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Load balancing does not work over CEF-switched interfaces
Organizations use parallel links and load balance their network traffic over these multiple links. This makes higher bandwidth available to network users, in a scalable, cost-effective manner. Cisco Express Forwarding (CEF) is a Layer 3 (L3) IP switching technology that provides superior performance compared with other switching technologies, especially in networks with dynamic traffic patterns. With CEF switching enabled, load balancing over parallel links is possible. At times, load balancing over CEF-switched interfaces might not work as expected. This might be caused by these circumstances:
Routes not present in the routing table
CEF switching globally disabled
CEF switching disabled on some interfaces
CEF's default behavior of load balancing on a per-source-destination basis
For load balancing to work over CEF-switched interfaces, the basic requirements for load balancing must be met. Multiple parallel paths to a destination must be known and installed in the routing table. To display the current state of the routing table, issue the show ip route command from the user EXEC or privileged EXEC mode.
If multiple routes to a destination have been installed but load balancing over them is not working as expected, or if the traffic distribution is uneven, verify that CEF switching is enabled globally on each of the parallel interfaces. To verify CEF status on a router, issue the show ip cef command from the user EXEC or privileged EXEC mode. To verify CEF status on an interface, issue the show cef interface command from the user EXEC or privileged EXEC mode. To enable CEF on an interface on which it has been explicitly disabled, issue the ip route-cache cef command from the interface configuration mode. Next, verify if CEF is load balancing on a per-packet or per-source-destination basis. To verify if per-packet or per-source-destination switching is configured, issue the show cef interface command from the user EXEC or privileged EXEC mode. By default, CEF load balances on a per-source-destination basis, which might result in certain interfaces forwarding more traffic than others, despite the paths being of equal cost. To achieve a fair distribution of traffic over the parallel paths, configure per-packet load balancing. To enable per-packet load balancing on a CEF interface, issue the ip load-sharing per-packet command from the interface configuration mode.
Note: Per-packet load balancing might cause reordering of packets, which is not suitable for certain types of traffic, such as Voice over IP (VoIP) traffic.