Server Fault is a question and answer site for system and network administrators. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I've tried multiple pieces of DHCP client and software in attempting to setup a solution for allocating a /127 prefix to virtual machines so that each maintains its own layer 2 isolation. Because there would only be one host assigned to each network, a /64 is impractical.

While the prefix size could reasonably be somewhere in the /64-127 range, the crux of the problem has been the same regardless of the software used in configuring: the DHCP call to bring up the interface uses the address advertised by DHCPv6 and inserts two routes: the /127 given by the router advertising packets and a /64 as well.

Any thoughts on why I'm getting the additional route added across dhcp client vendors?

share|improve this question
Can you add your RA and DHCPv6 configs? – Sander Steffann Dec 11 '12 at 10:03

SLAAC only work with /64 networks because the host needs the right-most 64 bits for it's generated EUI64 address [1]. So if the host determines it needs to do SLAAC (either because your Router Advertisements indicate so, or the host chooses to ignore your RA flags) thwn it has no choice but to make a /64 address.

You need to make sure your hosts aren't deciding to create SLAAC addresses for themselves. [2]

[1] See "Stateless Address Assignment" on this page

[2] For Linux, this may work: Is there a way to disable IPv6 SLAAC on a per-interface basis in Debian?

share|improve this answer
There are two flags in a router advertisement: the "A" flag (use SLAAC), and the the "M" flag on (use DHCPv6). A is off, M is on. Router Advertisements must be in place regardless of the subnet size because DHCPv6 packets don't include router information the way that DHCPv4 packets do, nor do they include subnet prefix sizes. – Jeff Ferland Dec 17 '12 at 23:49
@JeffFerland You're right, so the OP probably has the A flag set in his/her RA's. I'll edit my answer to correct the discrepancy. – fukawi2 Dec 18 '12 at 3:28
Additionally, DHCPv6 does not provide routing information at all. That is left to the RA, which will be a /64. – bahamat Dec 18 '12 at 4:18
@fukawi2 I am the OP. The A flag is not set. – Jeff Ferland Dec 18 '12 at 7:34
@JeffFerland Whoops, my bad. So have you disabled SLAAC explicitly? – fukawi2 Dec 18 '12 at 8:25

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.