Troubleshooting guide

12-2
ATM and Layer 3 Switch Router Troubleshooting Guide
OL-1969-01
Chapter 12 Troubleshooting Layer 2 Interfaces
Layer 2 Switching and Bridging Overview
The switch router performs Layer 2 switching using the following functions:
• Places path destination address, source address, and VLAN information are stored in CAM tables.
• Sends MAC Address update information to the route processor via Cisco IOS Interprocess
Communications (IPC).
• Considers Ethernet processor interface learning as “normal”
• Updates the route processor from the Ethernet processor interface.
• Broadcasts use and Broute VC to communicate with ports on other modules in same bridge group.
• Sets spanning tree sets state of IOS-CPU.
• Transmitted by IOS-CPU, and received by a Layer 2 ASIC, bridge PDUs are tunneled in the IPC
messages.
• Receives Bridge PDUs on trunk ports, and preserves port and tag information.
• During transmission, bridge PDUs are tagged by the ASICs on trunk ports
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.