IPv6 WAN Prefix Length Setting in WebGUI (3004.388.8_2)?

[jksmurf]

I am not sure if this is 3004.388.8_2 related or just a an ASUS thing, but I was just curious as to why, for "Native" IPv6 connection type, the WebGUI IPv6 screen shows a WAN prefix Length of 56 even after you type in 64 and hit apply? Just cosmetic, or a default or Native is supplied by the ISP so it is ignored?

I did a wee search on "IPv6 AND 64 AND setting" without hitting exactly what I meant; many hits shows the LAN (not WAN) Prefix (48, 56 etc).

I asked the ISP what value this WAN (not LAN) setting should be and they came back and said 64, which is what I put in there.
I have not tried to put in 56 to see what System Log, IPv6 would show.

No biggie, it works (IPv6 test sites work perfectly), just thought if you typed in 64 and it was used that the figure in that option box would revert to 64?

k.

 

[dave14305]

“WAN Prefix Length” is what you ask your ISP to give you. They may respond with something less, which is what you see in your case. “Gimme a /56!” “Suck it, here’s a /64!”

 

[jksmurf]

“WAN Prefix Length” is what you ask your ISP to give you. They may respond with something less, which is what you see in your case. “Gimme a /56!” “Suck it, here’s a /64!”

Lol ... OK... maybe. They're probably all ROTFL right now... tech jokers.

TBH I have no idea what difference it makes, just seemed ...odd.

 

[Shino]

The router automatically obtains the prefix length assigned by the ISP. You can try disabling DHCP-PD.

 

[jksmurf]

You can try disabling DHCP-PD.

Thank you. It was a default setting, so I just left it, but found the residual "56" figure in the selection box odd, especially as the IPv6 Log tab shows that the ISP issued /64 does seem to be obtained by the Router. So I guess it is just cosmetic to the webGui page.

Does doing what you suggested have any disadvantages?

 

[Tvlz]

There is no problem.
Your ISP assigned a WAN prefix of /56, giving you 256 /64 LAN prefixes for your own use, to subnet your LANs (give vlans their own IPv6 subnet?).

 

[jksmurf]

There is no problem.
Your ISP assigned a WAN prefix of /56, giving you 256 /64 LAN prefixes for your own use, to subnet your LANs (give vlans their own IPv6 subnet?).

Hmm. But the ISP said the WAN Prefix was 64? I asked exactly that question.

 

[Ripshod]

If it causes no problems why worry?
My ISP is the same - /56 for WAN, which includes all the attached devices. The /64 you see is for link-local - LAN only, assigned by the router to the LAN devices, commonly starting with fe80 and the equivalent of IPv4's 192.168.50.0 subnet.

(edited to correct a glaring error)

 

[jksmurf]

If it causes no problems why worry?

I’m not too worried as I noted above, was just an observation really and I thought it might be a WebGUI bug so I thought I’d mention it. If it’s not a bug then no worries.

 

[dave14305]

I’m not too worried as I noted above, was just an observation really and I thought it might be a WebGUI bug so I thought I’d mention it. If it’s not a bug then no worries.

The definitive answer should be available. Please post the output of these commands:

grep "WAN Prefix Size Requested" /jffs/syslog.log-1 /jffs/syslog.log
nvram get ipv6_prefix_len_wan
nvram get ipv6_prefix_length
grep ^PREFIXES /tmp/wan0_bound6.env | grep -oE "/[0-9]+"

The firmware should be updating the WAN Prefix setting after receiving the prefix from the ISP. The LAN interface will always only use a /64 from whatever size PD they give you.

 

[jksmurf]

Please post the output of these commands:

Thank you for following up on this, much appreciated. I’m doing this SSH over Tailscale from 36000 ft so hopefully it works!

ASUSWRT-Merlin RT-AX86U 3004.388.8_2 Wed Jul 31 23:48:50 UTC 2024


Smurfxxxx@RT-AX86U:/tmp/home/root# grep "WAN Prefix Size Requested" /jffs/syslog.log-1 /jffs/syslog.log/jffs/syslog.log:Aug 29 08:14:01 dhcp6_client: WAN Prefix Size Requested:/56, Re


Smurfxxxx@RT-AX86U:/tmp/home/root# nvram get ipv6_prefix_len_wan

56


Smurfxxxx@RT-AX86U:/tmp/home/root# nvram get ipv6_prefix_length

64


Smurfxxxx@RT-AX86U:/tmp/home/root# grep ^PREFIXES /tmp/wan0_bound6.env | grep -oE

/56


Smurfxxxx@RT-AX86U:/tmp/home/root#

 

[dave14305]

Great! The syslog output was truncated, but it doesn't matter. I can see the ISP actually does give you a /56, which is nice of them. I do think there is an opportunity to update the IPv6 page in the System Log area to show the full delegated prefix from the WAN. Maybe instead of DHCP-PD being enabled or disabled, it could be "Disabled" or "ipv6_prefix/ipv6_prefix_len_wan" nvram variables.

 

[jksmurf]

syslog output was truncated

Smurfxxxx@RT-AX86U:/tmp/home/root# grep "WAN Prefix Size Requested" /jffs/syslog.log-1 /jffs/syslog.log/jffs/syslog.log:Aug 29 08:14:01 dhcp6_client: WAN Prefix Size Requested:/56, Received:/56

Interesting and thank you. So (please excuse the non-tech terminology) ISP said the WAN Prefix was /64, and even though I put in /64 and hit apply, it displays /56, which is what I actually get?

Is this correctly reflected in the other screenshot I posted in the first post (not the screen you input that parameter, the other one)?

 

[dave14305]

ISP said the WAN Prefix was /64, and even though I put in /64 it displays /56, which is what I actually get?

Is this correctly reflected in the other screenshot I posted in the first post (not the screen you input that parameter, the other one)?

It's a questionable UI (someone sent this to Merlin, IIRC) where you set the desired prefix, and the nvram gets overwritten with the actual prefix. So in your case, you perhaps already had a /56, entered a /64 and applied, but the ISP gave you the same /56 back and the firmware overwrote that field back to 56.

The ISP-delegated prefix size is not visible on the second screenshot. Hence my suggestion to repurpose the DHCP-PD field to display it. In many cases, it will look the same as the LAN Prefix, since 64 is the default value in the firmware for the WAN Prefix Length. But if you get a larger prefix, you wouldn't really know it.

In most cases it's moot because the firmware doesn't know how to allocate another slice of a /60 or /56 prefix delegation to another interface, at least as far as I know. Not sure if 3006.102 SDN can handle that.

 

[jksmurf]

So in your case, you perhaps already had a /56, entered a /64 and applied, but the ISP gave you the same /56 back and the firmware overwrote that field back to 56.

Ah, righty-o.

In most cases it's moot

Fair enough

Hence my suggestion to repurpose the DHCP-PD field to display it.

That’s a nice idea; would clear up a bit of confusion.

Thank you once again.

It’s a pretty cosmetic query I guess and most folks wouldn’t probably be phased by it if it just works, but I quite like to know if I’m given a specific value for a parameter, that it shows up somewhere and says yup it’s right, or as you’ve explained, no it wasn’t what you were told but the value given is fine. All good ta.