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Author Topic: Advice for MTU of 1500 on Bt Business FTTH/FTTP  (Read 3538 times)

btsimonh

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Advice for MTU of 1500 on Bt Business FTTH/FTTP
« on: October 07, 2017, 09:48:53 AM »

Hi All,

We've just 'upgraded' from bt residential to bt business on fibre to home (mainly to get a static IP address!).
There's a Huawai HG8240 GPON and a BT Smarthub 6 on the line.
We'd like to run a reliable webserver behind the connection, and past experience has shown that ideally we'd like an MTU of 1500 through to the internet, as MTU path discovery is not to be trusted.
Currently, it looks like the Smarthub is using an MTU of 1500 on the ethernet, and so losing 8 bytes to PPPoE, resulting in an overall MTU of 1492.

Looking at the specs for the Huawai GPON, it looks like it has lots of configuration options, but I'm assuming these can only be configured by BT.
I'd be quite happy to run LEDE on an alternative router to the Smarthub, and have successfully established a PPPoE connection to the GPON from a BTHHv5a running LEDE (openwrt); however, the BTHHV5 LEDE ethernet is limited to 1500 MTU (driver issues), so can't do the mini-jumbo frames required.

I know we could go and buy another LEDE capable router which is capable of 1508 on the WAN ethernet, which then SHOULD allow 1500 all the way...

Questions:
1. Has anyone run 1508 mini jumbo frames against the BT Huawei HG8240 GPON with success?  What router did you use?
2. Can BT configure the GPON for a different mode (maybe just routing the ethernet, rather than providing a PPPoE server? if so, what's recommended, what do I ask BT for?

thanks in advance,

Simon
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smf22

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Re: Advice for MTU of 1500 on Bt Business FTTH/FTTP
« Reply #1 on: October 07, 2017, 04:08:58 PM »

Another option is to use the TCP Maximum Segment Size (MSS) clamping or MSS adjust feature. The feature works by the router looking for the TCP 'three way handshake' and adjusting the MSS in the TCP header to a value that you specify within the router configuration. There's a video TCP MSS clamping what is it and why do we need it? that talks to the MTU problem and how the MSS clamping works. I host a web server from my home LAN and use this feature to reduce the 'normal' IPv4 TCP MSS from 1460 to 1452 bytes and have never seen any issues with connectivity to the server.

I'm not sure if the standard BT Hubs have the capability, but if you're looking at alternative routers or the HH5 with LEDE, then check to see if they support the feature.
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BT FTTC 80/20 Huawei Cab - Zyxel VMG8924-B10A bridge mode + Ubiquiti EdgeRouter X

ejs

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Re: Advice for MTU of 1500 on Bt Business FTTH/FTTP
« Reply #2 on: October 07, 2017, 04:16:02 PM »

Wouldn't setting the MTU to 1492 on the web server achieve the same result as having the router adjust the TCP MSS?
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btsimonh

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Re: Advice for MTU of 1500 on Bt Business FTTH/FTTP
« Reply #3 on: October 08, 2017, 09:58:27 AM »

@smf22 - yes, MSS clamping should in theory do it, but If I can achieve 1500, then I'll feel it's a whole lot better (personal bad experience with MTU restricted pipes ;) )..
Just heard on the LEDE forum that the HG8240 may not support 1508 on the wire...  Anyone have any direct experience of this?
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smf22

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Re: Advice for MTU of 1500 on Bt Business FTTH/FTTP
« Reply #4 on: October 08, 2017, 11:14:22 AM »

Not sure if you'd seen it, but the user @jonS posted over at the BT Community Forum in MTU 1500 on FTTP/FTTH  that he was able to set his router PPPoE connection as 1508 bytes MTU and send 1472 byte pings with Don't Fragment. He doesn't state explicitely, but presumably those pings were from a client on his LAN.
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BT FTTC 80/20 Huawei Cab - Zyxel VMG8924-B10A bridge mode + Ubiquiti EdgeRouter X

btsimonh

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Re: Advice for MTU of 1500 on Bt Business FTTH/FTTP
« Reply #5 on: October 08, 2017, 11:30:45 AM »

@smf22 - good spot!  must have missed the notification on that reply... (as you can see, i've been researching for some time!).
This gives me more confidence to go off and find a router which will do 1508 on the wire for pppoe, preferably running LEDE...  or maybe I'll just have to go back to my lantiq mods and try to find out why they did not work.
(funny - did look to that forum when thinking where to post my question, but could not remember user & pwd!)
« Last Edit: October 08, 2017, 11:36:42 AM by btsimonh »
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btsimonh

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Re: Advice for MTU of 1500 on Bt Business FTTH/FTTP
« Reply #6 on: October 08, 2017, 08:15:08 PM »

A solution: I've added a usb gigabit adaptor to my LEDE BTHHV5a - seems to work as an ethernet port with larger MTU; will see if I can try agains the HG8240 tomorrow....

Test Successful; with the gigabit ethernet set to an MTU of 2000, and the PPPoE on LEDE set to 1508, we're getting the full 1500 bytes to the internet via the HG8240 on BT Business FTTH/FTTP.  We also see an IPV6 address (but not exactly sure how to interpret this)... 

Just to be absolutely clear.
Huawei HG8240 connected to fibre (BT Business near IP14; debenham exchange).
BT Homehub 5 Type A with LEDE 17.01.3 installed.
USB gigabit ethernet (ASIX) plugged into the LEDE router with driver installed, connected to HG8240.
PPPoE using the BT business username and password as given in connection email.
All working at MTU of 1500 (i.e. ethernet at 1508 between LEDE and HG8240).
« Last Edit: October 10, 2017, 09:40:12 PM by btsimonh »
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