RFC 3627 (rfc3627) - Page 2 of 6
Use of /127 Prefix Length Between Routers Considered Harmful
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RFC 3627 /127 Prefix Length Considered Harmful September 2003 2. Scope of this Memo This memo does not advocate the use of long prefixes, but brings up problems for those that do want to use them, for one reason or another. Detailed discussion on what is the "right" solution is out of the scope; it is not the goal of this memo to try to find the "best" addressing solution for everyone. 3. Problem with /127 and Two Routers Note that this problem does not exist between a router and a host, assuming the PREFIX::0/127 address is assigned to the router. Using /127 can be especially harmful on a point-to-point link when Subnet-router anycast address is implemented. Consider the following sequence of events: 1. Router A and Router B are connected by a point-to-point link. 2. Neither has anything configured or set up on this link. 3. 3ffe:ffff::1/127 address is added to Router A; now it performs Duplicate Address Detection (DAD) [NDISC] for 3ffe:ffff::1. Router A also adds the Subnet-router anycast address 3ffe:ffff::0/127. (DAD is not performed for anycast addresses.) 4. Now Router B has been planned and configured to use 3ffe:ffff::0/127 as its unicast IPv6 address, but adding it will fail DAD, and Router B does not have any address. Similar scenarios also happen during router reboots, crashes and such. The usability of subnet-router anycast address between two routers on a point-to-point link is very questionable, but it is still a mandated feature of [ADDRARCH]. Workarounds for this are presented in the next section. As of yet, this kind of unexpected behavior hasn't been seen at large perhaps because the Subnet-router anycast address hasn't been implemented or too widely used. Savola Informational
