Chapter 6. Multilayer Switching

Terms you'll need to understand:

• Internal and external route processors (RPs)

• Routed interface and Switched Virtual Interface (SVI)

• Router-on-a-stick

• Multilayer switching (MLS)

• Application-specific integrated circuit (ASIC)

• Centralized, distributed, and topology-based switching

• NetFlow and Cisco Express Forwarding (CEF) switching

• Forward Information Base (FIB) and adjacency tables

• Ternary CAM (TCAM) or CEF table

• ARP throttling

Techniques you'll need to master:

• Understand the issues when routing between VLANs

• Configuring routing between VLANs on an internal RP used routed and SVI interfaces

• Configuring routing between VLANs on a router-on-a-stick

• Understand centralized, distributed, and topology-based switching

• Using NetFlow and CEF switching

• Know the fields rewritten in hardware in the Ethernet frame and IP packet

• Enabling and verifying CEF operation

VLANs contain broadcasts, which enable you to scale your switched networks to much larger sizes. One of the downsides of this is that to pass traffic between VLANs (different subnets), you'll need a Layer 3 device, such as a traditional router or multilayer switch. This Layer 3 switching process is defined in the network layer of the seven-layer OSI Reference Model. This chapter covers both a traditional approach to this problem, using external routers, as well as multilayer switching (MLS) with internal routers.

Because of the shift toward client/server applications, the deployment of bandwidth-intensive multicasting applications, the need for improved response time, the need for high-speed switching, and the centralization of servers have all become critical components in a network design. To provide the necessary scalability in today's large campus environments, new technologies are needed to enhance both Layer 2 and Layer 3 performance.

The last chapter discussed many enhancements that you can configure on your switches to deal with STP. With the introduction of MLS, networks can scale their applications to any size and contain their broadcast and multicast problems. This enables them to take advantage of Layer 2 switching speeds and price and still take advantage of the redundancy, convergence, and load balancing of Layer 3 routing protocols, such as IS-IS, OSPF, and EIGRP.