On Wed, Nov 02, 2011 at 10:52:57AM -0700, David Stevens wrote:

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This address seems not to be explicitly mentioned in any RFC, but RFC 
4291 says "All routers are required to support the Subnet-Router 

anycast 

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addresses for the subnets to which they have interfaces."

That this directly contradicts RFC 2526 which specifies the
subnet-router anycast address to be either ::ffff:ffff:ffff:ff80 or
::fcff:ffff:ffff:ff80 depending on the phase of the moon (well,
interface type actually, but same thing. Also, the /64 <> /10
distinction would matter here.)

        The subnet-router anycast address is defined in section 2.6.1 of 
RFC 4291 to be "all 0's" for the prefix. The definition above is for
reserved anycast addresses. RFC 2526 says "IPv6 defines a required
Subnet-Router anycast address [3] for all routers within a subnet prefix,
and allows additional anycast addresses to be taken from the unicast
address space. This document defines an additional set of reserved
anycast addresses...".

Argh. I got thoroughly confused. Please ignore everything I said.

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[...] only one router will receive the packet [...]

        The host implementation is very straightforward. Not every host
on a segment has to use the *same* host for an anycast address (it's
kind of the point that it won't, in fact). A host simply needs to
do a solicitation for the anycast address and keep the first one that
answers (by definition, the "closest").

Right. Sorry. I was half-asleep when I wrote my previous mail and
clearly didn't think through the entire problem. I should probably stop
writing mails when not at least 80% awake :).

Neighbor Discovery can indeed select one of the anycast hosts and talk
to it using its lower-layer address, and no other host should
receive/process the packet.

Going back to Andreas's original question about Subnet-Router Anycast
for fe80::/64 (or /10), RFC 4291 says
   +------------------------------------------------+----------------+
   |                   subnet prefix                | 00000000000000 |
   +------------------------------------------------+----------------+

   The "subnet prefix" in an anycast address is the prefix that
   identifies a specific link.

But fe80::/64 does not identify a specific link, as it is link-local and
would specify all links but not one specifically. So, fe80:: is not a
Subnet-Router anycast address, I'd say.


Hoping I was awake enough this time...


-David