Segment Routing v6 Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Releases

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Segment Routing v6 Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Releases

SRv6 Locators

Updated: September 8, 2026

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Explains how SRv6 locators organize SID address spaces and provide the addressing structure required for SID allocation, routing, and forwarding within an SRv6 domain.

An SRv6 locator is a unique IPv6 address prefix that

identifies a block of Segment Identifiers (SIDs) in an SRv6-enabled network
belongs to a specific node in an SRv6-enabled network, and
enables routing and forwarding based on SRv6 SIDs.

A SID is a 128-bit value in SRv6 networks that represents a specific instruction or path in the network. For more information, see Segment Identifiers.

Structure of a SRv6 locator

The structure of an SRv6 locator consists of the most significant bits of the SID and serves as the address of a specific SRv6 node. The locator can be further divided into:

SID block: The most significant portion that designates the SRv6 domain.This field is the SRv6 network designator and is a fixed or known address space for an SRv6 domain. This is the most significant bit (MSB) portion of a locator subnet.
Node Id: This field is the node designator in an SRv6 network and is the least significant bit (LSB) portion of a locator subnet.

Types of locators

SRv6 supports several locator types to identify nodes and services within an SRv6 domain. Each locator type addresses specific requirements in network design and operation. The main SRv6 locator types are Base Format locator (F1), uSID locator (F3216), and Anycast locator.

Table 1. Comparison of SRv6 locator types
Feature	Base Format Locator (F1)	uSID Locator (F3216)	Anycast Locator
Definition	The Base Format locator is a standard SRv6 locator that identifies a specific router or node within the SRv6 domain. It uses a prefix to indicate the node and a function identifier (Func ID) to specify what action the node should perform.	The uSID locator is an advanced SRv6 locator that compresses multiple SRv6 segments into a single IPv6 address.	An anycast locator is a shared locator that identifies a group of nodes (Anycast group). All nodes in the group advertise the same locator, allowing traffic to be routed to the nearest node.
Components	Locator prefix and Function identifier (Func ID)	Locator prefix Micro-SID stack (uSIDs)	Anycast locator Prefix Function identifier (Func ID)
Format	`<Locator Prefix>::<Function ID>`	`<Locator Prefix>::<uSID1>:<uSID2>:<uSID3>`	`<Anycast Locator Prefix>::<Function ID>`
Example	`2001:db8:1::1` Prefix: 2001:db8:1::/48 Func ID: 1	`2001:db8:2::100:200:300` Prefix: 2001:db8:2::/48 uSIDs: 100, 200, 300	`2001:db8:3::2` Prefix: 2001:db8:3::/48 advertised by multiple nodes Func ID: 2
Usecase	Node Identification Simple SRv6 Deployments	Efficient Path Encoding Service Chaining	Redundancy and Load Balancing Exit Points in SRv6 Domains

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