IPv6 Default Router Preference

The IPv6 default router preference feature provides a coarse preference metric (low, medium, or high) for default devices.

Finding Feature Information

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Information About IPv6 Default Router Preference

Default Router Preferences for Traffic Engineering

Hosts discover and select default devices by listening to Router Advertisements (RAs). Typical default device selection mechanisms are suboptimal in certain cases, such as when traffic engineering is needed. For example, two devices on a link may provide equivalent but not equal-cost routing, and policy may dictate that one of the devices is preferred. Some examples are as follows:

  • Multiple devices that route to distinct sets of prefixes—Redirects (sent by nonoptimal devices for a destination) mean that hosts can choose any device and the system will work. However, traffic patterns may mean that choosing one of the devices would lead to considerably fewer redirects.

  • Accidentally deploying a new device—Deploying a new device before it has been fully configured could lead to hosts adopting the new device as a default device and traffic disappearing. Network managers may want to indicate that some devices are more preferred than others.

  • Multihomed situations—Multihomed situations may become more common, because of multiple physical links and because of the use of tunneling for IPv6 transport. Some of the devices may not provide full default routing because they route only to the 6-to-4 prefix or they route only to a corporate intranet. These situations cannot be resolved with redirects, which operate only over a single link.

The default router preference (DRP) feature provides a basic preference metric (low, medium, or high) for default devices. The DRP of a default device is signaled in unused bits in RA messages. This extension is backward compatible, both for devices (setting the DRP bits) and hosts (interpreting the DRP bits). These bits are ignored by hosts that do not implement the DRP extension. Similarly, the values sent by devices that do not implement the DRP extension will be interpreted by hosts that do implement it as indicating a “medium” preference. DRPs need to be configured manually.

How to Configure IPv6 Default Router Preference

Configuring the DRP Extension for Traffic Engineering

Perform this task to configure the DRP extension to RAs in order to signal the preference value of a default router.

SUMMARY STEPS

    1.    enable

    2.    configure terminal

    3.    interface type number

    4.    ipv6 nd router-preference {high | medium | low


DETAILED STEPS
     Command or ActionPurpose
    Step 1 enable


    Example:
    Router> enable
     

    Enables privileged EXEC mode.

    • Enter your password if prompted.

     
    Step 2 configure terminal


    Example:
    Router# configure terminal
     

    Enters global configuration mode.

     
    Step 3 interface type number


    Example:
    Router(config)# interface gigabitethernet 0/0/0
     

    Specifies the interface type and number, and enters interface configuration mode.

     
    Step 4 ipv6 nd router-preference {high | medium | low


    Example:
    Router(config-if)# ipv6 nd router-preference high
     

    Configures a DRP for a router on a specific interface

     

    Configuration Examples for IPv6 Default Router Preference

    Example: IPv6 Default Router Preference

    The following example displays the state of the DRP preference value as advertised by this device through an interface: 

     
  Device# show ipv6 interface gigabitethernet 0/1

  GigabitEthernet0/1 is up, line protocol is up
    IPv6 is enabled, link-local address is FE80::130
    Description: Management network (dual stack)
    Global unicast address(es):
      FEC0:240:104:1000::130, subnet is FEC0:240:104:1000::/64
    Joined group address(es):
      FF02::1
      FF02::2
      FF02::1:FF00:130
    MTU is 1500 bytes
    ICMP error messages limited to one every 100 milliseconds
    ICMP redirects are enabled
    ND DAD is enabled, number of DAD attempts: 1
    ND reachable time is 30000 milliseconds
    ND advertised reachable time is 0 milliseconds
    ND advertised retransmit interval is 0 milliseconds
    ND router advertisements are sent every 200 seconds
    ND router advertisements live for 1800 seconds
    ND advertised default router preference is Low
    Hosts use stateless autoconfig for addresses.

    The following example displays the state of the DRP preference value as advertised by other devices: 

    Device# show ipv6 routers

  Router FE80::169 on GigabitEthernet0/1, last update 0 min
    Hops 64, Lifetime 1800 sec, AddrFlag=0, OtherFlag=0, MTU=1500
    Preference=Medium
    Reachable time 0 msec, Retransmit time 0 msec
    Prefix FEC0:240:104:1000::/64 onlink autoconfig
      Valid lifetime 2592000, preferred lifetime 604800

    Additional References

    Related Documents

    Related Topic

    Document Title

    IPv6 addressing and connectivity

    IPv6 Configuration Guide

    Cisco IOS commands

    Cisco IOS Master Commands List, All Releases

    IPv6 commands

    Cisco IOS IPv6 Command Reference

    Cisco IOS IPv6 features

    Cisco IOS IPv6 Feature Mapping

    Standards and RFCs

    Standard/RFC

    Title

    RFCs for IPv6

    IPv6 RFCs

    MIBs

    MIB

    MIBs Link

    No new or modified MIBs are supported by this feature, and support for existing MIBs has not been modified by this feature.

    To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:

    http:/​/​www.cisco.com/​go/​mibs

    Technical Assistance

    Description

    Link

    The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password.

    http:/​/​www.cisco.com/​cisco/​web/​support/​index.html

    Feature Information for IPv6 Default Router Preference

    The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

    Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/​go/​cfn. An account on Cisco.com is not required.
    Table 1 Feature Information for IPv6 Default Router Preference

    Feature Name

    Releases

    Feature Information

    IPv6 Default Router Preference

    12.2(33)SRA

    12.2(33)SXH

    12.2(46)SE

    12.2(46)SG

    12.4(2)T

    15.0M

    15.0(2)SG

    3.2.0SG

    Cisco IOS XE Release 3.9S

    This feature provides a basic preference metric (low, medium, or high) for default devices.

    In Cisco IOS XE Release 3.9S, support was added for the Cisco ISR 4400 Series Routers.

    In Cisco IOS XE Release 3.9S, support was added for the Cisco CSR 1000V.

    The following commands were introduced or modified: ipv6 nd router-preference, show ipv6 interface, show ipv6 router.