Bridging LAN Over WAN

Bridging for me is simply straightforward here in Bridging we are NOT rewriting the Layer 3 packet simply forwarding  the Layer 3 information in Layer 2 frame intact from one end to other.

Today we are discussing Bridging same subnet over the WAN links , however i have taken the subnet in different VLANs just for fun to prove the reachability not depends on the local VLANs used in the LAN atmosphere.

Here i have assigned 192.168.10.1/24 to PC-1 and this is in VLAN 10 , altogether the other side PC-2 ip add is 192.168.10.2/24 and this is in VLAN 20.

We are using IRB (Integrated Routing and Bridging) for this.It mean we can Bridge and Route at the same time on the same interface.

On Router-1 and Router-2

bridge irb
bridge 1 protocol ieee
bridge 1 route ip

Then we assign appropriate interfaces into “bidge-group 1” on R1 and R2.

R1(config)#int fa0/0
R1(config-if)#bridge-group 1
R1(config-if)#int s0/0
R1(config-if)#bridge-group 1
R1(config-if)#end

R2(config)#int fa0/0
R2(config-if)#bridge-group 1
R2(config-if)#int s0/0
R2(config-if)#bridge-group 1
R2(config-if)#end

Then we will create a Virtual BVI interface on R1 and R2 to talk to the routed Serial interface and assign the ip range used as default-gateway for PC-1 and PC-2.
R1(config)#int bvi 1
R1(config-if)#ip add 192.168.10.254 255.255.255.0
R1(config-if)#end
R1#

R2(config)#int bvi 1
R2(config-if)#ip add 192.168.10.253 255.255.255.0
R2(config-if)#end
R2#

Other relevant configs are simple not any fancy stuffs used.

PC-1#sh ip route
Default gateway is 192.168.10.254
PC-2#sh ip route
Default gateway is 192.168.10.253

PC-1#sh run int fa0/0
interface FastEthernet0/0
ip address 192.168.10.1 255.255.255.0
no ip route-cache
duplex auto
speed auto
end

PC-2#sh run int fa0/0
interface FastEthernet0/0
ip address 192.168.10.2 255.255.255.0
no ip route-cache
duplex auto
speed auto
end

We will wait for some time for MAC address learning purpose and issue the command ” show bridge 1 verbose ” on R1 and R2 , this is very helpful command in Bridging Scenarios.

R1#sh bridge 1 verbose

Total of 300 station blocks, 297 free
Codes: P – permanent, S – self

BG Hash      Address      Action  Interface      VC    Age   RX count   TX count
1 00/0   c201.080a.0000 forward  Serial0/0       –      2         11          9
1 00/1   c205.080a.0000 forward  Serial0/0       –      1         13         10
1 00/2   c204.080a.0000 forward  FastEthernet0/0 –      1         26         18

Flood ports (BG 1)           RX count    TX count
FastEthernet0/0                     3           3
Serial0/0                           3           3

The configuration look healthy so far let us check the other side R2.

R1#sh arp
Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet  192.168.10.1            8   c204.080a.0000  ARPA   BVI1
Internet  192.168.10.254          –   c200.080a.0000  ARPA   BVI1

R2#sh bridge 1 verbose

Total of 300 station blocks, 297 free
Codes: P – permanent, S – self

BG Hash      Address      Action  Interface      VC    Age   RX count   TX count
1 00/0   c200.080a.0000 forward  Serial0/0       –      4          5          4
1 00/1   c205.080a.0000 forward  FastEthernet0/0 –      3         19         14
1 00/2   c204.080a.0000 forward  Serial0/0       –      3         20         15

Flood ports (BG 1)           RX count    TX count
FastEthernet0/0                     3           3
Serial0/0                           3           3

R2#sh arp
Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet  192.168.10.2            9   c205.080a.0000  ARPA   BVI1
Internet  192.168.10.253          –   c201.080a.0000  ARPA   BVI1

Here also it looks we are learning MAC addresses from port FastEthernet 0/0 and Serial 0/0 , lets run our final verification command from PC-1 altogether over the WAN links crossing R1 and R2 over the other side PC-2.

PC-1#ping 192.168.10.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.10.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 8/15/32 ms

PC-2#trace 192.168.10.1

Type escape sequence to abort.
Tracing the route to 192.168.10.1

1 192.168.10.1 44 msec *  20 msec

Indeed we are successful in Bridging LAN over the WAN.

Happy Bridging the Love across Borders 🙂

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Afroz Ahmad

I'm Afroz, a Network Engineer and CCIE with 17 years of experience in Computer Networking, Data Centers, Telecom, Internet Services and ISPs. Currently, I work as a Network Designer for a leading ISP equipment vendor. I share my expertise through my blog and courses, aiming to share my knowledge and simplify complex networking concepts for both beginners and experienced professionals.

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