MPLS - Segment Routing over MPLS

Segment Routing (SR) is a type of source routing. SR simplifies traffic engineering and enhances scalability by allowing source devices to define the path that traffic takes through the network using a sequence of instructions (segments).

SR in an MPLS environment is used to simplify and enhance the capabilities of network traffic engineering by using a source routing paradigm. The routers in the path from Customer Edge (CE), Provider Edge (PE) to Provider (P), each perform functions based on the labels that have been assigned to the traffic.

Here's a basic example of how you might configure SR on a network consisting of a CE router (CE1), a PE router (PE1), and a Provider router (P1). For this configuration, we'll assume you're using Cisco routers and the IOS XR software, which is commonly used in service provider environments.

Assumptions

  • Interface names are placeholders: Adjust them according to your actual interface naming conventions.
  • Router IDs and addresses are just examples.
  • This setup assumes the use of IS-IS as the Interior Gateway Protocol (IGP).

Configuration Steps

1. Enable Segment Routing on all routers

CE1 Configuration
router isis 1
 is-type level-2-only
 net 49.0001.0000.0000.0001.00
 segment-routing mpls
 interface GigabitEthernet0/0/0
  point-to-point
  address-family ipv4 unicast
   metric 10
PE1 Configuration
router isis 1
 is-type level-2-only
 net 49.0001.0000.0000.0010.00
 segment-routing mpls
 interface GigabitEthernet0/0/0
  point-to-point
  address-family ipv4 unicast
   metric 10
 interface GigabitEthernet0/0/1
  point-to-point
  address-family ipv4 unicast
   metric 10
P1 Configuration
router isis 1
 is-type level-2-only
 net 49.0001.0000.0000.0100.00
 segment-routing mpls
 interface GigabitEthernet0/0/0
  point-to-point
  address-family ipv4 unicast
   metric 10
 interface GigabitEthernet0/0/1
  point-to-point
  address-family ipv4 unicast
   metric 10

2. Configure MPLS on interfaces

You need to enable MPLS on all interfaces that will carry MPLS traffic.

PE1 Configuration
interface GigabitEthernet0/0/0
 mpls ip
interface GigabitEthernet0/0/1
 mpls ip
P1 Configuration
interface GigabitEthernet0/0/0
 mpls ip
interface GigabitEthernet0/0/1
 mpls ip

3. Verify and Test

After configuration, you should verify that MPLS labels are being distributed correctly and that the IS-IS is functioning with Segment Routing. Use commands like show mpls ldp neighbor and show isis database to check connectivity and label information.

This is a very basic setup to get you started with Segment Routing in an MPLS network. Depending on your specific requirements, you might need additional configurations like traffic engineering, more specific routing policies, or configurations for resiliency and failover.