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Understand SRv6 Static Segment Identifier

Segment Routing for IPv6–Traffic Engineering (SRv6-TE) is a source routing architecture that leverages IPv6 extension headers to encode network paths. Unlike traditional routing protocols that rely on hop-by-hop forwarding decisions, SRv6-TE allows the source node to specify the complete path through the network using a list of segment identifiers (SIDs).

For a comprhensive overview on SRv6 network programming in IS-IS networks and the related terminology, see How to Enable SRv6 Network Programming in IS-IS Networks and Example: Configuring SRv6 Network Programming in IS-IS Networks

In Segment Routing for IPv6–Traffic Engineering (SRv6-TE), a segment identifier (SID) represents a 128-bit IPv6 address that directs traffic to a specific node and function in the SRv6-TE network. The Static Segment Identifier (SID) Configuration feature enhances network traffic management by allowing static allocation of SIDs for micro and classic locators. This functionality is pivotal for environments where traditional Interior Gateway Protocols (IGP) like IS-IS are not utilized, providing an alternative path computation method. By enabling the advertisement of these static SIDs over BGP, you can facilitate segment routing traffic engineering, improving both efficiency and flexibility.

Key SRv6 Concepts

  • Segment Identifier (SID): An IPv6 address that identifies a specific instruction or segment in the network path.

  • Locator: The portion of the SID that identifies the network location.

  • Static SID: A manually configured SID that does not require IGP advertisement.

  • A SID automatically allocated and advertised by routing protocols like IS-IS.

Benefits of Segment Routing for IPv6 Static Segment Identifier Configuration

  • Enhance network traffic management by allowing the use of static SIDs in environments lacking traditional IGP routing protocols, ensuring effective path computation and traffic engineering.

  • Facilitate segment routing traffic engineering by enabling static SID advertisement over BGP, which improves network efficiency and flexibility.

  • Provide robust operational support through SRv6 OAM features like ping and traceroute, aiding in effective network troubleshooting and maintenance.

  • Support configuration for both primary and backup interfaces, ensuring network resilience and continuity in case of interface failures.

  • Allow integration of static SIDs with dynamic tunnels for Layer 3 VPN services, supporting service resolution and maintaining network service quality.

Configuration Overview

The SRv6 Static SID Configuration feature allows you to define static SIDs in your network, providing precise control over traffic management without the need for traditional IGP protocols like IS-IS. Use static SIDs in the following scenarios:

  • Networks without IGP protocols (IS-IS, OSPF) where dynamic SID allocation is not available.

  • Traffic engineering paths that require explicit, unchanging segment identifiers.

  • Testing and validation environments where predictable SID values simplify troubleshooting.

  • Border routers connecting to external networks where you want manual control over advertised segments.

Configuring Flavors for Micro-SID Locators

Flavors modify how the SRv6 data plane processes SIDs. You configure flavors at the locator level, and all SIDs under that locator inherit the configured behavior. Choose the appropriate flavor based on your network requirements:

  • Use PSP to reduce processing overhead at the final destination.

  • Use USD when you need to decapsulate the outer IPv6 header.

  • Use USP for standard segment routing header removal.

SRv6 manager supports micro-sid locators under the [edit routing-options source-packet-routing srv6] hierarchy.

Allocating Node SIDs for Micro-SID Locators

A node SID identifies the router itself within the SRv6 domain. When you configure a micro node SID, the SRv6 manager allocates the SID and can advertise it via BGP. If IS-IS is also configured, it installs remote micro node SIDs in the inet6.3 routing table for traffic engineering purposes.

To allocate a node SID for a micro-SID locator:

Allocating Adjacency SIDs for Micro-SID Locators

An adjacency SID represents a specific link to a neighboring router. Unlike node SIDs that identify destinations, adjacency SIDs direct traffic over explicit links, which is useful for traffic engineering and fast reroute scenarios. You can configure up to eight backup interfaces to provide protection in case the primary interface fails.

To allocate an adjacency SID for a micro-SID locator:

The following is a sample configuration of adjacency SIDs for micro-SID:

The backup statement is used to indicate that the interface is going to act as a protection for the adjacency SID. This allows to form a non-weighted ECMP for the adjacency SID and in the event of primary interface going down, the PFE switches the traffic to backup.

The next-hop statement is required in the case of broadcast links where the next hop address must be explicitly mentioned for proper forwarding.

Allocating END SIDs and END-X SIDs for Classic Locators

SRv6 manager supports allocation of static and dynamic END SIDs and END-X SIDs for classic locators. You can now allocate static END SIDs and END-X SIDs with SRv6 as the application. Flavors for classic SIDs can be configured as follows:

You can allocate END SIDs for classic locators along with flavors PSP, USD and USP. If IS-IS is configured, it installs remote END SIDs in the inet6.3 table. Remote END SIDs allocated by SRv6 manager is advertised through BGP export policy and is installed in inet6.0 table.

You can allocate END-X SIDs for classic locators along with flavors PSP, USD and USP. Static SRv6 routes for END-X SIDs is updated on any interface events. Like micro-adjacency SIDs, when the primary or backup interface goes down, the corresponding next hop is removed from the route and is added back once the interface becomes available. Like micro adjacency SIDs, END-X SIDs supports backup interfaces and next-hop. For a static END-X SID, up to eight interfaces can be configured as backup.

The following is a sample configuration of END-X SIDs:

Considerations for Combining IS-IS and Static SIDs

It is possible for both IS-IS and SRv6 manager to configure static SIDs for the same locator. However, conflicts can arise when both attempt to allocate the same SID values. Understanding these interactions helps you avoid configuration errors. Consider the following when configuring both IS-IS and static SIDs:

  • When both IS-IS and SRv6 manager configure the same static micro adjacency SID, classic adjacency SID, or classic node SID, a commit check fails due to duplicate allocation. To resolve this, ensure that IS-IS and static SID configurations use different SID values within the same locator..

  • Micro node SID can be configured by both IS-IS and SRv6 manager without commit check failures. When both are configured, the IS-IS SID route takes precedence over the SRv6 manager route due to protocol preference. This allows IS-IS to dynamically manage the node SID while maintaining a static fallback configuration.

  • IS-IS can configure non-zero algorithm SIDs for flexible algorithm support, but SRv6 manager only installs algorithm-0 (default) SIDs in the inet6.0 table, even when an algorithm is configured for the locator. IS-IS installs non-zero algorithm SIDs in the inet6color.0 table. This separation allows both static and dynamic SIDs to coexist using the same locator while supporting different traffic engineering algorithms..