Few topologies support MLS. Due to the nature of MLS, only certain system topologies allow candidate and enable packets to transit the router and switch properly. If both candidate and enable packets cannot be identified, no complete flow cache entry can be made. Acceptable topologies include the following:
Router on a stick This includes one router (internal RSM/MSFC or external) and one switch. The router has a single connection to the network, which is the stick (see Figure 7.7).
Figure 7.7: Router on a stick
Multiple switches, one router This is acceptable if only one switch connects to the router and the switches are connected via an ISL trunk (see Figure 7.8).
Figure 7.8: Multiple switches, one router
The second of the Cisco MLS implementations involves a process called Cisco Express Forwarding (CEF), which we will discuss in the next section.