I have a RB751g-2hnd. I'm operating via the web interface:

I have removed ether5 from the bridge.

switch chip: Type Atheros 8327
Under interfaces I created a VLAN admin-vlan1 on ether5 with a ID of 100.

under switch I created a VLAN on switch1 with mode=secure, vlan-header=add if missing, default-vlan-id=100 because ether5 is connected to a simple switch.
under IP I assigned an address to admin-vlan1 192.168.100.1/24 192.168.100.0 admin-vlan1 (I also tried without the VLAN created under interfaces and only a VLAN created on the switch chip)
I create a pool with a range in the 192.168.100.0 network.
I created a DHCP server for that pool and assigned to ether5 (I also tried it assigned to admin-vlan1).

When I turn off all VLAN related configs, that is from under interfaces and switch, the DHCP server sends out addresses and the gw address can be pinged from a host connected to the switch. What is it that I'm missing that causes things not to work as expected?

I want the DHCP server to provide addresses to what is connected to the simple switch on ether5 and for the subnet to work as VLAN 100.

I would appreciate any help with this.

From your post, I ge the feeling there is a bigger picture / goal missing, anyway.

Using info in your post, you do not need to do any config so on the chip level as it seems there will be no vlan switching.
Then add the vlan to ether 5, add IP address to vlan interface, same with DHCP server.
The simple switch, I assume it means unmanaged switch, frames coming from it is not tagged and ether5 is expecting tagged packets for DHCP to work, hence when removing the vlan config, all works. So you will need to add vlan config on the access ports on the simple switch

Read the whole article below, might make more sense then
https://wiki.mikrotik.com/wiki/Manual:I ... _Filtering

I'm acknowledging your reply and thank you.

Let me look at the article and I'll get back to you.

Recommend these references!
viewtopic.php?f=13&t=143620
https://wiki.mikrotik.com/wiki/Manual:Bridge_VLAN_Table