Jul 08,2008 00:00
While ISL encapsulates an Ethernet frame with a 30-byte header, IEEE 802.1Q simply adds an additional 4-byte Tag field to the Ethernet frame (EtherType 0x8100). The Tag field has three components in addition to the EtherType:
FCS is recomputed after the 4-byte tag is inserted. IEEE 802.1Q supports up to 4096 VLANs because of the 12-bit length. The IEEE 802.1Q tag is not inserted on the native VLAN, which is the VLAN that the port was assigned to before becoming a trunk port. Figure 4-6 illustrates the IEEE 802.1Q tag format.