Configuring Subinterfaces - Cisco Catalyst 2000 Configuration Handbook

the switch. (See the section "6-1: VLAN Configuration." in Chapter 6, "VLANs and
Trunking.")
Assign a protocol address to the interface:
2.
(interface) ip address address netmask
Use the appropriate command to configure the Layer 3 interface with network
addressing. The example here shows configuration of an IP address. See Step 3 of
section "5-2: Layer 3 Ethernet Interfaces" for other protocol options.
Enable the interface:
3.
(interface) no shutdown
The default status of many Layer 3 interfaces is shutdown, which is a disabled state.
To ensure that the interface is operational, you need to enable the interface with the
command no shutdown.

Configuring Subinterfaces

Create and access the subinterfaces:
1.
(global) interface type number.subnumber
In global configuration mode, use this command to create and access a subinterface.
The type is the controller type of the interface (for example, fastethernet or
gigabitethernet). The type can also be port-channel for a channeled connection. The
number specifies the location or logical number of the interface, and the
subnumber creates a logical Layer 3 subinterface off the main connection.
Specify an encapsulation and VLAN:
2.
(sub-interface) encapsulation {dot1q | isl} vlannumber [native]
In subinterface mode, you specify which VLAN is associated with a given subinter-
face using the encapsulation command. The type (dot1q or isl) depends on the type
of trunk connected to the router interface. The vlannumber option specifies which
VLAN is associated with the subinterface, that is, in which broadcast domain this
subinterface acts as a Layer 3 interface.
For dot1q trunks only, the option native specifies which one of the VLANs is the
native VLAN. This is important because native VLAN packets are not tagged as per
the 802.1Q specification.
Note Subinterfaces are used in configurations for routers or interfaces connected to a
trunk link. Layer 3 interfaces do not run the Dynamic Trunking Protocol (DTP), and any
switch connected to these interfaces must be configured in trunk on mode.
Assign a protocol address to the subinterface:
3.
(sub-interface) ip address address netmask
Chapter 5: Layer 3 Interface Configuration 83