How Segment Routing Works - Cisco ASR 9000 Series Configuration Manuallines

How Segment Routing Works

list of segments is encoded as a stack of labels. The segment to process is on the top of the stack. The related
label is popped from the stack, after the completion of a segment.
Segment Routing provides automatic traffic protection without any topological restrictions. The network
protects traffic against link and node failures without requiring additional signaling in the network. Existing
IP fast re-route (FRR) technology, in combination with the explicit routing capabilities in Segment Routing
guarantees full protection coverage with optimum backup paths. Traffic protection does not impose any
additional signaling requirements.
How Segment Routing Works
A router in a Segment Routing network is capable of selecting any path to forward traffic, whether it is explicit
or Interior Gateway Protocol (IGP) shortest path. Segments represent subpaths that a router can combine to
form a complete route to a network destination. Each segment has an identifier (Segment Identifier) that is
distributed throughout the network using new IGP extensions. The extensions are equally applicable to IPv4
and IPv6 control planes. Unlike the case for traditional MPLS networks, routers in a Segment Router network
do not require Label Distribution Protocol (LDP) and Resource Reservation Protocol - Traffic Engineering
(RSVP-TE) to allocate or signal their segment identifiers and program their forwarding information.
There are two ways to configure segment routing:
• SR-TE policy under "segment-routing traffic-eng" sub-mode
• TE tunnel with SR option under "mpls traffic-eng" sub-mode
Note
However, you can configure the above mentioned L2VPN and EVPN services using only "segment-routing
traffic-eng" sub-mode.
Each router (node) and each link (adjacency) has an associated segment identifier (SID). Node segment
identifiers are globally unique and represent the shortest path to a router as determined by the IGP. The network
administrator allocates a node ID to each router from a reserved block. On the other hand, an adjacency
segment ID is locally significant and represents a specific adjacency, such as egress interface, to a neighboring
router. Routers automatically generate adjacency identifiers outside of the reserved block of node IDs. In an
MPLS network, a segment identifier is encoded as an MPLS label stack entry. Segment IDs direct the data
along a specified path. There are two kinds of segment IDS:
• Prefix SID: A segment ID that contains an IP address prefix calculated by an IGP in the service provider
• Adjacency SID: A segment ID that contains an advertising router's adjacency to a neighbor. An adjacency
L2VPN and Ethernet Services Configuration Guide for Cisco ASR 9000 Series Routers, IOS XR Release 6.3.x
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core network. Prefix SIDs are globally unique. A prefix segment represents the shortest path (as computed
by IGP) to reach a specific prefix; a node segment is a special prefix segment that is bound to the loopback
address of a node. It is advertised as an index into the node specific SR Global Block or SRGB.
SID is a link between two routers. Since the adjacency SID is relative to a specific router, it is locally
unique.
A node segment can be a multi-hop path while an adjacency segment is a one-hop path.
L2VPN Services over Segment Routing for Traffic Engineering Policy