Hello all, RFC2373 (addrarch) defines Subnet-Router Anycast address:
--- 2.6.1 Required Anycast Address The Subnet-Router anycast address is predefined. Its format is as follows: | n bits | 128-n bits | +------------------------------------------------+----------------+ | subnet prefix | 00000000000000 | +------------------------------------------------+----------------+ [...] Packets sent to the Subnet-Router anycast address will be delivered to one router on the subnet. All routers are required to support the Subnet-Router anycast addresses for the subnets which they have interfaces. --- In practise this means that /127 prefix length must not be used in point-to-point links, because then the other end would always have to configure the subnet router anycast address as its _ipv6 address_. This is bad because one router of the two would respond to it (depending on DAD), not necessarily the one which configured it. Not nice. Example: 3ffe:ffff::2/127 configured for router A, and 3ffe:ffff::3/127 for router B. 3ffe:ffff::2 is also subnet-router anycast address, which router B should also recognize. Why everything still works: very few implementations do implement subnet-router anycast addresses. Recommendation: /64 or /126 for Point-to-point links. Another alternative: no subnet-router anycast address for those subnets where prefix >64 bits. Caveat: we've worked hard not to hardcode 64 in too many places.. Or is there something I've missed? If this would use a more "formal" approach, I could write a 2-page informational I-D of this (or more generally, on choosing p-t-p prefix length) I suppose. -- Pekka Savola "Tell me of difficulties surmounted, Netcore Oy not those you stumble over and fall" Systems. Networks. Security. -- Robert Jordan: A Crown of Swords -------------------------------------------------------------------- IETF IPng Working Group Mailing List IPng Home Page: http://playground.sun.com/ipng FTP archive: ftp://playground.sun.com/pub/ipng Direct all administrative requests to [EMAIL PROTECTED] --------------------------------------------------------------------
