Bridging Configuration

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C H A P T E R

Bridging Configurations 7-1

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 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 be ready to

proceed with configuring bridging.

About BridgingCisco IOS software supports transparent bridging for Ethernet. In addition, Cisco supports all themandatory 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.
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Configuring Bridging

Layer 3 Switching Software Feature and Configuration Guide7-2

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 BridgingTable 7-1 shows an example of configuring bridging for a router and an interface.

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 IOSdocuments on those subjects.

Table 7-1 Configuring a Bridge Group

Step Command Description1 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

The bridge priority command 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.

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.


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Administering Bridging

Administering BridgingOnce you have set up the switch router for bridging, you can administer its operation using the

commands in Table 7-2.

About Integrated Routing and BridgingYour 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 toexchange packets for a given protocol.

A BVI is a virtual interface within the campus switch router that acts like a normal routedinterface.

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 switchingsoftware supports the routing of IP andIPX between routedinterfaces andbridged

interfaces in the same router, in both fast-switching and process-switching paths.

Table 7-2 Administering Bridging

Command Displays

clear bridge bridge_group_number Remove any learned entries from the forwarding

database and clear 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 | interface address | mask Display classes of entries in the bridge forwarding

database.

show bridge bridge_group 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 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.


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Before Configuring IRBConsider the following before configuring IRB:

The default route/bridge behavior in a bridge group (when IRB is enabled) is to bridge allpackets. 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. Youcannot disable bridging for the nonroutable traffic.

When using IRB to bridgeand route a given protocol,no protocol attributes should be configuredon the bridged interfaces. Bridging attributes cannot be configured on the BVI.

Because bridges link several segments into one big and flat network and you want to bridge thepacket coming from a routed interface among bridged interfaces, the whole bridge group should

be represented by one network-layer segmentin the switch router, an interface.

The BVI has default data-link and network-layer encapsulations. These encapsulations are thesame as on the Ethernet, except that you can configure the BVI with some encapsulations that are

not supported on a normal Ethernet interface.

Configuring IRBConfiguring 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.

(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.

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 7-3 shows an example of defining a bridge group and configuring an interface.


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Configuring IRB

Table 7-4 shows an example of enabling and configuring IRB and BVI.

Table 7-5 shows an example of verifying the IRB configuration.

Table 7-3 Configuring Bridge Groups and Routed Interfaces

Step Command Description

1 Router(config)# bridge bridge_group

protocol ieee

From global configuration mode, define one or

more bridge groups.2 Router(config)# interface fa x/0/z

Router(config-if)#

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 7-4 Configuring IRB and BVI

Step Command Description

1 Router(config)# bridge irb Enable IRB. Allows routing of traffic from the

bridged interfaces.

2 Router(config)# interface bvi bridge_group Configure the BVI by assigning the

corresponding bridge groups number to the

BVI. Each bridge group can only have one

corresponding BVI.

3 Router(config)# bridge bridge_grouproute

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 itscorresponding bridge group to other routed

interfaces.

4 Router(config-if)# ip address

ip_address_mask

Configure protocol addresses on routed

interfaces. This step shows an example for IP.

5 Router(config)# Optionally, you can configure additional

routing attributes to the BVI.

6 Router# copy running-config startup-config Save your configuration changes to NVRAM.

Table 7-5 Verifying the IRB Configuration

Command Displays

show interfaces bvi interface_name BVI information, such as the BVI MAC address and processing statistics

show interfaces irb 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


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When you have completed the configuration tasks for bridging, see Chapter 8, EtherChannel

Configurations.
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Bridging Configuration

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