Troubleshooting guide

CHAPTER
12-1
ATM and Layer 3 Switch Router Troubleshooting Guide
OL-1969-01
12
Troubleshooting Layer 2 Interfaces
This chapter provides troubleshooting information about connectivity and performance problems in the
Layer 2 network connections of an ATM switch router and includes the following sections:
• Layer 2 Switching and Bridging Overview, page 12-1
• Troubleshooting Layer 2 Switching, page 12-3
• Troubleshooting Integrated Routing and Bridging, page 12-13
• Troubleshooting Trunk Port Problems, page 12-16
• Troubleshooting Fast EtherChannel Problems, page 12-16
Note For detailed cabling and hardware information for each port adapter, refer to the Catalyst 8540 CSR
Route Processor and Interface Module Installation Guide.
Layer 2 Switching and Bridging Overview
This section provides some overview information about Layer 2 switching and bridging.
Layer 2 Switching
The difference between Layer 2 and Layer 3 switching is the type of information inside the frame that
is used to determine the correct output interface. With Layer 2 switching, frames are switched based on
MAC address information. With Layer 3 switching, frames are switched based on network-layer
information.
Layer 2 switching does not look inside a packet for network-layer information as does Layer 3 switching.
Layer 2 switching is performed by looking at a destination MAC address within a frame. It looks at the
frame destination address and sends it to the appropriate interface if the switch knows the destination
address location. Layer 2 switching builds and maintains a switching table that keeps track of which
MAC addresses belong to each port or interface.
If the Layer 2 switch does not know where to send the frame, it broadcasts the frame out to all its ports
on the network, to learn the correct destination. When the frame reply is returned, the switch learns the
location of the new address, and adds the information to the switching table.