Configuring Bridging

The switch router can be configured to serve as both an IP and IPX router and a MAC-level bridge, bridging any traffic that cannot otherwise be routed. For example, a router routing IP traffic can also bridge the Digital local-area transport (LAT) protocol or NetBIOS traffic.

To configure bridging, you must perform the following tasks:

• In global configuration mode:

  • Select Spanning-Tree Protocol.

  • Assign a priority to the bridge (optional).

• In interface configuration mode:

  • Determine which interfaces belong to the same bridge group.

  These interfaces will be part of the same spanning tree. This allows the switch router to bridge all nonrouted traffic among the network interfaces comprising the bridge group. Interfaces not participating in a bridge group cannot forward bridged traffic.
  

  If the packet's destination address is known in the bridge table, it is forwarded on a single interface in the bridge group. If the packet's destination is unknown in the bridge table, it is flooded on all forwarding interfaces in the bridge group. The bridge places source addresses in the bridge table as it learns them during the process of bridging.
  

  A separate spanning-tree process runs for each configured bridge group. Each bridge group participates in a separate spanning tree. A bridge group establishes a spanning tree based on the BPDUs it receives on only its member interfaces.
  

  • Assign a cost to the outgoing interface (optional).

For additional transparent bridging configuration tasks, such as configuring bridged VLANs and routing between VLANs, as well as adjusting the Spanning-Tree Protocol, refer to the Cisco IOS documents on those subjects.

Because bridging is in the data-link layer (Layer 2) and routing is in the network layer (Layer 3), they have different protocol configuration models. With IP, for example, bridge group interfaces belong to the same network and have a collective IP network address. In contrast, each routed interface represents a distinct network and has its own IP network address. Integrated routing and bridging uses the concept of a Bridge Group Virtual Interface (BVI) to enable these interfaces to exchange packets for a given protocol.

Layer 3 switching software supports the routing of IP and IPX between routed interfaces and bridged interfaces in the same router.

• The default route/bridge behavior in a bridge group (when IRB is enabled) is to bridge all packets. Make sure you explicitly configure routing on the BVI for protocols that you want routed.

• Packets of nonroutable protocols such as local-area transport (LAT) are always bridged. You cannot disable bridging for the nonroutable traffic.

• Protocol attributes should not be configured on the bridged interfaces when using IRB to bridge and route a given protocol. Bridging attributes cannot be configured on the BVI.

• A bridge links several network segments into one large, flat network. To bridge a packet coming from a routed interface among bridged interfaces, the whole bridge group should be represented by one interface.

Step 1 Configure bridge groups and routed interfaces.

(a) Enable bridging.

(b) Assign bridge groups to interfaces.

(c) Configure routing for desired protocols.

Step 2 Configure IRB and the BVI.

(a) Enable IRB.

(b) Configure the BVI.

(c) Enable the BVI to accept routed packets.

(d) Enable routing on the BVI for desired protocols.

Step 3 Verify IRB configuration.

The packet is routed to the BVI and forwarded to the bridging engine. From the bridging engine, the packet exits through a bridged interface. Similarly, packets that come in on a bridged interface but are sent to a host on a routed interface go first to the BVI. Then the BVI forwards the packets to the routing engine before sending them out on the routed interface.

To define a bridge group and configure an interface in the bridge group, use the following steps beginning in global configuration mode.

Step	Command	Purpose
1	2948G-L3(config)# bridge bridge-group protocol ieee	Defines one or more bridge groups.
2	2948G-L3(config)# interface f1 2948G-L3(config-if)#	Enters interface configuration mode to configure the Fast Ethernet interface.
3	2948G-L3(config-if)# bridge-group bridge-group	Assigns a bridge group number to the designated interface.
4	2948G-L3(config-if)# end 2948G-L3#	Returns to global configuration mode.

To enable and configure IRB and BVI, use the following steps beginning in global configuration mode.

Step	Command	Purpose
1 .	2948G-L3(config)# bridge irb	Enables IRB. Allows routing of traffic from the bridged interfaces.
2 .	2948G-L3(config)# interface bvi bridge-group 2948G-L3(config-if)#	Configures the BVI by assigning the corresponding bridge group's number to the BVI. Each bridge group can only have one corresponding BVI.
3 .	2948G-L3(config-if)# ip address ip-address ip-address-subnet-mask	Configures protocol addresses on routed interfaces. This step shows an example for IP.
4 .	2948G-L3(config-if)# exit 2948G-L3(config)#	Exits interface configuration mode.
5 .	2948G-L3(config)# bridge bridge-group route protocol	Enables a BVI to accept and route routable packets received from its corresponding bridge group. You must issue this command for each protocol that you want the BVI to route from its corresponding bridge group to other routed interfaces.
6 .	2948G-L3(config)# end 2948G-L3#	Exits interface configuration mode.
7 .	2948G-L3# copy running-config startup-config	Saves your configuration changes to NVRAM.