Bridging Configurations

About Bridging

Configuring Bridging

Administering Bridging

About Integrated Routing and Bridging

Before Configuring IRB

Configuring IRB

Bridging Configurations

This chapter describes how to configure bridging for your switch router. For further information about the commands used in this chapter, refer to the command reference publications in the Cisco IOS documentation set and to "Command Reference."

This chapter includes the following sections:

Note You are at Step 5 in the suggested process for configuring your switch router (see Table 2-1). You should have already configured the networking and routing protocols and should now be ready to proceed with configuring bridging.

About Bridging

Cisco IOS software supports transparent bridging for Ethernet. In addition, Cisco supports all the mandatory Management Information Base (MIB) variables specified for transparent bridging in RFC 1286.

Cisco IOS software bridging functionality combines the advantages of a spanning-tree bridge and a full multiprotocol router. This combination provides the speed and protocol transparency of an adaptive spanning-tree bridge, along with the functionality, reliability, and security of a router.

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).

Configuring Bridging

Table 6-1 shows an example of configuring bridging for a router and an interface.

Table 6-1: Configuring a Bridge Group

Step Command Purpose

1 .

Router(config)# bridge bridge-group-number protocol {ieee | dec} #

From global configuration mode, assign a bridge group number and define a Spanning-Tree Protocol as either the IEEE 802.1D standard or DEC.
Note The IEEE 802.1D Spanning-Tree Protocol is the preferred way of running the bridge.

2 .

Router(config)# bridge bridge-group-number priority number

Assign a specific priority to the bridge, assisting in the spanning-tree root definition. The lower the priority, the more likely the bridge will be selected as the root.

3 .

Router(config)# interface fa x/0/z

Enter Ethernet interface configuration mode to configure the Fast Ethernet interface.

4 .

Router(config-if)# bridge-group bridge-group-number

Assign a network interface to a bridge group.

5 .

Router(config-if)# interface fa x/0/y

If you need to assign additional interfaces to a bridge group, then choose the next interface and assign it to a bridge group.

6 .

Router(config-if)# Ctrl-Z

Return to privileged EXEC mode.

7 .

Router# copy running-config startup-config

Save your configuration changes to NVRAM.

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:

Switching Services Configuration Guide
Bridging and IBM Networking Configuration Guide
Network Protocols Configuration Guide

Administering Bridging

Once you have set up the switch router for bridging, you can administer its operation using the commands in Table 6-2.

Table 6-2: Administering Bridging

Command Purpose

clear bridge bridge-group-number

Remove any learned entries from the forwarding database and clears the transmit and receive counts for any statically configured forwarding entries.

clear vlan statistics

Remove virtual LAN statistics from any static or system configured entries.

show bridge [bridge-group-number]

Display classes of entries in the bridge forwarding database.

show bridge [bridge-group-number] circuit-group [circuit-group] [src-mac-address] [dest-mac-address]

Display the interfaces configured in each circuit group and show whether they are participating in load distribution.

show bridge group verbose

Display extended information about configured bridge groups.

show bridge vlan

Display IEEE 802.10 transparently bridged virtual LAN configuration.

show span

Display the spanning-tree topology known to the Catalyst 8500 campus switch router.

show vlans

Display a summary of virtual LAN subinterfaces.

Command	Purpose
clear bridge bridge-group-number	Remove any learned entries from the forwarding database and clears the transmit and receive counts for any statically configured forwarding entries.
clear vlan statistics	Remove virtual LAN statistics from any static or system configured entries.
show bridge [bridge-group-number]	Display classes of entries in the bridge forwarding database.
show bridge [bridge-group-number] circuit-group [circuit-group] [src-mac-address] [dest-mac-address]	Display the interfaces configured in each circuit group and show whether they are participating in load distribution.
show bridge group verbose	Display extended information about configured bridge groups.
show bridge vlan	Display IEEE 802.10 transparently bridged virtual LAN configuration.
show span	Display the spanning-tree topology known to the Catalyst 8500 campus switch router.
show vlans	Display a summary of virtual LAN subinterfaces.

About Integrated Routing and Bridging

Your network may require you to bridge local traffic within several segments while having hosts on the bridged segments reach the hosts or routers on routed networks. For example, if you are migrating bridged topologies into routed topologies, you may want to start by connecting some of the bridged segments to the routed networks.

Using the integrated routing and bridging (IRB) feature, you can route a given protocol between routed interfaces and bridge groups within a single switch router. Specifically, local or unroutable traffic will be bridged among the bridged interfaces in the same bridge group, while routable traffic will be routed to other routed interfaces or bridge groups.

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.

A BVI is a virtual interface within the campus switch router that acts like a normal routed interface. A BVI does not support bridging, but it actually represents the corresponding bridge group to routed interfaces within the switch router. The interface number is the link between the BVI and the bridge group.

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.

Before Configuring IRB

Consider the following before configuring IRB:

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.

Configuring IRB

Configuring integrated routing and bridging consists of the following two key tasks and subtasks:

Step 1 Configure bridge groups and routed interfaces.

(a) Enable bridging.

(b) Assign bridge groups to interfaces.

Step 2 Configure IRB and the BVI.

(a) Enable IRB.

(b) Configure the BVI.

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

Step 3 Verify IRB configuration.

When you configure the BVI and enable routing on it, packets that come in on a routed interface destined for a host on a segment that is in a bridge group complete the following process.

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.

Table 6-3 shows an example of defining a bridge group and configuring an interface.

Table 6-3: Configuring Bridge Groups and Routed Interfaces

Step Command Purpose

1
Router(config)# bridge bridge-group protocol {ieee | dec}

From global configuration mode, define one or more bridge groups.

2
Router(config)# interface fa x/0/z

Enter Ethernet interface configuration mode to configure the Fast Ethernet interface.

3
Router(config-if)# bridge-group bridge-group

Assign a bridge group number to the designated interface.

4
Router(config-if)# exit

Return to global configuration mode.

Step	Command	Purpose
1	Router(config)# bridge bridge-group protocol {ieee \| dec}	From global configuration mode, define one or more bridge groups.
2	Router(config)# interface fa x/0/z	Enter Ethernet interface configuration mode to configure the Fast Ethernet interface.
3	Router(config-if)# bridge-group bridge-group	Assign a bridge group number to the designated interface.
4	Router(config-if)# exit	Return to global configuration mode.

Table 6-4 shows an example of enabling and configuring IRB and BVI.

Table 6-4: Configuring IRB and BVI

Step Command Purpose

1
Router(config)# bridge irb

Enable IRB, allowing routing of traffic from the bridged interfaces.

2
Router(config)# interface bvi bridge-group

Configure the BVI by assigning the corresponding bridge group's number to the BVI. Each bridge group can only have one corresponding BVI.

3
Router(config-if)# ip address ip-address subnet-mask

Configure protocol addresses on routed interfaces. This step shows an example for IP.

Optionally, you can configure additional routing attributes to the BVI.

4
Router(config-if)# exit

Exit interface configuration mode.

5
Router(config)# bridge bridge-group route protocol

Enable 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
Router(config)# end

Exit global configuration mode.

7
Router# copy running-config startup-config

Save your configuration changes to NVRAM.

Step	Command	Purpose
1	Router(config)# bridge irb	Enable IRB, allowing routing of traffic from the bridged interfaces.
2	Router(config)# interface bvi bridge-group	Configure the BVI by assigning the corresponding bridge group's number to the BVI. Each bridge group can only have one corresponding BVI.
3	Router(config-if)# ip address ip-address subnet-mask	Configure protocol addresses on routed interfaces. This step shows an example for IP. Optionally, you can configure additional routing attributes to the BVI.
4	Router(config-if)# exit	Exit interface configuration mode.
5	Router(config)# bridge bridge-group route protocol	Enable 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	Router(config)# end	Exit global configuration mode.
7	Router# copy running-config startup-config	Save your configuration changes to NVRAM.

Table 6-5 shows an example of verifying the IRB configuration.

Table 6-5: Verifying the IRB Configuration

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

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

When you have completed the configuration tasks for bridging, see "EtherChannel Configurations."

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