5.2 Multilayer Switching

Multilayer switching is a relatively new technology that combines a router and a switch into one device, giving an administrator the best of both worlds . More and more enterprise networks are switching from routers in their core to multilayer switches. Multilayer switches offer a lower per-port cost than a typical router, while providing enhanced services, such as dynamic routing protocols and ACL support.

A multilayer switch can work in one of two ways:

1. It can be a chassis that has both switch and route processor cards.

2. It can be workgroup-style switch that has a CPU, or multiple CPUs, dedicated to routing, as well as an Application Specific Integrated Circuit (ASIC) for the switching fabric.

Multilayer switching provides many benefits over a traditional routed network. The primary advantage is that a multilayer switch allows administrators to collapse their network, creating fewer layers, while still maintaining the same level of redundancy. In Figure 5.1, the network consisted of two layers : access and core. If that network were to be redone using a traditional network configuration, another layer would need to be added into the mix, as in Figure 5.4.

Figure 5.4. A traditional network with redundancy

The more layers a network contains, the greater the security risks. Not only are you forced to keep track of more devices, but there are more points of entry for an attacker. This type of network design is also difficult to expand. If the network grows at a fast pace, it will eventually outgrow the two core switches, and will be forced to add two additional switches, which means you will have to add more to your routers. Router expansion is generally limited, and new cards for routers are significantly more expensive than new ports for a switch.

A complex network is also difficult to troubleshoot; the more layers administrators have to sift through to find a problem, the longer it takes, and more downtime is experienced . The other problem with a complex network infrastructure is that it is easier to make a mistake that disables the entire network. When troubleshooting one problem, it is very easy to "correct" a configuration error that causes a much bigger problem. With proper documentation, this should never happen, but in reality even the most conscientious network administrator can make mistakes.

Aside from the security advantages to a multilayer-switched topology, there is a performance advantage. Multilayer switches are generally faster than traditional routers. When traffic is sent through a router, each packet is inspected and forwarded using the router's processor. Multilayer switches do not work in the same way; they perform an operation often referred to as "route once, switch many." A packet enters the switch, and is fed to the route processor (the router portion of the switch). The route processor learns the new route and sends the information back down to the switch. The switch then forwards all packets with the same header information to the same destination. The routing decision needs to be made only once; from then on the packets stay only within the ASIC. Packets are almost all forwarded at wire speed, which is why there is such a performance increase.

The route processor of a multilayer switch has the full capabilities of a router. Multilayer switches are capable of performing OSPF, BGP, and other advanced routing protocols, as well as often being able to support more ACLs than a traditional router.

This type of switching has become so popular that some networking companies, Extreme Networks and Foundry Networks in particular, no longer make distinctions between routers and switches, because the same underlying technology is used in both devices.