than a simple hub can achieve. This ability enables a switch to take the place of another device, called a
repeater. A repeater is a device dedicated only to data regeneration. Repeaters allow data to be forwarded
over greater distances, allowing the data to overcome regular data distance limitations for the type of physical
media being used.
By filtering frames and regenerating forwarded frames and packets, the switch can split the network into
many separate collision domains. This splitting allows for greater distances, dramatically lower collision rates,
and higher numbers of nodes on the network. Each VLAN in the network is its own broadcast domain, and
each port is its own collision domain. In a shared or flat network, every node is part of the same collision and
broadcast domain.
Switches do not have the ability to forward frames based on Layer 3 addresses or the ability to forward data
from one VLAN to another. A switch must forward these frames to a Layer 3 device for a routing decision.
This device can be an external or internal route processor. An external route processor is your typical router.
An internal router processor is considered a “router on a stick”; it is a module inside of the switch, but the
switch must access it remotely to make forwarding decisions. Let’s take a look at an internal route processor.
Internal Route Processors
When a switch receives a packet from a port on one VLAN destined for the port of another VLAN, the switch
must find a path on which to send the frame. Switches work at Layer 2 and are designed to isolate traffic to
collision domains or subnets; they cannot by default forward data from one VLAN to another VLAN or
network without some other Layer 3 devices. The Layer 2 device known as a “router on a stick” is used to
route the data and create routing tables of other networks and devices.
Route processors can be used to route data between foreign VLANs and other logically segmented parts of the
network, such as subnets. They also route data to remote WAN segments, networks, or the Internet.
Quite a few types of route processors are available for Catalyst switches. They include:
Route Switch Module (RSM)• Route Switch Feature Card (RSFC)• Multilayer Switch Module (MSM)• Multilayer Switch Feature Card (MSFC)•
Note InterVLAN routing using RSM, RSFC, MSM, and MSFC will be covered in Chapter 6.
How InterVLAN Routing Works
Layer 3 routing takes place between VLANs. This can become a challenging problem for an administrator to
overcome. As you already learned, there are two types of route processors: external and internal. An external
route processor uses an external router (such as the Cisco devices you are familiar with) to route data from
one VLAN to another VLAN. An internal route processor uses internal modules and cards located inside the
switch route data between VLANs.
Each type of Layer 3 routable protocol that does not have to be IP can have its own mapping for a VLAN. In
an IP network, each subnetwork is mapped to an individual VLAN. In an IPX network, each VLAN is
mapped to the IPX network number. With AppleTalk, a cable range and AppleTalk zone name are associated
with each VLAN.
By configuring VLANs, you control the size of your broadcast domains and keep local traffic local. However,
when an end station in one VLAN needs to communicate with an end station in another VLAN, this
communication is supported by interVLAN routing. You configure one or more routers to route traffic to the
appropriate destination VLAN.
Figure 5.4 shows Switch 1 handling traffic for a PC in VLAN 1 and Switch 2 handing traffic for VLAN 2.
The router has an ISL−configured interface connecting both switches.
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