Overview of Segment Routing
Segment routing is a technique by which the path followed by a packet is encoded in the packet itself, similar to source routing. A node steers a packet through a controlled set of instructions, called segments, by prepending the packet with a segment routing header. Each segment is identified by a segment ID (SID) consisting of a flat unsigned 32-bit integer.
Border Gateway Protocol (BGP) segments, a subclass of segments, identify a BGP forwarding instruction. There are two groups of BGP segments: prefix segments and adjacency segments. Prefix segments steer packets along the shortest path to the destination, using all available equal-cost multi-path (ECMP) paths.
Border Gateway Protocol - Link State (BGP-LS) is an extension to BGP for distributing the network’s Link-State (LS) topology model to external entities. BGP-LS advertise routing updates only when they occur which uses bandwidth more effectively. They advertise only the incremental change to all routers as a multicast update. They use variable length subnet masks, which are scalable and use addressing more efficiently.
The segment routing architecture is applied directly to the MPLS data plane.
Segment Routing Global Block
The segment routing global block (SRGB) is the range of local labels reserved for MPLS segment routing. The default label range is from 16000 to 23999.
SRGB is the local property of a segment routing node. Each node can be configured with a different SRGB value, and hence the absolute SID value associated to a prefix segment can change from node to node.
The SRGB must be a proper subset of the dynamic label range and must not overlap the optional MPLS static label range. If dynamic labels in the configured or defaulted SRGB range already have been allocated, the configuration is accepted, and the existing dynamic labels that fall in the SRGB range will remain allocated to the original client. A change to the SRGB range results in the clients deallocating their labels independent of whether the new range can be allocated.
High Availability for Segment Routing
In-service software upgrades (ISSUs) are minimally supported graceful restart. During the graceful restart period, the previously learned route and label state are retained.