Trying to get ipv6 working with Comcast (virginia) on a Cisco RV345 - only pulling a /128 address on the WAN

[justinh]

Hi gurus - seeking some help please. Thanks in advance!

Goal: get ipv6 working on a home network with 3 vlans. My ipv4 setup is great. Behind the rv345 I have a centos and debian box, mac osx Catalina latest, and win10 64bit boxes.

On comcast residential xfinity, 1gbps service, virginia. Have my own modem ( Motorola MB8600 (DOCSIS 3.1) ) and cisco rv345 router with latest .19 firmware.

DHCPv6 is working to pull a ipv6 address from comcast. There are 2 addresses pulled.

fe80::xxxx:xxxx:fe0d:9a39/64

2001:xxx:xxxx:xx:xxxx:xxxx:50bd:9b41/128

Here's the problem. Looks like the rv345 is seeking a single IPv6 address (IA_NA), and I need to get it to ask for a prefix (IA_PD), say /64. How can this be done on the RV345?

Thanks!

[Martin Aleksandrov]

Hello Justin,

Did you try with DHCP_PD (Prefix Delegation) option in the IPv6 DHCP settings on the WAN interface? The DHCPv6 prefix delegation is supported by most ISPs who provide native IPv6 for consumers on fixed networks. In your case, the delegating router (Comcast) should require only the identity of the requesting router (RV345) to choose a prefix for delegation.

Regards,

Martin

[justinh]

Hi @Martin Aleksandrov 

Thanks for the reply. I am using dhcp_pd as per the two screenshots (attached and here: https://imgur.com/a/ay8tPGw).

I thought the dhcp_pd was to support the vlans to get the right dynamic address? Is my setup incorrect? (Fwiw it is working but the problem remains that once comcast changes my ipv6 it will all stop working b/c I'm setting it static instead of dynamic)

What should I put in the dhcp_pd field on the wan to get a ipv6 /64 dynamic address pulled from comcast? Thanks!

[Martin Aleksandrov]

Hi Justin,

You are in the right direction. The idea behind the DHCP Prefix Delegation is to automate the process of an ISP of assigning prefixes to a customer for use within the customer's network. Once the ISP has delegated prefixes to a customer, the customer may further subnet and assign prefixes to the links in the customer’s network. So in your case, PD occurs between a provider edge (PE- Comcast) device and customer premises equipment (CPE - RV345) using DHCPv6 prefix delegation option.  Therefore you should put in the dhcp_pd field the prefix given from your ISP. Then in your VLAN table on RV345 you can subnet and assign prefixes by choosing Prefix from DHCP_PD option (IPv6 Address/Prefix Lenght Menu) in the prefix field. Of course, you can enable the DHCPv6 server as well.

[justinh]

Thanks. This is good.

Any idea what the PD might be for comcast? or maybe you know it for other providers and I can reverse engineer for comcast?

I've tried searching online and have contacted support (2 levels up from your standard l1 team) and you would have thought I was asking a nuclear physics question. Nobody had a clue at comcast.

Thanks!

[Martin Aleksandrov]

Hello Justin,

You mean you've contacted Comcast support and got no assistance? In general, a delegating router is provided with IPv6 prefixes to be delegated to requesting routers so you should get this prefix delegation from Comcast. What you can do is to contact our technical support and ask them to doublecheck your configuration and then to collaborate with the Comcast tech support to solve the IPv6 Comcast Connectivity issue. 

I have found a similar issue with Comcast in our Cisco Community you may want to take a look:

https://community.cisco.com/t5/ipv6/residential-ipv6-comcast-connectivity/td-p/2092409

Regards,

Martin