Configuring OSPF SR

Module Introduction

Before you start the hands-on part of this module, you should load the appropriate configuration and verify that the testbed is up and running by executing the corresponding robot file:

student@tour:~/trainings_resources/robot$ robot mpls_sr_isis/mpls_sr_ospf_setup.robot

In order to get a better understanding, the lab setup is shown in the picture below.

mpls sr ospf lab
Figure 1. MPLS Segment Routing Lab Setup

Configuring Prefix Segments

A prefix segment is a global IGP segment that is associated with an IGP prefix and represented by the Prefix-SID. The corresponding index can be set at the set interface <instance> protocol ospf address-family ipv4 area <area> interface lo-0/0/0/0 segment-routing level.

Prefix-SIDs must be unique in the SR domain.
As of version 23.8.1, RBFS does not support IPv6 for OSPF.

In addition, a segment routing global block (SRGB) must be defined. The SRGB consists of a base value and a range which defines how many prefix-SIDs the block contains. The SRGB i s configured using the set instance <instance> protocol ospf address-family ipv4 segment-routing command hierarchy.

For sake of simplicity and troubleshooting, it is strongly recommended to use identical SRGBs on all nodes within the SR domain.
Exercise 1: Configuring OSPF Segment Routing

Configure OSPF Segment Routing on router R1. The SRGB base should be 12000. The SID for the IPv4 loopback address is 11.

Click to reveal the answer 
cfg> set instance default protocol ospf address-family ipv4 segment-routing status enable
cfg> set instance default protocol ospf address-family ipv4 segment-routing srgb base 12000
cfg> set instance default protocol ospf address-family ipv4 segment-routing srgb range 100
cfg> set instance default protocol ospf address-family ipv4 area 0.0.0.0 interface lo-0/0/0/0 segment-routing ipv4 index 11
cfg> commit

After enabling OSPF-SR, we should be able to see IPv4 labeled-unicast routes in the routing table. Let’s check:

cfg> show route ipv4 labeled-unicast prefix 192.168.0.4/32
cfg>

It seems, that no IPv4 labeled-unicast route was installed into the routing table. Let’s take a step back and examine the OSPF database:

cfg> show ospf database
Instance: default, Area: 0.0.0.0
  Type            Link State ID   Advertising Router          Age        Sequence   Checksum     Cost  Link Count
  <...>
  Opaque-Area     4.0.0.0         192.168.0.1                3600      0x80000004     0x81bf        -          -
  Opaque-Area     4.0.0.0         192.168.0.2                 529      0x80000003     0x95fb        -          -
  Opaque-Area     4.0.0.0         192.168.0.3                 530      0x80000003     0x8f01        -          -
  Opaque-Area     4.0.0.0         192.168.0.4                 542      0x80000003     0x8906        -          -
  Opaque-Area     4.0.0.0         192.168.0.5                 531      0x80000003     0x830b        -          -
  Opaque-Area     4.0.0.0         192.168.0.6                 530      0x80000003     0x7d10        -          -
  Opaque-Area     7.0.0.11        192.168.0.1                3600      0x80000004     0x7a58        -          -
  Opaque-Area     7.0.0.12        192.168.0.2                 529      0x80000003     0x943b        -          -
  Opaque-Area     7.0.0.13        192.168.0.3                 530      0x80000003     0xac1f        -          -
  Opaque-Area     7.0.0.14        192.168.0.4                 537      0x80000003     0xc403        -          -
  Opaque-Area     7.0.0.15        192.168.0.5                 531      0x80000003     0xdce6        -          -
  Opaque-Area     7.0.0.16        192.168.0.6                 530      0x80000003     0xf4ca        -          -

From the output above we can see that a number of new LSA messages of type opaque have been learned. Each router in the network advertises two opaque LSAs. Thus, new information has been learned and stored into the link-state database, but is not used. Let’s have a closer look at the router-LSAs once again:

cfg> show ospf database ls-id 192.168.0.1 detail
Instance: default, Area: 0.0.0.0 LSAs
  LSA ID: 192.168.0.1
    Advertising router: 192.168.0.1, LSA type: Router
    Sequence number: 0x80000005, Checksum: 0x21cf, LSA age: 1094
    Length: 84, Options: *|-|-|-|-|-|E|*, Flags: -|-|-|-             (1)
    <...>

cfg> show ospf database ls-id 192.168.0.4 detail
Instance: default, Area: 0.0.0.0 LSAs
  LSA ID: 192.168.0.4
    Advertising router: 192.168.0.4, LSA type: Router, Router ID: 192.168.0.3
    Sequence number: 0x80000006, Checksum: 0x2650, LSA age: 1104
    Interface: hostif-0/0/2/0, Neighbor address: 172.16.0.6
    Length: 84, Options: *|O|-|-|-|-|E|*, Flags: -|-|-|-             (2)
    <...>
1 The O flag in the options field is not set.
2 The O flag in the options field is set.

If you compare the router-LSA of router R1 and R2, you recognize that the O (opaque) field in the options field of your router is not set. Therefore, the opaque information is not used. In order to fix this, we need to enable

Exercise 2: Configuring OSPF Segment Routing

On R1, enable the opaque-capability in the OSPF configuration.

Click to reveal the answer 
cfg> set instance default protocol ospf address-family ipv4 opaque-capability enable
cfg> commit

Now that we have enabled the opaque capability, we should see IPv4 labeled-unicast routes:

cfg> show route ipv4 labeled-unicast prefix 192.168.0.4/32
Instance: default, AFI: ipv4, SAFI: labeled-unicast
Prefix/Label                             Source            Pref    Next Hop                                 Interface                      Label
192.168.0.4/32                           ospf              10      172.16.0.2                               hostif-0/0/1/0                 16014
                                                                   172.16.0.6                               hostif-0/0/2/0                 16014

Module Summary

This module outlined how to configure MPLS SR using extensions of OSPF.

If you have completed the exercise, you can check the results by executing

student@tour:~/trainings_resources/robot$ robot mpls_sr_isis/mpls_sr_ospf_verify.robot