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Wednesday, January 23, 2013

Testing DHCPv6: Part 2

In a previous post, I talked about getting a DHCPv6 client on a linux image to use with Qemu and GNS3. That post was mainly focused on documenting the steps to get the DHCPv6 client on the host and ultimately working. I neglected to talk about the DHCPv6 server that I configured on the Cisco router.

Of course, I was just bit by a minor configuration miss that I learned in that exercise and quickly forgot until gently reminded as I watched my DHCPv6 simulation NOT work.

The DHCPv6 server configuration on the router is pretty simple:

ipv6 dhcp pool DHCPv6
 address prefix 2001:DB8:A64:6800::/64
 dns-server 2001:4860:4860::8888
 domain-name dynamips.com

But what I forgot was that SLAAC is mandatory for IPv6 nodes and will work as long as router advertisements are present. So simply setting the M-bit in the router advertisements to force DHCPv6 is not enough. You need to NOT advertise the IPv6 prefix on the interface.

So the interface configuration looks like:

interface FastEthernet2/0
 ipv6 address 2001:DB8:A64:6800::1/64
 ipv6 enable
 ipv6 nd prefix 2001:DB8:A64:6800::/64 no-advertise
 ipv6 nd managed-config-flag
 no ipv6 redirects
 no ipv6 unreachables
 ipv6 dhcp relay destination 2001:DB8:A01:100::1

In the above example, I'm sending the DHCPv6 requests to the DHCPv6 server running on another Cisco router. I've set the M-bit and I've also stopped router advertisements of the prefix with the 'ipv6 nd prefix 2001:DB8:A64:6800::/64 no-advertise' command.

And now it works!

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