why would you bridge an interface

[odub85]

i am still new to cisco.... and am looking at over some different configs for routers.. what would be the reasoning you would create a bvi interface instead of just using the vlan?

 

[doublea1535]

Basically a VLAN will turn a switch into many virtual switches. A BVI can make it seem that multiple interfaces on different routers are actually part of the same "switch".

For example assume you had two routers, A and B. Each one has a serial WAN connection (T1) to each other, and a ethernet LAN. If you wanted you could put all 4 interfaces into a BVI and it would appear to the end-users at both sites that they were on the same local segment as the other hosts. This is very useful if you need something to work within the same Ethernet segment (ie, it does not pass any L3 hops).

HTH

 

[ADB100]

On routers and older Layer-3 switches you didn't have the option of using VLAN's as such; Bridge-Groups and BVI's were your only choices. They are effectively the same concept though -

Current Layer-3 switch (3550 etc) config for 2 ports in the same VLAN:

vlan 10
name Data-VLAN
!
interface FastEthernet0/1
switchport mode access
switchport access vlan 10
!
interface FastEthernet0/2
switchport mode access
switchport access vlan 10
!
interface Vlan 10
ip address 10.1.1.1 255.0.0.0
!

Older Layer-3 switch (8510, 2948-L3) config for 2 ports in the same VLAN:

bridge 1 protocol ieee
!
interface FastEthernet0/1
bridge-group 1
!
interface FastEthernet0/2
bridge-group 1
!
interface BVI 1
ip address 10.1.1.1 255.0.0.0
!

HTH

Andy

 

[Belushi]

Another reason is that some protocols aren't routable. Bridging interfaces together helps get past some of those issues.