Ccna

C H A P TER

7

Bridging Configurations
This chapter describes how to configure bridging for your switch router. For more information about the Cisco IOS commands used in this chapter, refer to the Cisco IOS command references.This chapter includes the following sections:

• • • •

About Bridging Configuring Bridging Administering Bridging About Integrated Routing and Bridging

Note Youare 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 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) variablesspecified 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 IPXrouter 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).

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InInterface 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.
Bridging Configurations 7-1

Configuring Bridging

If thepacket'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 eachconfigured 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 7-1 shows an example of configuring bridging for a router and an interface.

Table 7-1 Step
1

Configuring a Bridge GroupDescription 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.

Command
Router(config)# bridge bridge_group_number protocol {ieee | dec} #

2

Router(config)# bridge bridge_group_number priority number

The bridge prioritycommand assigns 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. Enter Ethernet interface configuration mode to configure the Fast Ethernet interface. Assign a network interface to a bridge group. If you need to assign additional interfaces to a bridge group, then choose the next interfaceand assign it to a bridge group. Return to privileged EXEC mode. Save your configuration changes to NVRAM.

3 4 5

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

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

6 7

Router(config-if)# Ctrl-Z Router# copy running-config startup-config

For additional transparent bridging configuration tasks, such as...