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

Key concepts of SRv6 uSID

Updated: September 8, 2026

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Defines the terminology and carrier structure used to encode, organize, and process micro-instructions within an SRv6 packet.

The table summarizes the key concepts and roles within the SRv6 network programming framework.

Table 1. Understanding the key concepts of SRv6 uSID

Term

Description

uSID

An identifier that specifies a micro-segment.

A uSID is linked to a specific behavior, which corresponds to the SRv6 function, such as a Node SID or an Adjacency SID, associated with the given identifier.

The node where a uSID is instantiated is referred to as the "Parent" node.

uSID carrier

A 128-bit IPv6 address (carried in either in the packet destination address or in the SRH) in this format.

<uSID-Block><Active-uSID><Next-uSID>...
<Last-uSID><End-of-Carrier>...
<End-of-Carrier>

The uSID carrier format specifies the type of uSID carrier supported in an SRv6 network. The format specification includes block size and ID size.

The uSID carrier format is specified using the notation "Fbbuu" , where “bb” is size of block and “uu” is the size of the ID.

uSID block

The uSID block is an IPv6 prefix that defines a block of SRv6 uSIDs. This can be an IPv6 prefix allocated to the provider (for example, /22, /24, and so on), or it can be any well-known IPv6 address block generally available for private use, such as the ULA space FC/8, as defined in IETF draft RFC4193.

An SRv6 network may support more than a single uSID block.

The length of block [prefix] is defined in bits. From a hardware-friendliness perspective, it is expected to use sizes on byte boundaries (16, 24, 32, and so on).

uSID ID

The length of the uSID ID is defined in bits. From a hardware-friendliness perspective, it is expected to use sizes on byte boundaries (8, 16, 24, 32, and so on).

Active uSID

The first uSID that follows the uSID block is called the Active uSID.

Next uSID

The next uSID after the Active uSID is called the Next uSID.

Last uSID

The last uSID in the carrier before the End-of-Carrier uSID is called the Last uSID.

End-of-Carrier

A globally reserved uSID that marks the end of a uSID carrier. The End-of-Carrier ID is 0000. All empty uSID carrier positions must be filled with the End-of-Carrier ID; therefore, a uSID carrier can have more than one End-of-Carrier.

Structure of micro-sid carriers in SRH

This example shows an SRH with three micro-SID carriers, which can carry up to 18 micro-instructions.

Micro-SID Carrier1: {uInstruction1, uInstruction2… uInstruction6}

Micro-SID Carrier2: {uInstruction7, uInstruction8… uInstruction12}

Micro-SID Carrier3: {uInstruction13, uInstruction14… uInstruction18}

This illustration represents the structure of an SRH containing three micro-SID or segment identifier carriers. Each Micro-SID carrier acts as a container for a set of micro-instructions, with a maximum capacity of 6 micro-instructions per carrier. These micro-instructions are sequentially organized across the carriers.

  • Micro-SID Carrier 1 contains the first 6 micro-instructions (uInstruction1 to uInstruction6).

  • Micro-SID Carrier 2 contains the next set of 6 micro-instructions (uInstruction7 to uInstruction12).

  • Micro-SID Carrier 3 contains the final 6 micro-instructions (uInstruction13 to uInstruction18).

This structure allows up to 18 micro-instructions to be stored and processed efficiently within the SRH. Organizing the instructions into manageable groups makes interpretation and execution easier.