As mentioned earlier, a port that carries data from multiple VLANs is called a trunk. A trunk port can be on a switch, a router, or a server.

A trunk port can use one of two protocols: Inter-Switch Link (ISL) or IEEE 802.1q.

ISL is a Cisco-proprietary trunking protocol that involves encapsulating the data frame between an ISL header and trailer. The header is 26 bytes long; the trailer is a 4-byte cyclic redundancy check (CRC) that is added after the data frame. A 15-bit VLAN ID field is included in the header to identify the VLAN that the traffic is for. (Only the lower 10 bits of this field are used, thus supporting 1024 VLANs.)

The 802.1q protocol is an IEEE standard protocol in which the trunking information is encoded within a Tag field that is inserted inside the frame header itself. Trunks using the 802.1q protocol define a native VLAN. Traffic for the native VLAN is not tagged; it is carried across the trunk unchanged. Thus, enduser stations that don't understand trunking can communicate with other devices directly over an 802.1q trunk, as long as they are on the native VLAN. The native VLAN must be defined to be the same VLAN on both sides of the trunk. Within the Tag field, the 802.1q VLAN ID field is 12 bits long, allowing up to 4096 VLANs to be defined. The Tag field also includes a 3-bit 802.1p user priority field; these bits are used as class of service (CoS) bits for quality of service (QoS) marking. (Chapter 6, "Quality of Service Design," describes QoS marking.)

The two types of trunks are not compatible with each other, so both ends of a trunk must be defined with the same trunk type.

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