ISL and 8021Q Concepts

1ISL originally supported only normal-range VLANs, but was later improved to support extended-range VLANs as well.

1ISL originally supported only normal-range VLANs, but was later improved to support extended-range VLANs as well.

ISL and 802.1Q differ in how they add a header to the Ethernet frame before sending it over a trunk. ISL adds a new 26-byte header, plus a new trailer (to allow for the new FCS value), encapsulating the original frame. This encapsulating header uses the source address (listed as SA in Figure 2-3) of the device doing the trunking, instead of the source MAC of the original frame. ISL uses a multicast destination address (listed as DA in Figure 2-3) of either 0100.0C00.0000 or 0300.0C00.0000.

802.1Q inserts a 4-byte header, called a tag, into the original frame (right after the Source Address field). The original frame's addresses are left intact. Normally, an Ethernet controller would expect to find either an Ethernet Type field or 802.3 Length field right after the Source Address field. With an 802.1Q tag, the first 2 bytes after the Address fields holds a registered Ethernet type value of 0x8100, which implies that the frame includes an 802.1Q header. Because 802.1Q does not actually encapsulate the original frame, it is often called frame tagging. Figure 2-3 shows the contents of the headers used by both ISL and 802.1Q.

Figure 2-3 ISL and 802.1Q Frame Marking Methods KEY POINT

ISL Header 26 bytes

Encapsulated Ethernet Frame

CRC 4 bytes

Figure 2-3 ISL and 802.1Q Frame Marking Methods KEY POINT

ISL Header 26 bytes

Encapsulated Ethernet Frame

CRC 4 bytes