Local-Area Networks and LAN Switching - CCIE Routing and Switching Exam Certification Guide - page 153

ISL Frame Fields (Continued) Table 4-10

ISL Field Description

RES 16-bit reserved field, set to zero for Ethernet.

Encapsulated
Frame

Encapsulated Ethernet, Token Ring, FDDI, or ATM frame, including its own CRC,
completely unmodified.

CRC 32-bit CRC value calculated on the entire encapsulated frame from the DA field to the

encapsulated frame field.

ISL Configuration
On router interfaces, subinterfaces are created for each VLAN. On each subinterface, the
encapsulation is set to ISL to enable ISL trunking to the switches. As shown in Example 4-14,
each subinterface is configured with a different VLAN number with the encapsulation isl vlan-
number command.

Example 4-14 ISL Configuration

interface fastethernet 1/1.1
encapsulation isl 1
ip address 192.168.1.1 255.255.255.0
!
interface fastethernet 1/1.2
encapsulation isl 2
ip address 192.168.2.1 255.255.255.0
!
interface fastethernet 1/1.3
encapsulation isl 3
ip address 192.168.3.1 255.255.255.0

On Catalyst, use the set trunk command. Because ISL is the default, you do not use the isl
keyword:

set trunk mod/port [on | desirable | auto | negotiate] isl

IEEE 802.1q
The IEEE 802.1q standard trunks VLANs over FE and GE interfaces and can be used in a
multivendor environment. IEEE 802.1q uses one instance of STP for each VLAN allowed in
the trunk. Similar to ISL, 802.1q uses a tag on each frame with a VLAN identifier. Unlike ISL,
802.1q uses an internal tag.

IEEE 802.1q also provides support for the IEEE 802.1p priority standard. A priority field is
included in the 802.1q frame.