IPv6 Anycast



  • I have some ipv6 anycast issues/questions.

    1. Reserved highest 128 addressing:

    I am working towards making a IPv6 address allocation standard and want to make sure I do not step on any of the reserved toes. I am a bit confused on the anycast side of things and I am hoping an expert may be able to shed some light.

    RFC2526 states that for subnets with the interface identifier field in EUI-64 format the universal/local bit needs to be set to 0. Where I am confused is with how that is represented in the actual IPv6 address. In the EUI-64 format if the universal/local bit is set to 0 the address is globally administered and if it is set to 1 it is locally administered. However, an IPv6 address is usually created following the modified EUI-64 format in which the universal/local bit is inverted, so that 0 means locally administered and 1 means globally administered.

    So do I take the format in RFC2526 exactly, so that I am actually using a 0 in the universal/local bit which makes sense as the IP is being manually assigned (locally administered). If my subnet prefix is 2001:490:10:c4a::/64, then the reserved anycast range would be what is below.

    2001:490:10:c4a:FDFF:FFFF:FFFF:FF80
    2001:490:10:c4a:FDFF:FFFF:FFFF:FFFF

    However, this contradicts the RFC2526 statement, "Within each subnet, the highest 128 interface identifier values are reserved for assignment as subnet anycast addresses." If the highest 128 interface identifier values are reserved for assignment as subnet anycast addresses, then bits 65 - 121 should all be 1making reserved anycast range what is below.

    2001:490:10:c4a:FFFF:FFFF:FFFF:FF80
    2001:490:10:c4a:FFFF:FFFF:FFFF:FFFF

    Can anyone confirm which of these are correct or if I am just way off in left field in my understanding. Also, since the 72nd is a 1, I am guessing that the anycast rules override the multicast rules for the anycast range.

    1. Reserved Subnet-Router anycast address:

    Also as I understand it, 2001:490:10:c4a::/64 is reserved as the Subnet-Router anycast address. Recently, use of smaller subnets like /127 have been approved for point to point links. Does anyone know how this would effect the use of 2001:490:10:c4a:: in a 2001:490:10:c4a::/127. Should 2001:490:10:c4a::/127 just not be used or the fact that it is not a /64 make it OK.

    1. Usage:

    In my reading regarding using anycast addresses, it seems they are useless without router involvement. The examples I have seen use Cisco routers and all the examples show the use of a dedicated subnet for anycast. Is the use of a dedicated subnet for anycast required? I have a working example below and an example below. I would like to know if the modified example should work? If not, is that a Cisco software limitation?

    Example:
    interfase FastEthernet1/1
    no ip address
    duplex auto
    speed auto
    ipv6 address 2001:15::1/64
    ipv6 address 2345::/64 anycast
    ipv6 ospf 1 area 3

    Could it be like this?
    interfase FastEthernet1/1
    no ip address
    duplex auto
    speed auto
    ipv6 address 2001:15::1/64
    ipv6 address 2001:15:/64 anycast
    ipv6 ospf 1 area 3

    1. pfSense:

    Can pfSense act as an anycast router?

    Thanks for any help!



  • Normally with anycast addresses you need a routing broker as BGP/OSPF.
    Anycast is no different from the normal ipv6 networking apart from the routing daemon handling the finding of the nearest neighbor by keeping the routing table correct.


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