Understanding Adjacency Segments, Anycast Segments, and Configurable SRGB in SPRING
Segment routing (SR) or Source Packet Routing in Networking (SPRING) is a control-plane architecture that enables an ingress router to steer a packet through a specific set of nodes and links without relying on the intermediate nodes in the network to determine the actual path it should take. SPRING enables automation of a network by using a software-defined network (SDN) controller for traffic steering and traffic engineering in a WAN packet network. To steer packets through the specified set of nodes and links, the ingress router prepends packets with segments that contain an appropriate combination of tunnels. Each segment is associated with an identifier, which is referred to as the segment identifier (SID). An ordered list of segments is encoded as a stack of labels. Every node in the segment routing domain is allocated labels based on the availability of the dynamic label range. A segment routing global block (SRGB) is the range of label values reserved for segment routing.
Starting in Junos OS Release 17.2R1, you can define the SRGB for the IS-IS protocol, and provide prefix anycast segments in addition to node segments to prefixes that are advertised by the IS-IS protocol through policy configuration. Junos OS also extends support to SPRING anycast segments and configurable adjacency segment indexes for the IS-IS protocol.
Benefits of Anycast Segments, Adjacency Segments, and Configurable SRGB
With the support for anycast prefix segments on Junos OS, you can configure multiple routers to advertise the same prefix with the same SID, which facilitates load balancing.
Configuring the adjacency hold time helps retain segments for a specified period of time after a link flaps and ensures faster convergence after a link fails.
Configuring the SRGB label range ensures that the labels are more predictable across segment routing domain.
Configurable Segment Routing Global Block
A segment is encoded as an MPLS label. An ordered list of segments is encoded as a stack of labels. Every node in the segment routing domain is allocated labels by the node label manager based on the index range configured for source packet routing. These labels are allocated to the node segment based on the availability of the dynamic label range managed by node label manager. An SRGB is the range of label values used in segment routing. You can configure an available SRGB label range for the IS-IS and OSPF protocols so that the labels are predictable across segment routing domains. Ensure that the configured SRGB labels are not used by any other application.
Adjacency Segments and Prefix Segments
A node steers a packet to its destination through an ordered list of instructions, called segments. Essentially, segment routing engages interior gateway protocols (IGPs) such as IS-IS and OSPF to advertise two types of network segments:
Adjacency segments—A strict forwarded single-hop tunnel that carries packets over a specific link between two nodes, irrespective of the link cost.
Prefix segments—A multihop tunnel that uses equal cost multi-hop aware shortest path links to reach a prefix. The prefix SID supports both IPv4 and IPv6 prefixes. A node segment is a special case of prefix segment that uses shortest path links between two specific nodes. An anycast segment is also a type of prefix segment that identifies a set of routers to advertise the same prefix with the same SID value.
Configurable Adjacency Segment Hold Time
The IS-IS protocol creates adjacency segments per adjacency, level, and address family (one each for IPv4 and IPv6). An MPLS label is allocated for each adjacency segment that gets created. These labels are allocated after the adjacency status of the segment changes to the up state. Starting in Junos OS Release 17.2R1, you can configure a hold time to ensure that IS-IS does not release the segments immediately after a link flaps or goes down, but retains them for the configured hold time duration. The default hold time for adjacency segments in IS-IS protocol is 300 seconds.
The OSPF protocol creates adjacency segments per adjacency. To ensure adjacency segments are retained during adjacency or link flaps, the adjacency segments are not released immediately during the link down. The default hold time for adjacency segments in OSPF protocol is 180 seconds.
Prefix Segment Index
Currently, Junos OS enables you to configure a SPRING node SID for IPv4 and IPv6 address families for each routing instance. This node SID is attached to an IPv4 and IPv6 router ID if the router ID is configured on the loopback interface. Otherwise, the lowest IP address assigned to the loopback interface is chosen as the node SID. Configuring a node SID through policy allows you to choose the loopback address that gets the node SID. If the node SID configuration exists and a policy is defined for node SID selection for the same prefix, then the policy configuration takes precedence.
Starting in Junos OS Release 17.2R1, you can designate prefix segment indexes to prefix SIDs, both anycast and node SIDs, that are advertised in IS-IS through policy configuration. Remote routers use this index to consolidate prefixes into respective SRGBs and to derive the segment identifier and forward the traffic destined for a specific prefix. After the prefix segment indexes are provisioned, the devices running Junos OS advertise them in one or more of the following IS-IS TLV types by using a new Prefix-SID Sub-TLV (type 3):
IP Prefix TLV (type 135)
MT IP Prefix TLV (type 235)
IPV6 Prefix Reachability TLV (type 236)
MT IPV6 Prefix Reachability TLV (type 237)
Starting in Junos OS Release 19.1, you can similarly designate prefix segment indexes to prefix SIDs, both anycast and node SIDs, that are advertised in OSPF through policy configuration. Remote routers use this index to consolidate prefixes into respective SRGBs and to derive the segment identifier and forward the traffic destined for a specific prefix.
Anycast Segments
An IGP anycast segment is an IGP prefix segment that identifies a set of routers. An anycast segment enforces forwarding based on the equal-cost multipath-aware shortest-path toward the closest node of the anycast set. Within an anycast group, all the routers advertise the same prefix with the same SID value, which facilitates load balancing.