 field already present in Ethernet frames. The ISL FCS frame computes a checksum based on the frame in- cluding the ISL header; the Ethernet FCS checksum does not include this header. Adding more information to an Ethernet frame can be problematic. If an Ethernet frame has been created at the maximum size of 1,518 bytes, ISL will add an extra 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 more headers to a frame, 802.1Q inserts data into existing headers. An additional four-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 four bytes are added, the maximum size for an 802.1Q frame is 1,522 bytes. This may result in "baby giant" frame errors, though the frames will still be supported on Cisco devices. How Trunks Work 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 Nexus, only support 802.1 1Q. In some cases, the blade within a switch chassis may be the deciding factor. As an example, some 10 Gb blades available for the Catalyst 6509 only support 802.1Q, while the switch itself supports 802.1Q and ISL; in this case, 802.1Q must be used. In many installations, you can use either protocol, and the choice is not important. When you're trunking between Cisco switches, there is no real benefit of using one protocol 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. is better because the frames are smaller and there is no encapsulation. What usually