Why does the Internet disappear when the EOIP tunnel is up?

Question

System administration

Between two offices on Mikrotiks, pptp vpn was raised, on top of it there was an eoip tunnel. On one router, as soon as the tunnel comes up, LAN clients cannot access some sites. For example, Yandex, Google, work, but Sberbank, Drome - no, etc. As soon as the tunnel is turned off - everything works fine. The router settings are standard, so I won’t describe it, there can’t be an error. When the tunnel is enabled, the trace is lost somewhere on the provider's gateways. All in all, it's kind of a mess.
Where to dig and what to do?

Answer 1

Looks like an MTU issue.