I have recently switched to a Juniper Networks SRX300 Gateway/Security device.

The SRX is connected to a Vigor 130, with the early 2017 firmware upgrade and is running PPPoE in bridge mode, as I receive multicast from BTTV in UHD (4K allegedly).

I now have issues with knowing what MSS/MTU values to set on the 130. I have BT's Infinity 2 service (80/20), together with the UHD BTTV STB.

In the past I have streamed 4K@60fps from YouTube, now I cannot stream anything with a resolution of more than 480i, which is not a solution.

I believe I need to be able to increase the MSS value on the SRX300, which means I need a higher MTU. I do not know what the maximum MTU that BT's network accepts, so some guidance would be much appreciated.

All suggestions gratefully received ;-[]

It's an old post, but in case anyone comes across it. This is documented in the the BT Wholesale Broadband Connect (WBC) Products, Service Description in Section 3.5.1 PPPoE Aspects:

"With PPPoE the Maximum Transmission Unit (MTU) is 1492 bytes (i.e. the 1500 byte maximum data size of Ethernet less the PPP header overhead)."

Regards

I'm afraid thats not quite right - the SIN you have linked to is for the ADSL2 service. The OP's question was about BT VDSL which is covered by SIN 498: http://www.sinet.bt.com/sinet/sins/pdf/498v7p3.pdf

SIN498 section 2.1.2 EthernetFrameSize: The maximum supported Ethernet frame size is 1530 bytes (excluding IFG and pre- amble and single/double tag – see 2.1.3).

This is to allow support of full 1500 ethernet frames across the PPPoE connection. For this to work you need:

• your router that is connected to the 130 to be capable of supporting baby jumbo frames


• to configure the MTU of your routers ethernet interface to be 1508 to allow for the addition of the PPPoE header


• to configure your PPPoE connection to have a MTU of 1500


• your ISP to have 1500 MTU support downstream of their BT interconnect

The modem baby jumbo frame requirement is part of the BT certification, so if your modem has passed 498 like the Vigor 130, then you should be able to use 1500 MTU. I've been running an Ubiquiti EdgeRouter-X and Vigor 130 connected to Aquiss via BT VDSL2 for the last year with 1500 MTU on both IPv4 and IPv6 from my LAN to the internet. I will admit though if you are not sure, 1492 is always a safe fallback :wink:

Hope that helps,
Jonathan

@JonathanP Thanks for correcting... and educating