Tag Archives: neighbor cache

  • Know Thy (IPv6) Neighbor

    In IPv6 there are four [tables | lists | data structures] used as part of the packet forwarding process (e.g. Neighbor Discovery and the Conceptual Sending Algorithm).  They are:

    • Neighbor Cache
    • Destination Cache
    • Prefix List
    • Default Router List

    These data structures are currently defined by RFC 4861 (and previously by RFC 1970 and RFC 2461, both now obsolete).  RFC 4861 does a good job describing what these data structures are for and how they should be used but does not, as is typical (and appropriate) to RFC’s, provide any specifics on how they are implemented by each operating system.

    The Neighbor Cache, whose closest conceptual relative in IPv4 is the ARP cache, is primarily a list of IPv6-to-MAC address mappings.  If that were all that was true we could say that the ARP cache and the Neighbor Cache were the same thing.  But they are not.  A Neighbor Cache is much more than an ARP cache.  The Neighbor Cache contains the following information: