Troubleshooting guide

12-4
ATM and Layer 3 Switch Router Troubleshooting Guide
OL-1969-01
Chapter 12 Troubleshooting Layer 2 Interfaces
Troubleshooting Layer 2 Switching
Layer 2 unknown unicast switching is accomplished by the following:
• Having all unknown unicast packets sent over the broadcast VC
• Depending on the IOS version installed, Layer 2 broadcasts are sent differently:
– With Release 12.0(4a)W5(11a) and earlier, Layer 2 broadcasts are sent to the route processor
– With Release 12.0(5)W5(13) and later, Layer 2 broadcasts are sent using the P2MP (BCAST)
VC only
Note The route processor is not reachable using any of the bridged interfaces. Administrative tasks to the
route processor are preformed using the Console or route processor Ethernet interface
Figure 12-2 Layer 2 Learning Process
Bridging over Fast EtherChannel
MAC address learning in the switch router occurs differently depending on the IOS version installed.
For Cisco IOS Release 12.0(4a)W5(11a) and earlier, MAC address learning occurs as follows:
• The first member of Fast EtherChannel learns the source address via incoming traffic (Egress
Learn).
• Other members learn the source address (Egress Learn) via IPCs from the Route Processor. These
are static entries and will not age out in the CAM.
• Static entries are deleted explicitly via IPCs from route processor.
For Cisco IOS Release 12.0(5)W5(13) and later, MAC address learning occurs as follows:
• The first member of a Fast EtherChannel learns the source address via Traffic (Egress Learn).
• Other members learn the source address (Egress Learn) from the first member, via IPCs over the
P2MP VC.
• Layer 2 entries age out in the CAM.
Route Processor
CAM
CAM entry
00-e1-00-00-00-00 2/3
Bridge
table
Port
50431