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Tune the IPv6 neighbor discovery process

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Eventually we will all switch over to IPv6 as more and more things become addressable over IP networks. IPv6 has...

a new protocol (RFC 2461) called Neighbor Discovery (ND) that is used to find nodes' Layer 2 addresses. In that regard it is similar to IPv4's Address Resolution Protocol. A brief description of the protocol, Neighbor advertisement and Neighbor solicitation may be found at: http://www.ip6.com/us/book/Chap2.pdf. A much more complete description may be found in the book "IPv6 for Cisco IOS", by multiple authors, from Syngress Press, or you can see it online.

There are times when the Neighbor Discovery process can hang up or take a long time to resolve. However, there are some steps you can use to tune this process so that any node failure is more quickly communicated. Assuming that you are working with a Cisco router, the parameters you should work with are:

  • ipv6 nd reachable-time. This sets how long a node has to respond. For advertisements the default is 0 ms and for discovery it is 3000 ms. If you lower the value of the discovery, you get a faster response but experience more traffic.
  • ipv6 nd ra-interval. This parameter sets advertisement intervals, with a 200 second default.
  • ipv6 nd ra-lifetime. The router lifetime value is how long an advertisement stays active. The default is 1800 seconds.
  • ipv6 nd dad attempts. The DupAddrDetectTransmit parameter controls the number of solicitation messages active on an interface when address detection is active on a unicast IPv6 interface. The default is one message, but the range is 0 to 600.
  • ipv6 nd prefix advertisement. This setting controls which IPv6 prefixes to use in router advertisements.
  • ipv6 nd suppress-ra. With this command you can suppress advertisements on your LAN. Here router advertisements occur if uincast routing is used. The command with no parameter applied to any non-LAN interface will enable router advertisements.
  • ipv6 nd manage-config-flag. With this command you can set the managed address configuration flag in advertisements. This also lets hosts know if they should obtain their addresses using DHCPv6 or not.

Barrie Sosinsky is president of consulting company Sosinsky and Associates (Medfield MA). He has written extensively on a variety of computer topics. His company specializes in custom software (database and Web related), training and technical documentation.


This was last published in December 2002

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