We have an issue where the router (2865) drops packets on the LAN resulting in very slow internet.
Ping times are all over the place from a laptop or PC directly connected to the router with nothing else connected.
Strangely if we disconnect the WAN the ping times on the LAN interface goes back to normal.
We swapped in a 2927 and it does exactly the same, wildly varying ping times on LAN until the WAN is disconnected.
Anyone any ideas?

Does the fact that this happens with two different routers suggest that it may be something (unusual?) in your configuration that's at the root of this?

Out of interest, which firmware are you running on the 2927?

Sounds like your WAN and LAN are using the same subnet address!

Try changing the LAN subnet address, or, try changing the address of your WAN/Modem’s address to a different network address.

This may be why your pings are all over the place.

I.e, if your LAN is 192.168.0.1/24, make sure your WAN device is using 192.168.1.0 or 192.168.2.1 or anything other than the same subnet as your LAN.

This is a total guess btw, as the info you have given is slightly limited.

LAN IP and WAN IP are completely different so that is not the issue, we have tried changing internal LAN IP of the router and it makes no difference.

Router WAN is connected to an ISP suppled router in what we assume is bridged mode, we have no access to the ISP router, it is part of a fully managed leased line service.

Do you have any VPN connections running when you're trying this ping test?

You could try a trace route (when the WAN is connected obviously) and see if that highlights the problem. You'll need something more sophisticated to trace a packet across your LAN though.

I have seen pings go crazy when using 'Hardware Acceleration' over 'IPSec' before, and that's not pinging a device at the end of the VPN tunnel either, it's worth checking that setting if you're using VPN.

Have you tried a FULL reset of the router (after making a backup of its config first of course) to see if that gives you stable ping results once the WAN is re-established and working again?

Also, have you tried a non DrayTek router in place of the DrayTek setup, to rule that out, by means of the same problem being observed?

I am having the same issue on Vigor 2962  with intermittent periods of packet loss and high ping rtts on both WAN and LAN. Web gui also becomes unresponsive. This can last for hours and then goes away. Power cycling routers and switches has no effect. RTTs go from 30-60 ms to 300-2000 with long periods of drops. It's as if the router is totally screwed up.

It had been working flawlessly for months and then this started happening in June and has occurred 4 or 5 times since. I bought another 2962 (both latest f/w) and same thing happens. Thinking it was something internal, I replaced switches  and some cables with new to no avail.

WAN 1 is FTTP and just added Three 5G on WAN3. Just discovered that everything goes back to normal when the WAN 1 ethernet is unplugged and the WAN3 service kicks in, so I'm thinking it's either the WAN1 cable or the ONT. No signs of high traffic, so don't think it's DDOS. ISP says no fault on WAN and no DDOS.

Any ideas? I think it must be something physical to cause the router to behave so badly. I have just changed WAN 1 port from Auto to 1G AN and will monitor. Is it worth contacting Draytek support or trying to get a replacement ONT?