|
North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical Use of IPv6 /127 prefixes
I've run into an interesting question regarding the widespread practice of using /127 prefixes under IPv6. I'm posting this to nanog instead of 6bone because I want to ask nanog IPv6 people. Allocating a /127 is a common convention under IPv6 to provide globally routable addresses for a point to point connection, such as a tunnel. Many tunnel brokers use it, etc. I know that its use is widespread on the links between cisco routers at the core the IPv6 Internet without obvious trouble. However, it would appear to be in conflict with anycast addresses (btw, anycast addresses are not the same as IPv4 broadcast addresses). RFC 2373 section 2.6.1 regarding "Required Anycast Address": The "subnet prefix" in an anycast address is the prefix which identifies a specific link. This anycast address is syntactically the same as a unicast address for an interface on the link with the interface identifier set to zero. 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. So what is anycast used for? I've see lots of links where the the addresses used are NNNN:NNNN:NNNN:NNNN:NNNN:NNNN:NNNN:000E/127 and NNNN:NNNN:NNNN:NNNN:NNNN:NNNN:NNNN:000F/127 which works, yet shouldn't if the 000E address is allways anycast on both sides of the link. Why do /127s work for point to point links? Mike. +------------------- H U R R I C A N E - E L E C T R I C -------------------+ | Mike Leber Direct Internet Connections Voice 510 580 4100 | | Hurricane Electric Web Hosting Colocation Fax 510 580 4151 | | [email protected] http://www.he.net | +---------------------------------------------------------------------------+
|