f the route cache being invalidated. Process switching must also be used to resolve these differences. Optimum Switching Optimum switching uses a multiway tree instead of a binary tree to record and retrieve information in the route cache. For the purposes of IP addresses, a multiway tree is much faster because each octet can only be one of 256 values. A binary tree is designed to support any type of value, SO there is no limit to its potential size. Figure 15-5 shows a multiway tree as it might appear for optimum switching. The root branches into 256 nodes numbered 0-255. Each node then branches into an additional 256 nodes. This pattern continues for four levels-one for each octet. The nodes in grey show how an IP address would be matched. The example used here is the address 3.253.7.5. Dec: 3.253.7.5 Figure 15-5. Optimum-switching multiway tree Interrupt Context Switching This type of tree is much faster for IP address lookups than a binary tree because the table is optimized for the IP address format. Information for each route (prefix) or IP address is stored within the final node, as shown in Figure 15-6. Because the size of this information can be variable, and each node may or may not contain informa- tion, the overall table size is also variable. This is a drawback: searching the tree is not as efficient as it might be if every node were of a known static size. Dec: 3.253.7.5 Forwarding information Forwarding information Forwarding information Forwarding information Forwarding information Figure 15-6. Data stored in optimum-switching multiway tree Because the relevant data is stored in the nodes and has no direct relationship to the RIB or the ARP cache, entries are aged and rebuilt through process switching in the same manner used with fast switching. Optimum switching is available only on high-end routers, such as models in the 7500 and 7600 series. Cisco Express Forwarding Cisco Express Forwarding (CEF) is the switching path of choice on any router that supports it. CEF is the default switching path o