Integrated Routing and Bridging
Sorry for the lack of updates lately but I spent the whole last week skiing and recharging my
batteries and now I’m back fully motivated to continue my path to the lab.
This time we will be talking about Integrated Routing and Bridging (IRB). Before studying for
the lab I had never used this feature. I’m not sure why we would use this feature in a
production network, maybe because we need to bridge two networks instead of routing
them due to some badly written application. If you have used it in real networks please post
in the comments. It is fair game for the lab so we need to know about it.
IRB is a feature used on routers that lets us bridge between a bridged domain and a
routed domain. Remember that in order for a VLAN to span a router the router must
be able to forward frames from one interface to another while maintaining the VLAN
header. If a network protocol is configured on a router interface (IP) it will terminate
the VLAN. This means that the VLAN header will not be maintained. When configuring
IRB we will be using a Bridged Virtual Interface (BVI), this can be compared to a SVI
on a switch. A BVI gives the bridged interfaces a connection to the routed world.
When IRB is configured and traffic comes in on a routed interface (IP address configured)
that is destined for a host in the bridge group the traffic will first be routed to the BVI.
The packet will then be forwarded to the bridging engine which forwards it through a
bridged interface, the forwarding is based on the destination MAC address. If a packet
comes in on a bridged interface destined for a host in a routed network the traffic will
first go to the BVI and then be sent to the routing engine before it sends it out the
routed interface. If bridging between two interfaces with no routed protocols the traffic
will not pass the BVI interface. Think of the bridge-group as an external switch and
the BVI lets us connect this external switch to the router.
The image below describes the scenario. R1 and R3 are in different VLANs but in
the same subnet, we need communication between the two routers. Between the
routers we have a couple of switches.
The configuration on R1 and R3 is straightforward. They have physical interfaces
with an IP address.
R1 is connected to SW1 and R3 to SW3. The switch configuration is just a basic access port.
Router R6 is connected to SW2 and it needs a trunk port.
Now we need to configure R6 to bridge between the two different VLANs. We start by activating IRB.
Then we need to tie the interfaces to the bridge-group.
Now we create a BVI interface in the subnet.
Lastly we need to activate spanning-tree and activate routing for the bridged interfaces.
So using IRB we can both bridge and route between interfaces on a
router, something that is not possible otherwise.
Finally, these are some useful commands to show what is going on when using IRB.