problematic. If an Ethernet frame has been created at the maximum size of 1,518 bytes, ISL will add an additional 30 bytes, for a total frame size of 1,548 bytes. These frames may be counted as "giant" frame errors, though Cisco equipment has no problem accepting them. 802.1Q 802.1Q takes a different approach to VLAN tagging. Instead of adding additional headers to a frame, 802.1Q inserts data into existing headers. An additional 4-byte tag field is inserted between the Source Address and Type/Length fields. Because 802.1Q has altered the frame, the FCS of the frame is altered to reflect the change. Because only 4 bytes are added, the maximum size for an 802.1 1Q frame is 1522 bytes. This may result in "baby giant" frame errors, though the frames will still be supported on Cisco devices. Which Protocol to Use Why are there two protocols at all? There is a history at work here. Cisco developed and implemented ISL before 802.1Q existed. Older switches from Cisco only support ISL. Oddly enough, other switches, like the Catalyst 4000, only support 802.1Q. In some cases, the blade within a switch chassis may be the deciding factor. As an example, the 10-Gb blade available for the Catalyst 6509 only supports 802.1Q, while the switch itself supports 802.1Q and ISL; in this case, 802.1Q must be used. In many installations, either protocol can be used, and the choice is not important. When trunking between Cisco switches, there is no real benefit of using one proto- col over the other, except for the fact that 802.1Q can support 4,096 VLANs, whereas ISL can only support 1,000. Some purists may argue that ISL is better because it doesn't alter the original frame, and some others may argue that 802.1 1Q is better because the frames are smaller, and there is no encapsulation. What usually ends up happening is that whoever installs the trunk uses whatever protocol she is used to. Cisco has recommendations on how to set up trunks between Cisco switches. This document (Cisco document ID 24067) is titled "System Requirements to 