Layer 3 Switching Software Feature and Configuration Guide, 12.0(4a)WX5(11a)
Bridging Configurations
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Bridging Configurations

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:

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


Table 7-1: Configuring a Bridge Group
Step Command Description

1.

Router(config)# bridgebridge_group_numberprotocol {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)# bridgebridge_group_numberprioritynumber

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)# interfacefa 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)# interfacefa 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 and routing between VLANs, as well as adjusting the Spanning-Tree Protocol, refer to the Cisco IOS documents on those subjects.

Administering Bridging

Once you have set up the switch router for bridging, you can administer its operation using the commands in Table 7-2.


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.


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.

  • When using IRB to bridge and route a given protocol, no protocol attributes should be configured on 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 the packet coming from a routed interface among bridged interfaces, the whole bridge group should be represented by one network-layer segment—in the switch router, an 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.

(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.


Table 7-3: Configuring Bridge Groups and Routed Interfaces
Step Command Description
1

Router(config)# bridgebridge_groupprotocolieee

From global configuration mode, define one or more bridge groups.
2

Router(config)# interfacefa x/0/z

Router(config-if)#

Enter Ethernet interface configuration mode to configure the Fast Ethernet interface.
3

Router(config-if)# bridge-groupbridge_group

Assign a bridge group number to the designated interface.
4

Router(config-if)# exit

Return to global configuration mode.


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


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 bvibridge_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)# bridgebridge_grouprouteprotocol

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.
4

Router(config-if)# ip addressip_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 shows an example of verifying the IRB configuration.


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


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