CEF and load sharing?

Question

What determines if packets get switched using per packet versus per destination load balancing and how does this behavior change with and without CEF turned on? What are the defaults?

Answer 1

Load sharing is either per packet or per destination. If you have 2 equal cost paths to the same network then per destination load sharing distributes the load over both links, alternating every time there's a different destination IP address. All packets to the first destination go over the first link, all packets to the 2nd destination go over the 2nd link, then back to the first link again for a 3rd destination IP address. Cisco Express Forwarding CEF is on by default and per destination is the default type of load sharing.

Note, if you're talking IPv6, then there is NO per packet load sharing, only per destination in IPv6, and also CEF is not enabled by default in IPv6, you can turn it on using the "ipv6 cef" command. This is the same as saying that "process switching" is on by default in IPv6.

So... if CEF is turned on for the inbound interface, then it gets CEF-switched. Period. If the packet cannot be CEF switched like with ACL logging, it gets punted down to fast switching or process switching. If you disable CEF and use process switching on the inbound interface by giving the command no ip route-cache, then it uses fast switching.