Overview of Segment Routing
Segment Routing (SR) is a scalable and flexible method of source routing. The source selects a path and encodes it as an ordered list of segments in the packet header. Any instruction has an identifier called a segment. The segment ID (SID) is a flat unsigned 32-bit integer that uniquely identifies each segment. The following are examples of segment instruction:
- Go to node N using the shortest path
- Follow Layer 1, Layer 2, and Layer 3 to node N through the shortest path to node M.
- Apply service S
The network no longer needs to keep a per-application and per-flow state with segment routing. Instead, it follows the packet’s instructions for forwarding.
The Cisco Intermediate System-to-Intermediate System (IS-IS) and Open Shortest Path First (OSPF) protocols are extended for Segment Routing. It works with either an MPLS (Multiprotocol Label Switching) or IPv6 data plane, and it combines with MPLS’ comprehensive multi-service capabilities, such as Layer 3 VPN (L3VPN), Virtual Private Wire Service (VPWS), Virtual Private LAN Service (VPLS), and Ethernet VPN (EVPN).
With no changes to the forwarding plane, segment routing can be deployed straight to the Multiprotocol Label Switching (MPLS) architecture. Segment routing makes better use of network bandwidth and has lower latency than traditional MPLS networks. An MPLS label is used to encode a segment. A stack of labels is used to represent an ordered list of segments. The section that needs to be processed is at the very top of the stack. Following the completion of a segment, the relevant label is popped from the stack.
With a new type of routing extension header, segment routing may be implemented to IPv6 architecture. An IPv6 address is used to encode a segment. An ordered list of segments is encoded as an ordered list of IPv6 addresses in the routing extension header. A pointer in the routing extension header indicates the segment to process. Following the completion of a segment, the pointer is incremented.
Segment Routing protects traffic without imposing any topological constraints. The network covers traffic against link and node failures without the need for additional network signaling. Existing IP fast re-route (FRR) technology, along with Segment Routing’s explicit routing capabilities, ensures complete protective coverage with optimal backup paths. There are no additional signaling requirements for traffic protection.
How Does Segment Routing Work?
A router in a Segment Routing network can choose any path to forward traffic, whether explicit or the shortest path, according to the Interior Gateway Protocol (IGP). Segments represent subpaths combined by a router to create a complete route to a network destination. The identifier for each segment is distributed throughout the network using new IGP extensions. Both IPv4 and IPv6 control planes can benefit from the extensions. Unlike standard MPLS networks, routers in a Segment Router network do not need to assign or broadcast their segment identities or program their forwarding information using Label Distribution Protocol (LDP) and Resource Reservation Protocol-Traffic Engineering (RSVP-TE).
A segment identification is associated with each router (node) and each link (adjacency) (SID). The shortest path to a router as determined by the IGP is represented by node segment identities, which are globally unique. A node ID is assigned to each router by the network administrator from a reserved block. On the other hand, an adjacency segment ID is locally meaningful and reflects a specific adjacency to a neighboring router, such as the egress interface. Outside of the restricted block of node IDs, routers construct adjacency identifiers automatically. A segment identifier is encoded as an MPLS label stack entry in an MPLS network. Data is directed along a particular path by segment IDs. Segment IDS can be of two types:
Prefix-SID: A segment ID in the service provider core network that contains an IP address prefix derived by an IGP. Prefix SIDs are one-of-a-kind internationally. A node SID is a type of prefix SID that includes the node’s loopback address. It’s described as an index into the SR Global Block, or SRGB, unique to each node.
Adjacency SID: A segment ID that contains the adjacency of an advertising router to a neighbor. A link between two routers is known as an adjacency SID. The adjacency SID is locally unique because it refers to a specific router.
Conclusion
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