DHCPv6-PD and changing prefix

  • I've noticed that if I reboot my firewall or even temporarily disconnect it from the cable modem, my IPv6 prefix will change.  I thought DHCPv6-PD had a mechanism to prevent this.  I have noticed a setting in "Advanced DHCP6 Client Configuration", "Identity Association Statement", where there's a check box for non-temporary address allocation.  Should this be checked?  What about the boxes beside it?  What, if anything, would go in them?

    tnx jk

  • I don't know what happened, but lately this no longer seems to be an issue.  In fact, there has been another change for the better.  For years, my IPv4 DHCP address was virtually static and the host name, based on modem and firewall MAC addresses was consistent, unless I changed hardware.  In that case, the IPv4 address would change too.  Today, I changed modems, as the one I had was locking up occasionally.  When I connected the new modem, I found the host name had changed as expected, but my IPv4 & IPV6 WAN addresses were the same as before, as was my IPv6 prefix.  So, whatever is different is working well!

  • For curiosity, I asked an engineer at my ISP (Telus) about this.

    Here is his response:

    Your prefix should only really change if the UUID in the dhcp solicit changes, or if your device is offline for a long period of time (a few days) then it may change.

    If you reboot your device, you should get the same prefix regardless of if you ask for it or not.

    Somewhat vague, but better than it seems to be elsewhere. It's a shame that ISPs haven't standardized on something more definitive.

  • When I had that problem, briefly disconnecting the Ethernet cable was enough to cause a change.  It is standardized in DHCPv6-PD.  I supposed it may have a lease time, as DHCP addresses do.  After all, with barely 4000 /48s for each person on earth, we'd soon run out, if unused prefixes weren't reclaimed.  ;)

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