3 engine. Step 6 The Layer 3 engine installs the resolved adjacency in the switch (removing the ARP throttling adjacency). Step 7 The switch forwards the packet to host B. Step 8 The switch receives a subsequent packet for host B (IP-B). Step 9 The switch performs a Layer 3 lookup and finds a CEF entry for host B. The entry points to the adjacency with rewrite information for host B. Step 10 The switch rewrites packets per the adjacency information and forwards the packet to host B on VLAN20.
Content 4.3 Deploying CEF-Based Multilayer Switching 4.3.4 Describing CEF Configuration Commands Use the commands in Figure to configure CEF and verify its operation. Figure describes the CEF configuration and verification commands.
Content 4.3 Deploying CEF-Based Multilayer Switching 4.3.5 Enabling CEF-Based MLS Hardware Layer 3 switching is permanently enabled on Cisco Catalyst 6500 Series Supervisor Engine 720s with Policy Feature Card 2 (PFC2) or PFC3, Multilayer Switch Feature Card 3s (MSFC3s), and Distributed Forwarding Cards (DFCs). No configuration is required, and CEF cannot be disabled. You can use the no ip cef command to disable CEF on the Cisco Catalyst 4000 or the no ip route-cache cef command on a Cisco Catalyst 3550 interface. If CEF is enabled globally, it is automatically enabled on all interfaces as long as IP routing is enabled on the device. You can then enable or disable CEF on an interface basis. Cisco recommends that CEF be enabled on all Layer 3 interfaces. If CEF is disabled on an interface, you can enable CEF as follows: Per-destination load balancing allows the router to use multiple paths to achieve load sharing. Packets for a given source-destination host pair are guaranteed to take the same path, even if multiple paths are available. This ensures that packets for a given host pair arrive in order. Per-destination load balancing is enabled by default when you enable CEF, and it is the load balancing method of choice for most situations. Because per-destination load balancing depends on the statistical distribution of traffic, load sharing becomes more effective as the number of source-destination pairs increases. The show ip cef command displays entries in the FIB.
Content 4.3 Deploying CEF-Based Multilayer Switching 4.3.6 Describing Common CEF Problems and Solutions CEF is the fastest means of switching Layer 3 packets in hardware. The CEF tables stored in hardware are populated from information gathered by the route processor. There are two primary steps in troubleshooting CEF operations: Troubleshooting CEF is, in essence, verifying that packets are indeed receiving the full benefit of CEF switching and not being punted to a slower packet switching or processing method. The Cisco term "punt" describes the action of sending a packet down to the next-fastest switching level. The following list defines the order of preferred Cisco IOS switching methods, from fastest to slowest: A punt occurs when the preferred switching method did not produce a valid path or, in CEF, a valid adjacency. If the CEF lookup process fails to find a valid entry in the FIB, CEF installs a punt adjacency to the less-preferred system. CEF punts all packets with that adjacency to the next-best switching mode to forward all the packets by some means, even if that means is less efficient. Figure describes some basic CEF problems and associated solutions.
Content 4.3 Deploying CEF-Based Multilayer Switching 4.3.7 Describing CEF Troubleshooting Commands The commands available to troubleshoot CEF are platform dependent. The commands in Figure can be used to troubleshoot CEF on the Cisco Catalyst 4500 series switch. You can use the show interface command with the | begin L3 argument to verify that Layer 3 traffic is being switched, thereby utilizing CEF. Use the show interfaces command with the | include switched command to show switching statistics at each layer for the interface and to verify that Layer 3 packets are being switched. Figure illustrates the command used to display detailed information about the adjacency table. Each time an adjacency entry is created, a Layer 2 data link–layer header for that adjacent node is pre-computed and stored in the adjacency table. This information is subsequently used for encapsulation during CEF switching of packets. The show adjacency detail command displays the information to be used during this Layer 2 encapsulation. The header information displayed should be the same as would be expected during normal (non-CEF) Layer 2 forwarding operations. Adjacency statistics are updated approximately every 60 seconds. The show cef drops command displays whether packets are being dropped because of incomplete or nonexistent adjacencies. The two known reasons for incomplete or nonexistent adjacencies are as follows: The debug facility can be used to display detailed information on CEF operations. Use the debug ip cef command to view CEF drops because of an incomplete adjacency. You can include arguments to limit the output, which reduces overhead and allows you to focus on a specific CEF operation. The following arguments limit the debug output:
Content 4.3 Deploying CEF-Based Multilayer Switching 4.3.8 Troubleshooting Layer 3 CEF-Based MLS The CEF tables stored in hardware are populated from information gathered by the route processor. To properly troubleshoot CEF operations, first ensure that the normal Layer 3 operations on the route processor are functioning properly so that the CEF tables are populated with accurate and complete information. Next, verify that information from the route processor has properly populated the FIB and adjacency table used by CEF to perform Layer 3 switching of packets. The steps below verify whether packet transfer between the following hosts is occurring using CEF: