Assign IP address to a bridge?

RouterOS Beginner Basics
1

Hi!

I understand the concept of bridges, but I am not sure what’s the benefit of assigning an IP address to a bridge.
What are the use cases in which I need to have an IP assigned to a bridge?

2

Bridge has a few personalities, neatly explained in this tutorial: http://forum.mikrotik.com/t/routeros-bridge-mysteries-explained/147832/1

One of personalities is interface allowing CPU to communicate with L2 network joined together by bridge (the switch-like personality). If CPU is to communicate on L3 with devices members of said L2 network, then this interface needs IP address. Of course, if device is used as switch, then bridge interface is not needed and thus doesn’t need IP address. But it gets handy if one wants in-band management of such switch.

3

The advantage is when you only have one LAN subnet and the router comes default configured to dish out IP addresses to all ports on the bridge with this subnet.
If you want to use vlans, take bridge off any DHCP assignments and attach all vlans to bridge and vlans get address, dhcp server, ip pool etc…

4

Thank you for your answers!

I’ve read through the long thread you mentioned, but did not fully understand, why I need an IP address on a bridge.
The following are my takeaways from that (please, correct me if I am wrong).

  • A bridge is a logical triple of a) a port to the CPU/RouterOS-mechanics, b) a port to the physical interface, and c) the virtual switch itself. For the user, it looks like a single entry in “/interface bridge”, but the bridge settings control different parts.
  • Assigning an IP to the bridge, set the IP to the port to the CPU to access it on L3.

Can you provide some basic real world examples on when I need L3 access to the bridge from the CPU?

5

Management of said device (used as switch) from network connected to one of bridged ports.

Routing between single off-bridge port (WAN) and bridged ports (LAN).

Etc.

6

That took me quite a while to understand this properly.
What helped me was to understand the RouterOS packet flow (https://help.mikrotik.com/docs/spaces/ROS/pages/328227/Packet+Flow+in+RouterOS) from the documentation.