Table of Contents

Configuring Bridging

Configuring Bridging

This chapter describes how to configure bridging for the Catalyst 2948G-L3 switch router. For more information about the Cisco IOS commands used in this chapter, refer to the Cisco IOS Command Reference publication.This chapter includes the following sections:

• About Bridging
  
  
• Configuring Bridging
  
  
• Monitoring and Verifying Bridging
  
  
• About Integrated Routing and Bridging
  
  
• Configuring IRB
  
  
• Monitoring and Verifying IRB
  
  

About Bridging

Cisco IOS software supports transparent bridging for Fast Ethernet and Gigabit 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

To configure bridging for a switch router on an interface, use the following steps beginning in global configuration mode.

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.

Monitoring and Verifying Bridging

Once you have set up the switch router for bridging, you can administer its operation using the following commands.

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 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.

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

Configuring IRB

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

Step 1 Configure bridge groups and routed interfaces.

Step 2 Configure IRB and the BVI.

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.

To define a bridge group and configure an interface in the bridge group, use the following steps beginning in global configuration mode.

To enable and configure IRB and BVI, use the following steps beginning in global configuration mode.

Monitoring and Verifying IRB

You can verify the IRB configuration by using the following commands.