Segment Routing (SR)
is a flexible, scalable way of doing source routing. The source chooses a path
and encodes it in the packet header as an ordered list of segments. Segments
are identifier for any type of instruction. Each segment is identified by the
segment ID (SID) consisting of a flat unsigned 32-bit integer. Segment
instruction can be:
With segment routing,
the network no longer needs to maintain a per-application and per-flow state.
Instead, it obeys the forwarding instructions provided in the packet.
Segment Routing relies
on a small number of extensions to Cisco Intermediate System-to-Intermediate
System (IS-IS) and Open Shortest Path First (OSPF) protocols. It can operate
with an MPLS (Multiprotocol Label Switching) or an IPv6 data plane, and it
integrates with the rich multi service capabilities of MPLS, including Layer 3
VPN (L3VPN), Virtual Private Wire Service (VPWS), Virtual Private LAN Service
(VPLS), and Ethernet VPN (EVPN).
Segment routing can be
directly applied to the Multiprotocol Label Switching (MPLS) architecture with
no change in the forwarding plane. Segment routing utilizes the network
bandwidth more effectively than traditional MPLS networks and offers lower
latency. A segment is encoded as an MPLS label. An ordered 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 routing can be
applied to the IPv6 architecture with a new type of routing extension header. A
segment is encoded as an IPv6 address. An ordered list of segments is encoded
as an ordered list of IPv6 addresses in the routing extension header. The
segment to process is indicated by a pointer in the routing extension header.
The pointer is incremented, after the completion of a segment.
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.