Table of Contents

Configuring Bridging

This chapter describes how to configure bridging for your switch router. Unless otherwise noted, the information in this chapter applies to the Catalyst 8540 CSR, Catalyst 8510 CSR, and Catalyst 8540 MSR with Layer 3 functionality. 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:

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

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Note   You are at Step 5 in the suggested process for configuring your switch router (see "Suggested Procedure for Configuring Your Switch Router" section). You should have already configured the networking and routing protocols and should now be ready to proceed with configuring bridging.

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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 port path cost on the outgoing interface (optional). When configuring POS uplink interfaces and ATM uplink interfaces in conjunction with parallel Gigabit Ethernet links, we recommend that you explicitly assign the port path cost to these interfaces because the default value might not yield the optimal spanning tree.
    
    

Configuring Bridging

To configure bridging for a router and an interface, perform the following steps, beginning in global configuration mode:

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:

• Cisco IOS Switching Services Configuration Guide
  
  
• Cisco IOS Bridging and IBM Networking Configuration Guide
  
  
• Cisco IOS Network Protocols Configuration Guide, Part I
  
  
• Cisco IOS Network Protocols Configuration Guide, Part II
  
  

Monitoring Bridging

Once you have set up the switch router for bridging, you can monitor 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 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.

Note   BVIs do not support IP multicast routing.

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.

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, perform the following steps, beginning in global configuration mode:

To enable and configure IRB and BVI, perform the following steps, beginning in global configuration mode:

For a complete configuration example of IRB using BVI, see the "Catalyst 8540 CSR with ISL, VLAN, and BVI with GEC" section.

To verify the IRB configuration, use the following commands:

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