MTU setting for Zen via CityFibre

I was wondering what the recommended MTU is for Zen users using our own modems via CityFibre. I've currently got it set to 1492 which is what I was using previously with my old FTTC connection however I'd read somewhere that CityFibre supports a larger MTU but before I go fiddling and breaking things I thought I'd ask here in case someone had some good advice.

Hi

Found an posting online that says the MTU can be set to 1508, so basically 1500 with the 8 bytes overhead for pppoe.

Regards

Andrew

I am not connected to CityFibre yet (iam awaiting one last connection from Zen before they can offer CityFibre to me yet)

I did a little looking around while I was waiting and saw two useful sources of info:

https://cdn.cityfibre.com/Partner-page/Downloads/CF-...

https://support.aa.net.uk/CityFibre_ONT

Both Suggest the Max MTU for CF is 1900, Hopefully Zen will support this too!

I tried to set 1900 on my Asus RT-AX86U but it only let me set up to 1492 (On Zen/CF)

In reply to a post by RichTea23:

Both Suggest the Max MTU for CF is 1900, Hopefully Zen will support this too!

The Internet in general does not run at 1900 MTU so no don't bother trying that.
1500 is still the correct size for layer 3 on a router.

1508 is mostly a special PPPoE use case between a router's ethernet interface and an xDSL modem (ADSL or FTTC) so that the PPP encapsulation is able to accommodate 1500 inside it.

If your FTTP also uses a PPPoE that's where you'd need to be sure the intervening connectivity to the provider supports 1508 until the PPPoE is decapsulated.
(With xDSL this happens before it even leaves your property as it goes out on the telephone cable twisted pair rather than as ethernet).

Other higher MTUs are really only viable for use within inside the LAN and only if you know you have at least 2 devices that can support it. Enterprises and other organsiations with multiple internal routers may also be able to route > 1500 internally if they know it is possible on their kit.

Edited by prlzx (Wed 14-Feb-24 21:53:37)

In reply to a post by timo_w2s:

I was wondering what the recommended MTU is for Zen users using our own modems via CityFibre. I've currently got it set to 1492 which is what I was using previously with my old FTTC connection however I'd read somewhere that CityFibre supports a larger MTU but before I go fiddling and breaking things I thought I'd ask here in case someone had some good advice.

1500 is the maximum anyone should be setting for the internet.
1492 is used because 8 bytes are used on PPPoE.

With Openreach FTTC and with PPP I used to set 1508 on specific interfaces on the router as that was 1500 + 8 for PPP, but the modem needed to support RFC 4638 (baby jumbo frames).

Some will say Cityfibre uses higher MTU because they make the incorrect assumption that some Cityfibre providers don't use PPP so it saves 8 bytes.
Well some Cityfibre providers DO use PPP, just as some Openreach providers DON'T.

If it's the same router you used 1492 with before then use 1492 again.

Zen's preferred router is the Fritz!Box. On the AVM website I found this:

Adjusting the MTU size in the FRITZ!Box

The FRITZ!Box does not offer the option to manually adjust the MTU size (Maximum Transmission Unit), i.e. to manually set the maximum size of user data that can be transmitted in a single data packet.

Manually adjusting the MTU size is also not necessary because the FRITZ!Box supports MSS clamping (Maximum Segment Size) that automatically adjusts the size of the data packets to the smaller MTU of the interconnected networks.

This means that there will be no performance loss due to the fragmenting or discarding of data packets.

Note:The maximum MTU is 1492 bytes for DSL connections established via PPPoE (Point to Point Protocol over Ethernet). The maximum MTU is 1500 bytes for cable and fiber optic connections. The internet service provider can reduce the MTU by using additional protocols.

In reply to a post by cjn:

Note:The maximum MTU is 1492 bytes for DSL connections established via PPPoE (Point to Point Protocol over Ethernet). The maximum MTU is 1500 bytes for cable and fiber optic connections. The internet service provider can reduce the MTU by using additional protocols.

And that's plain wrong from Fritzbox.

Openreach fully support RFC 4638 and so do many modems and routers. I ran a 1508 MTU (1500 after PPP is stripped) for years on FTTC.
I modified my Zyxel firmware to support it in bridge mode. Many others run the same kind of setup.

The Openreach Huawei Hg612 supported it, and A&A even advised you to set 1508 in your router.

AFAIK all the Openreach ONTs support it, my Huawei does.

Cityfibre support it also.

Here's a post where someone claims to have patched a Fritzbox 7530 to support 1508 MTU on PPP. It's recent and doesn't look tested.

In reply to a post by j0hn83:

And that's plain wrong from Fritzbox.

One of the downsides of using AVM's boxes is they are designed and engineering for the German market. Their statement may be correct for German DSL services, but is incorrect for Openreach DSL in the UK.

Since I know nothing and care less about MTU, I'll just return to my state of blissful ignorance and leave the experts to their debate.