Troubleshooting guide

11-2
ATM and Layer 3 Switch Router Troubleshooting Guide
OL-1969-01
Chapter 11 Troubleshooting Layer 3 Network Connections
Overview of Layer 3 Switching
Defining Layer 3 Switching
Layer 3 switching refers to a class of high-performance switch routers optimized for the campus LAN
or intranet, providing both wirespeed Ethernet routing and switching services.
A Layer 3 switch router performs the following three major functions:
• Packet switching
• Route processing
• Intelligent network services
Compared to other routers, Layer 3 switch routers process more packets faster by using
application-specific integrated circuit (ASIC) hardware instead of microprocessor-based engines.
Layer 3 switch routers also improve network performance with two software functions—route
processing and intelligent network services.
To simplify forwarding of the IP packets, route processing is usually executed during the initial call or
session setup. At that point, the Layer 3 enabled ATM switch router determines the appropriate route,
and forwards to the interfaces information describing the path to be used. In fact, data exchanged
between the communicating source and destination end nodes may never need to flow to or through a
conventional router.
Frame forwarding on subsequent packets in the same flow is performed using the Layer 3 switch
functions at the line card. Once the route has been determined, all subsequent frames in the flow are
simply switched or forwarded across the chosen path. This takes advantage of the high throughput and
low latency characteristics of switching by enabling the traffic to bypass the route processor once a path
calculation has been performed.
Understanding Packet Flow
Figure 11-1 shows and describes, in Steps 1 through 4, how the initial packet travels through the switch
Layer 3 route processor to set up the network route.
Note When making Layer 3 switching decisions, the route processor does not reference the switch fabric,
(that is, the PVC configuration). The interface map (where the switch maps an egress interface to a
Broute VC) is programmed when the switch is booted up. At that time, the PVCs are automatically
configured.