Very strange bandwidth issue between router and switch

Hi,

Just reaching out to the community brain to see if anyone has come across this before or if anyone has some idea what might be happening.

Topology:

New BT FFTP 550/75 connection, this feeds into a Vigor 2865 that then uplinks to a TP-Link managed PoE switch.

Issue:

Internet speed test (using multiple speed test sites to get a decent result) when a PC is plugged into the DrayTek LAN port it runs close to advertised, 450-480Mbps/70Mbps.  Plug the same PC into the switch and run the exact same speed tests and throughput drops to 150-170Mbps/60Mbps.

I then performed a simple LAN test by copying a 1Gb file between two PCs like so:

PC A -> Switch -> PC B  = 110Mb/sec
PC A -> DrayTek -> Switch -> PC B = 110Mb/sec

Baffled! if the LAN to LAN speed is OK why am I seeing a 50-60% drop in internet performance for devices connected to the switch.  For reference I did unplug all devices from the switch, just leaving the PC and the uplink as well to check that nothing was causing some kind of issue.

I've checked the switch for any bandwidth management/QoS etc, likewise I've done the same on the DrayTek and everything that should be disabled is.

Cheers
Rob
 

Packet loss on the Draytek - TPlink connection?

I would also expect slightly in excess of the BT nominal speed directly into the Draytek on multi-threaded tests

Do you have hardware acceleration active on the Draytek?

LAN side MTU mis-matches maybe ?

Packet loss is a possibility but I am not seeing anything in the switch logs. I might dig out WireShark and see if I can spot anything odd.

I've tried with hardware acceleration on and off, technically as we know the acceleration is only recommended on lines over 500Mbps, but the speed test results are consistent with it on or off.

I've not seen the BT line get over 500Mbps using the multiple speed test sites. My go to used to be Fast.com but recently that has been producing some really weird results, like my WiFi running at 1.4Gbps which is impressive as the WAPs are connected via standard gigabit ports and the FTTP is 900Mbps, so I've lost faith in that one now.

I was thinking MTU might be doing something odd, I've reduced the internet MTU to 1492. The switch doesn't appear to be configurable so I guess its fixed at 1500 although jumbo frames are enabled at 5018

I don't think the internet MTU will be having any effect. I'd be interested to know if removing jumbo frames from the switch has any effect.

or vice-versa enabling them on the router (think this has to be done via CLI though - so switch side will be simpler/faster)