Unlike ISL, IEEE 802.1Q is not a Cisco proprietary protocol. It can be used to carry the traffic of more than
one subnet down a single cable, and it is compatible with devices that are not running the Cisco IOS. 802.1Q
changes the frame header with a standard VLAN format, which allows multiple−vendor VLAN
implemen−tations. For example, a Bay Networks switch or a 3COM switch can work with a Cisco switch to
pass VLAN information on a trunk link.
Inter−Switch Link (ISL) Protocol
Cisco created the ISL protocol, and therefore ISL is proprietary to Cisco devices. Several NIC cards from
Intel and other companies support ISL trunking. If you need a non−proprietary VLAN protocol over Ethernet,
you will need to use the 802.1Q protocol.
Along with being proprietary to Cisco switches, ISL is used for Fast Ethernet and Gigabit Ethernet trunk links
only. ISL is a way of explicitly tagging VLAN information onto an Ethernet frame traversing the network
through trunk links. This tagging information allows VLANs to be multiplexed over a trunk link through an
external encapsulation method. By running ISL, you can interconnect multiple switches and still maintain
VLAN information as traffic travels between switches on trunk links. Along with switches, you can also use
ISL to create trunk links between two Cisco routers that support ISL, a switch and a router, and a switch and a
server that has a NIC that supports ISL.
On a trunk port, each frame is tagged as it enters the switch. Once the frame is tagged with the appropriate
VLAN information, it can go though multiple routers or switches without retagging the frame, which reduces
latency. It is important to understand that ISL VLAN information is added to a frame only if the frame is
forwarded out a port configured as a trunk link. The ISL encapsulation is removed from the frame if the frame
is forwarded out an access link.
ISL is an external tagging process. The original frame is not altered; it is encapsulated within a new 26−byte
ISL header. This tagging adds a new 4−byte frame check sequence (FCS) at the end of the frame, as shown in
Figure 5.2.
Figure 5.2: A typical ISL frame.
Remember, only a Cisco device or an ISL−aware NIC is capable of interpreting frames with an ISL frame tag.
By using ISL, the frame encapsulation means that the frame can violate the normal Ethernet maximum
transmission unit size of 1,518 bytes.
The ISL header, shown in Figure 5.3, is entered into the frame. The ISL header contains the following:
Figure 5.3: The ISL header inserted into an ISL encapsulated packet.
Destination address (DA)—A 40−bit multicast address set to 01−00−0c−00−00. This address signals
the receiver that this packet is in ISL format.
•
Frame type field—Indicates the media type the frame is supporting. The possible options are 0000 for
Ethernet, 0001 for Token Ring, 0010 for FDDI, and 0011 for ATM.
•
4−bit User field—Identifies one of four possible priorities of the frame: XX00 for normal, XX01 for
priority 1, XX02 for priority 2, and XX11 for the highest priority.
•
Source MAC address (SA)—Set to the sending switch port’s IEEE 802.3 MAC address. Some
receiving devices ignore the SA field.
•
16−bit LEN field—Shows the length of the packet in bytes minus the excluded fields. The excluded
fields are the CRC, DA, Type, User, Source Address, and LEN field itself. The total of the excluded
fields is 18 bytes. Therefore, the LEN field contains the total packet size minus 18 bytes from the
excluded fields.
•
802.2 LLC header—For ISL frames, this field is always set to AAAA03.•
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