Greetings. There is a problem with MTU on RB4011iGS+
The configuration has a bridge to which physical interfaces, VLAN and EOIP tunnels are added. On physical interfaces, MTU 9000 is set manually. MTU EOIP 1378 is set automatically, MTU VLAN 1500 is set manually. MTU Bridge 9000 is set manually. After rebooting the router, the MTU VLAN is set to the MTU EOIP size - 1378. If disable the VLAN and enable it again, the MTU VLAN is set correctly to 1500. What could be the problem? The problem is observed on firmware from 7.12 to 7.14

All devices in same L2 broadcast domain should use same MTU. And hence bridge will assume lowest possible MTU of all member ports. That's because L2 entities (bridges, switches) don't fragment frames, they can either forward them unaltered or drop them. If you want to have "transparent" bridging between local LAN and remote site using L2 tunnel (EOIP), then you have to set MTU of EOIP tunnel to whatever MTU is used in LAN (normally 1500) and let EOIP fragment large frames to fit into smaller payload packets (limited by MTU on WAN interface reduced by IP+EOIP overhead).

Let's say. Why then, after disabling / enabling vlan - mtu returns to the manually installed one?

I'd say it's a bug ... I'd somehow expect returning MTU to 1500 after disabling EOIP (if EOIP is carried over VLAN, then disabling VLAN will disable EOIP). But I wouldn't expect MTU to remain at 1500 after adding EOIP (with low MTU) back. So I'd say that the bug is that MTUs are not thoroughly checked in this case.

Out of curiosity: does connectivity between EOIP endpoints work correctly also is using packet sizes larger than EOIP MTU (and smaller than MTU on both subnets, which seems to be standard 1500)?

Out of curiosity: does connectivity between EOIP endpoints work correctly also is using packet sizes larger than EOIP MTU (and smaller than MTU on both subnets, which seems to be standard 1500)?

I tried increasing and decreasing the MTU at the ends of the tunnel and did not notice any obvious problems. But I prefer to leave automatic size calculation.

Automatic size calculation doesn't take into account that you joined eoip interface as port to a bridge. Automatic size calculation is fine if you use eoip interface as stand-alone interface where MTU size doesn't matter much as any conflicts can be resolved on IP layer.

But if it works for you, just keep using your config.