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.

• 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 port path cost on the outgoing interface (optional). When configuring POS uplink interfaces and ATM uplink interfaces in conjunction with parallel Gigabit Ethernet links, we recommend that you explicitly assign the port path cost to these interfaces because the default value might not yield the optimal spanning tree.

For additional transparent bridging configuration tasks, such as configuring bridged VLANs, routing between VLANs, and adjusting the Spanning Tree Protocol, refer to the following Cisco IOS publications:

Command	Purpose
clear bridgebridge-group	Removes any learned entries from the forwarding database and clears the transmit and receive counts for any statically configured forwarding entries.
clear vlan statistics	Removes virtual LAN statistics from any static or system configured entries.
show bridge [bridge-group]	Displays classes of entries in the bridge forwarding database.
show bridge [bridge-group] circuit-group [circuit-group] [src-mac-address] [dest-mac-address]	Displays the interfaces configured in each circuit group and shows whether they are participating in load distribution.
show bridge group verbose	Displays extended information about configured bridge groups.
show span	Displays the spanning tree topology known to the Catalyst 8500 campus switch router.
show vlans	Displays a summary of virtual LAN subinterfaces.

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, in both fast-switching and process-switching paths.

• 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.

• The 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 the bridged interfaces, the whole bridge group should be represented by one interface.

• The BVI has default data-link and network-layer encapsulations. These encapsulations are the same as on the Ethernet, except that you can configure the BVI with some encapsulations that are not supported on a normal Ethernet interface.

   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, perform the following steps, beginning in global configuration mode:

	Command	Purpose
Step 1	Router(config)# bridge bridge-group protocol {ieee | dec}	Defines one or more bridge groups.
Step 2	Router(config)# interface fastethernet slot/subslot/interface or Router(config)# interface gigabitethernet slot/subslot/interface Router(config-if)#	Enters Ethernet interface configuration mode to configure the Fast Ethernet interface.
Step 3	Router(config-if)# bridge-group bridge-group	Assigns a bridge group number to the designated interface.
Step 4	Router(config-if)# exit	Returns to global configuration mode.

To enable and configure IRB and BVI, perform the following steps, beginning in global configuration mode:

	Command	Purpose
Step 1	Router(config)# bridge irb	Enables IRB, allowing routing of traffic from the bridged interfaces.
Step 2	Router(config)# interface bvi bridge-group Router(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.
Step 3	Router(config-if)# ip address ip-address subnet-mask	Configures protocol addresses on routed interfaces. This step shows an example for IP. Optionally, you can configure additional routing attributes to the BVI.
Step 4	Router(config-if)# exit Router(config)#	Exits interface configuration mode.
Step 5	Router(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.
Step 6	Router(config)# end Router#	Exits global configuration mode. Optionally, you can configure additional routing attributes to the BVI at this point.
Step 7	Router# copy system:running-config nvram:startup-config	Saves your configuration changes to NVRAM.

Command	Purpose
show interfaces bvi interface-name	Displays BVI information, such as the BVI MAC address and processing statistics.
show interfaces irb	Displays the following BVI information: Protocols that this bridged interface can route to the other routed interface if this packet is routable Protocols that this bridged interface bridges Entries in the software MAC-address filter