Inter-Switch Link Protocol (ISL)

Inter-Switch Link Protocol (ISL)
Inter-Switch Link Protocol (ISL) is a way of explicitly tagging VLAN information onto an
Ethernet frame. This tagging information enables 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.
Cisco created the ISL protocol, and therefore ISL is proprietary to Cisco devices only. If you
need a nonproprietary VLAN protocol, use the 802.1Q, which is covered next in this chapter.
ISL is an external tagging process, which means that the original frame is not altered but
instead is encapsulated with a new 26-byte ISL header and a 4-byte frame check sequence (FCS)
field at the end of the frame. Because the frame is encapsulated with information, only ISL-aware
devices can read the frame. Token Ring devices can also be connected with the appropriate ports,
if VTP version 2 is being used. The size of the frame can be up to 1548 bytes long for Ethernet and
17,878 bytes for Token Ring.
On multi-VLAN (trunk) ports, each frame is tagged as it enters the switch. ISL NICs enable
servers to send and receive frames tagged with multiple VLANs, so the frames can traverse
multiple VLANs without going though a router, which reduces latency. This technology can
also be used with probes and certain network analyzers. In addition, it enables users to attach
to servers quickly and efficiently without going through a router every time they need to communicate
with a resource. Administrators can use the ISL technology to simultaneously
include file servers in multiple VLANs, for example.
It is important to understand that ISL VLAN information is added to a frame as soon as that
frame enters the switch. The ISL encapsulation is removed from the frame if the frame is forwarded
out an access link.
460 Chapter 14  VLANs, Trunks, and VTP
Preventing communication from one VLAN to another might be desirable, but
the network design might still require that some devices have access to all
VLANs. In addition to configuring a filter on a router, you can install a network
card that is ISL- or 802.1Q-capable. This enables an e-mail server or database
server to be directly connected to all VLANs without a router being involved.