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Virtual LANs

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What would happen if N VLANs on the switch could share a single

100Mbit connection to the router? The number of ports used for connectivity
would equal two, as opposed to (N*2), and the available number of ports for
servers and workstations would increase substantially.

In this section, the use of ISL was defined with a switch to router connection. ISL

should also be considered when the administrator wishes to connect multiple

switches that are members of the same VLAN.

Administrators must keep the following issues in mind when considering ISL:

ISL is available only on products that support ISL. Although a number
of other vendors have licensed ISL technology (including Intel), the
standard is proprietary to Cisco, and fewer vendors support the ISL
standard compared to 802.1q. In addition, with the release of 802.1q
and gigabit interfaces, Cisco has altered the default trunk encapsula-
tion in favor of 802.1q. Gigabit EtherChannel trunk links default to
802.1q, whereas non-EtherChannel gigabit ports negotiate ISL or
802.1q. FastEthernet ports, as of this writing, continue to default to
ISL.

ISL links must be point-to-point.

ISL should only be used on 100Mbit full-duplex or greater connec-
tions. Although it is possible to use ISL on 10Mbit links, the limited
bandwidth and other considerations make such a plan impractical.

ISL may require an upgrade of the IOS or memory on the router.

ISL can encapsulate Token Ring. This is referred to as ISL+.

ISL adds 30 octets to the original frame, which is encapsulated with-
out modification.

ISL includes a CRC value at the end of the frame.

Because ISL is an encapsulation of the original frame, an administrator

must consider the overhead generated to support the encapsulation. Fre-
quently, the available bandwidth is more than sufficient to cover this addi-
tional load. ISL adds 30 octets to the length of the original frame. In the case
of Ethernet, this results in a frame 1,548 octets long.

The ISL frame is shown in Figure 11.2.

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