Basic Concepts

Segment Category

An example of Prefix SIDs, Adjacency SIDs, and Node SIDs is shown in Figure 2-2.

Figure 2-2 Prefix SID, Adjacency SID and Node SID

In simple words, a prefix segment indicates a destination address, and an adjacency segment indicates a link over which data packets travel. The prefix and adjacency segments are similar to the destination IP address and outbound interface, respectively, in conventional IP forwarding. In an IGP area, a network element (NE) sends extended IGP messages to flood its own node SID and adjacency SID. Upon receipt of the message, any NE can obtain information about the other NEs.

In actual application, the prefix segment, adjacency segment, and node segment can be used independently or in combinations. The following three main cases are involved.

Prefix Segment

A prefix segment-based forwarding path is computed by an IGP using the SPF algorithm. In Figure 2-3, node Z is a destination, and its prefix SID is 100. After an IGP floods the prefix SID, all nodes in the IGP area lean the prefix SID of node Z. Each node runs SPF to compute the shortest path to node Z. Such a path is a smallest-cost path.

Figure 2-3 Prefix segment-based forwarding paths

If there are several paths have the same cost, they perform ECMP. If they have different costs, they perform link backup. The prefix segment-based forwarding paths are not fixed, and the ingress cannot control the whole forwarding path.

Adjacency Segment

In Figure 2-4, an adjacency segment is assigned to each adjacency. The adjacency segments are contained in a segment list defined on the ingress. The segment list is used to strictly specify any explicit path. This mode can better implement SDN.

Figure 2-4 Adjacency segment-based forwarding path

Adjacency Segment + Node Segment

In Figure 2-5, adjacency and node segments are combined to forcibly include a specific adjacency into a path. Nodes can use node segments to compute the shortest path based on SPF or to load-balance traffic among paths. In this mode, paths are not strictly fixed, and therefore, they are also called loose explicit paths.

Figure 2-5 Adjacency segment + node segment-based forwarding path

SR Forwarding Mechanism

SR can be used directly in the MPLS architecture, where the forwarding mechanism remains. SIDs are encoded as MPLS labels. The segment list is encoded as a label stack. The segment to be processed is at the stack top. Once a segment is processed, its label is removed from a label stack.

Label Conflicts and Handling Rules

Prefix segments are manually configured. These settings on different devices may conflict with one another. Label conflicts are as follows:

• Prefix conflict: The same prefix is associated with two different SIDs.

• SID conflict: The same SID is associated with different prefixes.

If label conflicts occur, handle prefix conflicts before SID conflicts and use the following rules to preferentially select a SID or prefix:

1. A prefix with a larger mask is preferred.

2. The prefix of a smaller value is preferred.

3. A smaller SID is preferred.

For example, label conflicts occur in the following four routes (in the form of prefix/mask SID):

• a. 1.1.1.1/32 1

• b. 1.1.1.1/32 2

• c. 2.2.2.2/32 3

• d. 3.3.3.3/32 1

The process of handling the label conflicts is as follows:

1. Prefix conflicts are handled. Routers a and b lead to a prefix conflict. Route a has a smaller SID than route b. Route a is preferred. After the conflict is handled, the following three routes are selected:

   • a. 1.1.1.1/32 1

   • c. 2.2.2.2/32 3

   • d. 3.3.3.3/32 1

2. SID conflicts are handled. Routes a and d lead to a SID conflict. Route a has a smaller prefix than route d, route a is preferred. After the conflict is handled, the following two routes are selected:

   • a. 1.1.1.1/32 1

   • c. 2.2.2.2/32 3