Trunking with the Cisco Catalyst 3560 Switch
In this section, you’ll look at one more switch—the Cisco Catalyst 3560. The configuration is
pretty much the same as it is for a 2960 with the exception being that the 3560 can provide
layer-3 services and the 2960 can’t. Plus, the 3560 can run both the ISL and the IEEE 802.1Q
trunking encapsulation methods—the 2960 can run 802.1Q only. With all this in mind, let’s
just take a quick look at the VLAN encapsulation difference regarding the 3560 switch.
With a higher end switch, you have the option to set the encapsulation method, as highlighted
in Table 6.5:
For the 3560, you have the encapsulation command that the 2960 switch doesn’t:
Core(config-if)#switchport trunk encapsulation ?
dot1q Interface uses only 802.1q trunking encapsulation
when trunking
isl Interface uses only ISL trunking encapsulation
when trunking
negotiate Device will negotiate trunking encapsulation with peer on
interface
Core(config-if)#switchport trunk encapsulation dot1q
Core(config-if)#switchport mode trunk
TABLE 6 . 5 Configuring Trunk Encapsulations
Command Meaning
switchport trunk
encapsulation
Allows the configuration of an encapsulation type. The type of
encapsulation used can vary depending on the device or the
type used on the other end of the trunk link.
TABLE 6 . 6 Encapsulation options
Command Meaning
dot1q Specifies the IEEE specification 802.1q encapsulation. This type
of encapsulation is an open standard and is supported by almost
all switches.
ISL Specifies the ISL standard of encapsulation that is a Cisco proprietary
standard of encapsulation. This encapsulation type is
being phased out of use. Cisco doesn’t even support it on their
lower-level switches any more.
As you can see, we have the option to add either the IEEE 802.1Q (dot1q) encapsulation
or the ISL encapsulation to the 3560 switch. After you set the encapsulation, you still have to
set the interface mode to trunk.