Cogent IPv6

Ray Soucy rps at
Thu Jun 9 17:19:17 UTC 2011

Don't assume that DHCPv6 is the same as DHCP.

DHCPv6 does not provide route information because this task is handled
by RA in IPv6.

An IPv6 RA has flags for Managed (M), Other (O), and Autonomous (A)
address configuration.  None of these flags are exclusive.

While most routers have the A flag set by default (which enables
stateless addressing) it can be disabled, and hosts will not pick up a
stateless address as a result.

The M flag tells hosts to make use of DHCPv6 for an address, and the O
flag tells hosts to make use of DHCPv6 for additional configuration,
such as DNS.

Most popular configurations:

You can use the A and O flag for stateless addressing with DHCPv6 for DNS.

You can use A, M, and O flags if you want every host to have a
stateless address, but want to use DHCPv6 to also give some hosts a
predictable address (e.g. for servers), and have them use DHCPv6 for
DNS information.

You can have only the M and O flags set and hosts will only use DHCPv6
for configuration.

Most routers also support relaying of DHCPv6 information to a central server.

For those who speak "Cisco" here is an example interface configuration
for DHCPv6 only.

 ipv6 address 2001:DB8:100::1/64
 no ipv6 unreachables
 ipv6 nd reachable-time 900000
 ipv6 nd prefix default 900 300 no-autoconfig
 ipv6 nd managed-config-flag
 ipv6 nd other-config-flag
 ipv6 nd router-preference High
 ipv6 nd ra interval 300
 ipv6 nd ra lifetime 300
 no ipv6 redirects
 ipv6 verify unicast source reachable-via rx
 ipv6 dhcp relay destination 2001:DB8:200::2
 ipv6 dhcp relay destination 2001:DB8:200::3

Leaving out the "no-autoconfig" will also allow stateless if your
prefix-length is 64.  If you don't have a 64-bit prefix stateless
won't work regardless of whether the A flag is set or not.

Also note, if using DHCPv6, a DUID is used instead of the MAC address,
though 2 out of 3 valid DUID formats include a MAC address of the host
and I haven't actually seen the 3rd implemented.  DUIDs are stored
after the first time they get generated, so if you're imaging hosts
you'll need to included deleting the DUID as part of your imaging
process, or you'll have conflicts.


On Thu, Jun 9, 2011 at 12:59 PM, Iljitsch van Beijnum
<iljitsch at> wrote:
> On 9 jun 2011, at 14:19, sthaug at wrote:
>> It is perfectly possible to use RA *only* for the default router, and
>> not announce any prefix at all. This implies a link-local next hop.
> Router advertisements always use the router's link local address, you can't get a router's global address from this. IPv6 routing protocols also pretty much only use link locals, so link local next hop and default routes are completely routine.

Ray Soucy

Epic Communications Specialist

Phone: +1 (207) 561-3526

Networkmaine, a Unit of the University of Maine System

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