Routing and Bridging on a Layer 2 or Layer 3 device

bridginglayer2layer3routing

The thing that intrigues me is, say I have a router and I configure it’s interface as a Layer 2 and have 2 Vlans on the same router. So if I want to transfer the data from one Vlan(say VLAN-10) to another(VLAN-20) I need a Router. So instead, I configure IRB(IntegratedRouting and Bridging) for each of the Vlans and give them iPs.
So my question is, if I now send a packet from VLAN-10 to VLAN-20 device, how will that happen? Do we need to give a static route from one IRB to another or configure any toting protocols?
Please tell me how the packet transfer place.

And if the roles are now reversed and the Switch is configured with IRB with the same VLAN, will it be capable of handling the routing with all the routing tables.
Can it replace the Routers and help to avoid using it?

Best Answer

if I now send a packet from VLAN-10 to VLAN-20 device, how will that happen?

The sending node needs to pass the packet to the gateway router. With a MAC-based L2 like Ethernet, the frame transporting the packet is addressed to the gateway's MAC. The router receives the frame, extracts the packet, looks at the destination IP, consults its routing table, and forwards the packet out the according interface. With a MAC-based L2, the frame carrying the packet is addressed to the destination's MAC as translated by ARP or NDP. If the destination isn't local to the router the frame is addressed to the next hop gateway.

Do we need to give a static route from one IRB to another or configure any toting protocols?

Usually, the router adds the local networks connected to any of its interfaces to its routing table automatically.

And if the roles are now reversed and the Switch is configured with IRB with the same VLAN, will it be capable of handling the routing with all the routing tables.

This works the same way, even if the outbound interface is the same as the inbound interface. Some routers don't forward packets back through their inbound interface by default and require explicit configuration.

Can it replace the Routers and help to avoid using it?

Inter-VLAN communication is only possible by using a router connected to these VLANs. If you want them bridged just change the ports to the same VLAN ID and use the same IP subnet.

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