Cisco Systems RJ-45-to-AUX Multilayer Switching, How MLS Works, Why Not Call Them Routers?, 227

Chapter 11: Multilayer Switching

In Depth

Earlier in this book, I told you that switches were Layer 2 devices and routers were Layer 3 devices, which fit nicely into those well−known seven layers. You know the seven—the Open System Interconnection (OSI) Reference Model layers. Oh, did I forget to mention the Multilayer Switching Engine, multilayer switches, and Layer 3 switches?

Do you recall when it was easy to remember that Layer 2 devices use physical addresses and Layer 3 devices use logical addresses? These distinctions will seem much blurrier after you read this chapter.

For now, let’s define what Multilayer Switching (MLS) does. MLS is a method for increasing the performance of IP routing. Whereas routers provide routing functionality with a central processing unit (CPU), MLS injects certain advanced modules—either available separately or built into switches—with Application−Specific Integrated Circuit (ASIC) silicon chips to handle Layers 3 and 4 routing. As a result, the switching hardware can handle the routing functions previously performed by routers at Layer 3. MLS combines the functions of switching and the functions of routing to increase the level of performance in the device.

Why Not Call Them Routers?

If MLS switches offer the same benefits as routers, why not just call them routers with a lot of interfaces? Although most multilayer switches are much faster and considerably less per port cost than routers, some MLS devices are simple, stackable workgroup switches that fall well short of the flexibility, protocol support, port density, and WAN features you find on most enterprise edition routers (with the exception of the Catalyst 6000, which now offers a FlexWAN Card).

Until the Cisco IOS version 12.1 was released, the only protocol supported by MLS was Internet Protocol (IP). Even now, MLS supports only IP and Internetwork Packet Exchange (IPX).

Let’s examine how MLS works and the components used in MLS.

How MLS Works

Before we can analyze how MLS works, we need to understand how a network sends traffic from point A to point B. Cisco’s implementation of MLS supports every Cisco routing protocol used in its product line, including the following:

∙Border Gateway Protocol (BGP)

∙Distance Vector Multicast Routing Protocol (DVMRP)

∙Internet Group Multicast Protocol (IGMP)

∙Open Shortest Path First (OSPF)

∙Protocol−Independent Multicast (PIM)

∙Routing Information Protocol (RIP)

However, Cisco’s implementation of MLS supports only two Layer 3 routable protocols: IP and IPX.

IP and IPX are connectionless protocols. This means they attempt to deliver every packet in a best−effort

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