How To Configure Cisco Router To Work As A Bridge

As we know, a router job is to do routing, and it's interfaces must be configured to work on non-overlapping subnets, but how can we deal with a special requirements to make the router work as if it was within a switched network?

For example:

One of your customers asked to help him setup the following topology

Load Balance <======> Internet Router <=======> Firewall

this could be due to a change in design and they router will stay in the picture while it will be passing traffic just like it is a Hub device.

But you need lower the routing on the Internet Router and use the same subnet on all of the three Devices, and let's say we will use 10.0.0.0/24 subnet to connect the three devices interfaces together.

You already finished the Load Balance interface G0/0 IP configuration
You also finished the Firewall G0/0 interface IP configuration

All is left to do is to make the router work on two interfaces that are facing Firewall and Load Balance as one IP address within the same subnet (10.0.0.0/24) and these interfaces of the router will be G0/0 and G0/1 and the IP addresses will be used are

LB IP is 10.0.0.1 mask 255.255.255.0
Internet RTR ip is 10.0.0.2 mask 255.255.255.0
Firewall IP is 10.0.0.3 mask 255.255.255.0

The solution to do this is configure the Internet RTR to work as bridge by configuring the BVI interface with the IP 10.0.0.2/24 as below:

bridge irb
bridge 1 protocol ieee
bridge 1 route ip
!
interface GigabitEthernet0/0
bridge-group 1
!
interface GigabitEthernet0/1
bridge-group 1
!
interface BVI 1
ip address 10.0.0.2 255.255.255.0

and then 
#interface range G0/0-1
#no shutdown

You can then verify the connectivity between the devices using ping

Also you can use #show interface bvi 1 on the internet router

Also :
RTR#show bridge 1 verbose

Total of 300 station blocks, 300 free
Codes: P - permanent, S - self


Flood ports (BG 1)           RX count    TX count
G0/0                                         2           2
G0/1                                         2           2

I hope this was useful!
Thanks

Samer R. Saleem