IP Addresses and Services Configuration Guide for Cisco 8000 Series Routers, IOS XR Releases

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IP Addresses and Services Configuration Guide for Cisco 8000 Series Routers, IOS XR Releases

Network stack IPv4 and IPv6

Updated: August 12, 2026

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Provides comprehensive instructions for configuring and monitoring IPv4 and IPv6 features on interfaces, covering addressing, dual-stack operation, IPv6 Neighbor Discovery behavior, and scaling and address-management options for IPv4 and IPv6 deployments.

Network stack IPv4 and IPv6 features are software functions that

  • configure and monitor Internet Protocol Version 4 (IPv4) and Internet Protocol Version 6 (IPv6)

  • support interface addressing, dual-stack operation, and IPv6 neighbor discovery, and

  • provide scaling and address-management options for IPv4 and IPv6 deployments.

An interface can use multiple IPv6 global addresses. However, multiple IPv6 link-local addresses on the same interface are not supported.

Network stack IPv4 and IPv6 exceptions

Use this reference to confirm exceptions before you operate IPv4 and IPv6 interfaces, neighbor entries, or address conflict policies.

Table 1. Network stack exceptions

Exception

Behavior

IPv6 neighbor commands

The clear ipv6 neighbors and show ipv6 neighbors commands include the location node-id keyword. If you specify a location, the command displays only neighbor entries in that location.

Stale IPv6 neighbor entries

The ipv6 nd scavenge-timeout command sets the lifetime for neighbor entries in the stale state. When the scavenge timer expires, the entry is cleared.

IPv4 and IPv6 interface commands

The show ipv4 interface and show ipv6 interface commands include the location node-id keyword. If you specify a location, the command displays only interface entries in that location.

IP address conflict policy

Cisco IOS XR software allows conflicting IP address entries at configuration time. If a conflict exists between two active interfaces, the software brings down the interface according to the configured conflict policy. By default, the higher interface instance is brought down.

For example, if HundredGigE0/0/0/1 conflicts with HundredGigE0/0/0/2, the software brings down the IPv4 protocol on HundredGigE0/0/0/2. IPv4 remains active on HundredGigE0/0/0/1.

IPv4 and IPv6 functionality

IPv4 and IPv6 functionality is dual-stack interface support that

  • lets an interface send and receive data on both IPv4 and IPv6 networks

  • helps existing IPv4 deployments transition to IPv6 while retaining IPv4 connectivity, and

  • uses IPv6 capabilities such as larger address space, simplified headers, prefix aggregation, and neighbor discovery.

IPv6 protocol context

IPv6, formerly named IPng, is a packet-based protocol used to exchange data, voice, and video traffic over digital networks. IPv6 was proposed when the 32-bit IPv4 addressing scheme became insufficient for Internet growth.

IPv6 adds a larger address space, a simplified main header, and extension headers. RFC 2460 describes the initial IPv6 specification from the Internet Engineering Task Force (IETF).

IPv6 design benefits

IPv6 design provides these benefits:

  • Larger address space for scale and global reachability.

  • Simplified packet header handling for more efficient packet processing.

  • Prefix aggregation, simplified network renumbering, and site multihoming for a more efficient addressing hierarchy.

  • Support for routing protocols such as Open Shortest Path First (OSPF) and multiprotocol Border Gateway Protocol (BGP).

IPv6 neighbor discovery role

The IPv6 neighbor discovery process uses Internet Control Message Protocol (ICMP) messages and solicited-node multicast addresses to determine the link-layer address of a neighbor on the same local link, verify neighbor reachability, and track neighboring routers.

Host route scale support

Host route scale support is a connected host-route scaling feature that

  • increases IPv4 and IPv6 host route scale values

  • uses an alternative programming method so that next-hop scale limitations are not applied to connected host routes, and

  • lets the router support both IPv4 and IPv6 host routes at a larger scale.

Table 2. Feature History Table
Feature Name Release Information Feature Description
Host route scale support Release 25.4.1

This feature provides an alternative programming approach to configure the connected host route, enhancing the scalability for both IPv4 and IPv6.

Introduced in this release on: Fixed Systems (8010 [ASIC: A100], 8200 [ASIC: P100], 8700 [ASIC: P100], 8700 [ASIC: K100])(select variants only*)

*This feature is supported on:

  • 8011-4G24Y4H-I

  • 8011-32Y8L2H2FH

  • 8011-12G12X4Y-A/D

  • 8212-48FH-M

  • 8711-32FH-M

  • 8712-MOD-M

The feature introduces the hw-module profile route scale host-route command.

Configure host route scale support

Configure host route scale support when connected IPv4 and IPv6 host routes require higher scale and must avoid next-hop scale limitations.

Procedure

  1. Configure maximum scaling for connected host routes.

    Example:

    Router# configure
Router(config)# hw-module profile route scale host-route
Router(config)# commit

  2. Reload the router or the affected line cards.

    You must reload the chassis or all affected line cards to activate or deactivate host-route scale mode.

  3. Verify the host-route scale status.

    Example:

    Router# show hw-module profile route-scale
--------------------------------------------------------------
Knob                 Status        Applied       Action
--------------------------------------------------------------
IPv6-Pfx-Expansion   Unconfigured  N/A           None
LPM-Tcam-Scale       Unconfigured  N/A           None
Wide-Entries-Shortened Unconfigured N/A          None
LPM-CEM-Scale        Unconfigured  N/A           None
LPM-Full-Scale       Unconfigured  N/A           None
Host-Route-Scale     Configured    Yes           None

IPv6 prefix scale expansion

IPv6 prefix scale expansion is a route-scale feature that

  • stores long connected IPv6 prefixes in Central Exact Match (CEM) memory

  • expands each /126 prefix into four /128 prefixes and each /127 prefix into two /128 prefixes, and

  • helps avoid Longest Prefix Match (LPM) memory allocation failures when many /126 and /127 IPv6 prefixes are present.

Table 3. Feature History Table
Feature Name Release Information Feature Description

IPv6 prefix scale expansion

Release 25.1.1

ntroduced in this release on: Fixed Systems (8010 [ASIC: A100])(select variants only*)

*This feature is supported on Cisco 8011-4G24Y4H-I routers.

IPv6 prefix scale expansion

Release 24.4.1

Introduced in this release on: Fixed Systems (8200 [ASIC: P100], 8700 [ASIC: P100, K100])(select variants only*); Modular Systems (8800 [LC ASIC: P100])(select variants only*)

*This feature is now supported on:

  • 8212-48FH-M

  • 8711-32FH-M

  • 8712-MOD-M

  • 88-LC1-36EH

  • 88-LC1-12TH24FH-E

  • 88-LC1-52Y8H-EM

IPv6 prefix scale expansion

Release 7.3.5

You can use Central Exact Match (CEM) memory to store /126 and /127 IPv6 prefixes when the prefixes do not fit in Longest Prefix Match (LPM) memory.

To store these prefixes in CEM memory, expand each /126 prefix into four /128 prefixes and each /127 prefix into two /128 prefixes.

In earlier releases, Cisco IOS XR software stored /126 and /127 IPv6 prefixes only in LPM memory.

This feature introduces these commands:

Guidelines for IPv6 prefix scale expansion

Supported routers

Use IPv6 prefix scale expansion only on Q100 or Q200 ASIC-based Cisco 8000 routers. To know more about Q100 and Q200 ASIC-based Cisco 8000 routers, see the Cisco 8000 Series Router data sheet.

  • Q100 ASIC-based Cisco 8000 routers support 256,000 Central Exact Match (CEM) scale entries.

  • Q200 ASIC-based Cisco 8000 routers support 608,000 CEM scale entries.

Reload the router after you configure IPv6 prefix scale expansion for the feature to take effect.

Configure IPv6 prefix scale expansion

Configure IPv6 prefix scale expansion when many /126 or /127 connected IPv6 prefixes must be stored in CEM memory instead of LPM memory.

Procedure

  1. Configure the IPv6 unicast connected-prefix route-scale profile.

    Example:

    Router# configure
Router(config)# hw-module profile route scale ipv6-unicast connected-prefix high
Router(config)# commit
Router(config)# end

  2. Reload the router for the feature to take effect.

    Example:

    Router# reload location all
Proceed with reload? [confirm] y

  3. Verify that the route-scale profile is applied.

    Example:

    Router# show hw-module profile route-scale
--------------------------------------------------------------
Knob          Status       Applied       Action
--------------------------------------------------------------
Route-Scale   Configured   Yes           None

  4. Verify the expanded IPv6 prefixes.

    Example:

    Router# show cef ipv6 2001:DB8:23:1::/126 hardware egress detail location 0/0/CPU0
leaf npd data:
Specific route may have overwritten NH for expanded prefix.
In that case, use specific route prefix for this command.
Expanded_Prefix0:{v6addr: 2001:0DB8:0023:0001:0000:0000:0000:0000}/128
Expanded_Prefix1:{v6addr: 2001:0DB8:0023:0001:0000:0000:0000:0001}/128
Expanded_Prefix2:{v6addr: 2001:0DB8:0023:0001:0000:0000:0000:0002}/128
Expanded_Prefix3:{v6addr: 2001:0DB8:0023:0001:0000:0000:0000:0003}/128

Router# show cef ipv6 2001:DB8:23:1::/127 hardware egress detail location 0/0/CPU0
leaf npd data:
Specific route may have overwritten NH for expanded prefix.
In that case, use specific route prefix for this command.
Expanded_Prefix0:{v6addr: 2001:0DB8:0023:0001:0000:0000:0000:0000}/128
Expanded_Prefix1:{v6addr: 2001:0DB8:0023:0001:0000:0000:0000:0001}/128
npu: 0, npu_rc: 0
npu: 1, npu_rc: 0