An Internal Route Processor

An Internal Route Processor
An
internal route processor
is a router on a card that fits inside the switch. This enables a switch
to route packets without having the packets leave the box that the switch resides in. You need to
add an internal route processor to a layer 2 device—for example, a 4000 Catalyst switch—to be
able to provide forwarding of layer 3 packets without an external router.
Adding an internal route processor makes a layer 2 switch into a multilayer switch and can
integrate layer 2 and layer 3 (and possibly layer 4) functionality in a single box. The 4000 series
uses a
Layer 3 Switching Module (L3SM)
, and the 6000 series uses the
Multilayer Switch Module
(MSM)
and the
Multilayer Switch Feature Card (MSFC)
to perform this function. The MSM and
MSFC—and older Route Switch Modules (RSMs) and Route Switch Feature Cards (RSFCs)—are
configured in exactly the same way on older switches.
The 4000 series router module (WS-X4232-L3) consists of a 4GB routing switch fabric with
4GB interfaces. Two of these gigabit connections appear on the front panel, making externally
F0/0.1
F0/0.2
F0/0.3
Trunked Link
VLAN 1
VLAN 2
VLAN 3
(Router on
a stick)
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Chapter 17 
Inter-VLAN Routing
accessible gigabit router ports, while the two remaining ports are connected internally to the
switch backplane. (There are also 32 10/100M ports, which are standard layer 2 ports and not
linked into the routing fabric.)
Most of the time, ports 3 and 4 are configured as part of the same channel, and subinterfaces
are added as needed using either ISL or 802.1Q encapsulation. The configuration of gigabit
ports 3 and 4 on the router module must be consistent with the configuration of port slot/1 and
slot/2 on the switch.

The traffic flow between the module and the switch can be seen using the global
commands
show interface port-channel
or
show interface gigabit
.
The L3SM is plugged directly into the switch and runs the Cisco IOS in order to perform
inter-VLAN communication. The 4000 series switch sees the RSM as a single trunked port with
a single MAC address. In other words, it appears as a router on a stick to the switch.