0, number of retransmission o
    First 0(0) /0(0) Next 0(0) /0(0)
    Last retransmission scan length is 0, maximum is 0
    Last retransmission scan time is l msec, maximum is 8 msec
    LS Ack list: NSR-sync pending 0, high water mark o
    Adjacency SIDs:
        Label: 24010,    Dynamic, Unprotected
        Label: 24011,    Dynamic,   Protected (Has backup)
    Neighbor Interface ID: 4
    Segment-Routing Enabled
      Strict SPF capable
      Start SID: 16000 Range Size: 8000
      Flex algo Enabled
    Capabilities advertised in RI LSA
      Graceful Restart capable
      Graceful Restart Helper capable
HostName: r5

Total neighbor count: 1