Basics Of Segment Routing covers that node steers a packet through an ordered list of instructions, called segments. A segment can represent any instruction, topological or service based. The advantage of using Segment Routing (SR-MPLS) over LDP and RSVP was clearly explained in earlier post of “Segment Routing Based MPLS Vs Classic MPLS”.

This post is used to learn and practice Segment Routing concepts and configuration on Cisco IOS-XR based platform. By the end of this example, one can easily understand how to enable SR with ISIS, SRGB, SIDs and it’s advertisements.

We will be using the below topology to understand how SR can be used in the network.

In the context of an IGP-based distributed control plane, two topological segments are defined: the IGP Adjacency segment and the IGP-Prefix segment. In the context of a BGP-based distributed control plane, two topological segments are defined: the BGP peering segment and the BGP-Prefix segment.
• IGP-Adjacency Segment: an IGP-Adjacency segment is an IGP segment attached to a unidirectional adjacency or a set of unidirectional adjacencies. By default, an IGP-Adjacency segment is local (unless explicitly advertised otherwise) to the node that advertises it. Also referred to as “Adj-SID”.

• IGP-Prefix Segment: an IGP-Prefix segment, also referred to as “prefix-SID”, is an IGP segment representing an IGP prefix. When an IGP-Prefix segment is global within the SR IGP instance/topology, it identifies an instruction to forward the packet along the path computed using the routing algorithm specified in the algorithm field, in the topology, and in the IGP instance where it is advertised. It is advertised as an index into the node specific SR Global Block or SRGB.

• IGP-Node Segment: an IGP-Node segment is an IGP-Prefix segment that identifies a specific router (for example, a loopback). Also referred to as “Node Segment”.

• SR Global Block (SRGB): the set of global segments in the SR domain (the range of labels reserved for segment routing). In SR-MPLS, SRGB is a local property of a node and identifies the set of local labels reserved for global segments. In SR-MPLS, using identical SRGBs on all nodes within the SR domain is strongly recommended. Doing so eases operations and troubleshooting as the same label represents the same global segment at each node. The SRGB default value is 16000 to 23999.

• SR Local Block (SRLB): local property of an SR node. In SR-MPLS, SRLB is a set of local labels reserved for local segments.