4-1 Transparent Bridging | Cisco Field Manual[c] Router Configuration

Not all network protocols can be routed, so it is sometimes necessary to bridge these protocols.
Cisco routers support transparent bridging to connect separate network interfaces in a single broadcast domain.
Cisco routers support DEC, IEEE 802.1d, and the vlan-bridge protocol.
Multiple bridge groups can be enabled on a router.

Configuration

Enable the bridging process, and select a spanning-tree protocol:
```
 (global)  bridge-group   number   protocol  [  ieee   dec   vlan-bridge  ] 
```
This command enables the bridging process by specifying a group to be associated with interfaces that will be a part of that broadcast domain or group. The keyword number specifies the broadcast group. An interface can be a member of only one group, but the router can have multiple broadcast groups containing different interfaces. The keyword protocol selects the spanning-tree protocol to be used by the bridge for that group.
Place the interfaces into a bridge group:
```
 (interface)  bridge-group   number  
```
This command places the selected interface in a bridge group. When two or more interfaces are placed in the same bridge group, they become members of the same broadcast domain, and traffic is bridged between those interfaces.
Note

Bridging is designed for protocols that a router cannot route. In other words, either the routing process has not been enabled on the router, or there is no way to route the protocol in question. IP routing is on by default on Cisco routers. If you want to bridge IP between interfaces, you must disable the IP routing process or configure IRB or CRB on the router. To disable IP routing, enter the following command:
```
 (global)  no ip routing  
```
Optional) Specify the bridge priority for the selected group:
```
 (global)  bridge   bridge-group   priority   number  
```
The priority is used to determine the selection of the root bridge. The device with the lowest bridge ID becomes the root device, and the priority is the leading value of the bridge ID. The number range for IEEE spanning tree is 0 to 65536; the default is 32768. The number range for DEC spanning tree is 0 to 255; the default is 128.
(Optional) Disable spanning tree on an interface:
```
 (interface)  bridge-group   bridge-group   spanning-disabled  
```
By disabling spanning tree on an interface, you stop the sending and processing of bridge protocol data units (BPDUs) for that interface. You must specify the group number that the selected interface is on with the bridge-group option.

Caution

By disabling spanning tree on an interface, you eliminate the router's capability to detect spanning tree loops. If you use this command, be certain that you have no loops . If there is a loop, this could cause your network serious problems!
(Optional) Set the spanning tree update interval:
```
 (global)  bridge   bridge-group   hello-time   seconds  
```
This sets the update time for spanning tree BPDUs in seconds for the specified group. The range is 1 to 10 seconds; the default is 1 second.
(Optional) Set the forward delay:
```
 (global)  bridge   bridge-group   forward-time   seconds  
```
This specifies how long the bridge waits before it begins forwarding packets on any bridge interface in a particular bridge group. When an interface becomes active or the max-age expires , this is how long the interface waits while receiving BPDUs and learning MAC addresses before it transitions into forwarding or blocking state. The range is 10 to 200 seconds. The default is 30 seconds.
(Optional) Set the spanning tree maximum aging timer:
```
 (global)  bridge   bridge-group   max-age   seconds  
```
This specifies for a bridge group the time in seconds that a bridge waits before it considers a forwarding bridge dead. When an interface is in blocking mode and it stops receiving BPDUs from another bridge that is forwarding for a given link, it must wait the period of the max-age timer until it moves into listen and learning mode (specified by the forward-time timer). The range is 10 to 200 seconds; the default is 15 seconds.

NOTE

The hello-time, forward-time, and max-age timers can be set on any bridge, but the root bridge controls them. If these timers are set on any bridge other than the root, they are changed to match the timers on the root. If you want to modify these timers, you must modify them on the root bridge.
(Optional) Place a MAC address in the forwarding table manually:
```
 (global)  bridge   bridge-group   address   mac-address  {  forward   discard  }   [  interface  ] 
```
The bridge dynamically builds a forwarding table by listening for a source address on a given interface. Using the bridge address command, you can specify a given MAC address in a bridge group and specify the action, forward or discard, that the bridge should take. If you specify forward, you must specify the interface that is attached to the segment leading to that device.
(Optional) Disable MAC from forwarding to learned addresses:
```
 (global)  no bridge   bridge-group   acquire  
```
Use this command to specify that the bridge should not forward to a dynamically learned address for a given bridge group. If you disable bridge acquire, the bridge will not send to learned addresses; it will forward to only statically assigned addresses.
(Optional) Specify how long a dynamic MAC entry is retained in the forwarding table:
```
 (global)  bridge-group   bridge-group   aging-time   seconds  
```
This command specifies, in seconds, how long a dynamically learned address remains in the MAC table for a specified group. If a frame bearing the source address in the table is not received in this amount of time, the address is flushed and has to be relearned. The range is 0 to 1,000,000 seconds; the default is 300 seconds.

Example

In this example, two bridge groups (broadcast domains) have been set up for Router Katie. E0 and E1 are members of one broadcast domain, and E2 and E3 are members of another domain. The DEC Spanning-Tree Protocol is specified for the group containing E2 and E3, and the IEEE Spanning-Tree Protocol is specified for E0 and E1. The priority is set to 0 to allow this bridge to become the root for that spanning tree. The forward-delay, max-age, and hello timers have also been set for the IEEE group on this bridge. For group 2, the aging-time has been set to 10 minutes. Figure 4-1 shows the network topology for this example.

Figure 4-1. Transparent Bridging Example

graphics/04fig01.gif

  interface ethernet 0   bridge-group 1   interface ethernet 1   bridge-group 1   interface ethernet 2   bridge-group 2   interface ethernet 3   bridge-group 2   no ip routing   bridge 1 protocol ieee   bridge 1 forward-time 35   bridge 1 max-age 12   bridge 1 hello-time 3   bridge 1 priority 0   bridge 2 protocol dec   bridge 2 aging-time 600