Prefix delegation to multiple interfaces

elscorcho · Dec 23, 2021, 4:56 AM

I have service with Mediacom. My WAN interface configured for DHCP6 and my LAN interface configuration set to Track Interface using the IPv6 interface WAN and IPv6 Prefix ID of 0. This works fine and my LAN interface status page reports an IPv6 address with a /56 subnet mask.

My question is how to delegate an IPv6 to a second interface. I tried selecting Track Interface on WAN and choosing an IPv6 Prefix ID of 1, but I get the following error:
The specified IPv6 Prefix ID is out of range. (wan) - (0) - (0)

Setting Prefix ID back to 0 results in:
This track6 prefix ID is already being used in LAN.

It's as if the entire /56 is currently assigned to the LAN interface under Prefix ID 0, leaving none available for other interfaces to use. So how can I split the /56 and use it on my second interface?

MrPete · Dec 23, 2021, 6:22 AM

@elscorcho Something is strange. Subnets are /64 each, so you should have 256 (00-ff) available with /56 allocated to you.

I assume you rebooted after the initial setup?

Oh... try setting the LAN to 1 instead of 0... I think 0 is special.

JKnott · Dec 23, 2021, 11:37 AM

@elscorcho

Each interface requires a unique prefix ID. With a /56 your choices are 0 to ff.

elscorcho · Dec 23, 2021, 3:52 PM

@mrpete I cannot set LAN prefix to anything other than 0 or I get:

The specified IPv6 Prefix ID is out of range. (wan) - (0) - (0)

My clients on the LAN interface are also reporting a prefix len of /56 so I think LAN prefix is using the entire /56 and I don't know how to split it into /64's for use on other interfaces.

I also tried rebooting my router. I received a new PD with len /56 but still the same issues as described above.

Here's software version info if that helps:
2.5.2-RELEASE (amd64)
built on Fri Jul 02 15:33:00 EDT 2021
FreeBSD 12.2-STABLE

JKnott · Dec 23, 2021, 5:25 PM

@elscorcho

What do you have for DHCPv6 Prefix Delegation size on the WAN page? With a /56, it should be 56.

elscorcho · Dec 23, 2021, 8:54 PM

@jknott That was it, thanks! I had it set to /64 so I changed to /56 and the second interface allowed me to set the prefix ID.

JKnott · Dec 23, 2021, 9:03 PM

@elscorcho

Good. That setting allows you to choose whatever size prefix you want, up to whatever your ISP provides. So, if you choose 64, then you only get a single /64.

I also have a /56, with a /64 assigned to my main LAN, guest WiFi, test LAN, a downstream Cisco router and I can then route another /64 to the Cisco router.

With IPv6, you get gazillions of addresses, so no need to worry about how many you use.