Segment Routing Deployment - Control and Data Plane

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.

Enter the following commands to enable Segment Routing on Node S1, S2, L3, L4 and P5 as per above figure.

       

router isis 
address-family ipv4 unicast   
segment-routing mpls 
! 
interface Loopback0   
address-family ipv4 unicast    
prefix-sid index 
!  
segment-routing 
!

How does the SID encoding works:
Prefix SID
• Label form SR Global Block (SRGB)
• SRGB advertised within IGP via TLV
• Prefix-SID can be configured as an absolute value or an index
• In the protocol advertisement, Prefix-SID is always encoded as a globally unique index. Index represents an offset from SRGB base, zero-based numbering, i.e. 0 is 1st index E.g. index 1, SID is 16,000 + 1 = 16,001

       
Prefix-Sid index 1 means that S1 router will be having 16000+1=16001 prefix-sid. Similar way, S2 will have Prefix-Sid index 2 means S2 
router will be having 16000+2=16002 prefix-sid.

Adjacency SID - This is used for physical links
• Locally significant
• Automatically allocated by the IGP for each adjacency
• Always encoded as an absolute (i.e. not indexed) value

Below is the output on L4 node which clearly shows that every router is able to generate the SR value by using Prefix-Sid Index value

Show ipv4 interface brief

Output of Show route shows that the destination 5.5.5.5 is getting load balanced and no FRR is configured for the same.

Out of Show cef shows that destination 5.5.5.5 is using SR labeled stack.

Output of Show mpls forwarding plane - This is how the data plane is working. Isn't it like MPLS, the only difference is that under prefix-ID - SR Pfx is used.

       
Where possible, it is recommended that identical SRGBs be configured on all nodes in an SR domain.  This simplifies troubleshooting as the same 
label will be associated with the same prefix on all nodes.  In addition, it simplifies support for anycast.

Output of show mpls forwarding labels 16005 detail

Output of "Traceroute sr-mpls" shows that SR label 16005 is used for forwarding and PHP is taken care by the second last node.

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