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Chapter 7

Multi-Layer Switching (MLS)

MLS Requirements

Cisco Catalyst switches require additional hardware to see the packet header
information. Catalyst 5000 switches use the NetFlow Feature Card (NFFC)
to gather this information and cache it. Catalyst 6000 series switches use the
Multilayer Switch Feature Card (MSFC) and the Policy Feature Card (PFC)
to gather and cache header information. There is a detailed process, which
will be discussed later in the chapter, that allows switches to establish flows.

MLS requires three components to function in any network (we have

already briefly discussed two of them):

Multilayer Switching Protocol (MLSP) is a protocol that runs on the
router and allows it to communicate to the MLS-SE regarding topol-
ogy or security changes.

Multilayer Switching Route Processor (MLS-RP) can be an MLS-
capable router or an RSM installed in the switch.

Multilayer Switching Switching Engine (MLS-SE) is an MLS-capable
switch (a 5000 with an NFFC or a 6000 with an MSFC and PFC).

Now that you have a basic understanding of what MLS does and what is

required for MLS to function in a network, let's get into the nitty-gritty of
how it works.

MLS Procedures

We discussed the three required components of MLS. It is important to
understand how they work together to enable layer 3 switching. Let's look
at a sample network topology that will support MLS. Figure 7.2 shows a
simple architecture of a router and a switch with two connected hosts on the
switch. Again, the hosts have different VLAN assignments, requiring the
router's intervention to route packets. Notice that the figure depicts the main
interface with two subinterfaces, FE0/0.2 and FE0/0.3.

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