carries traffic for multiple VLANs, each frame must be “marked” with a VID so that it is differentiated from frames coming from other VLANs. This marking or frame identification is accomplished through a trunking protocol. Frame identification uniquely assigns an ID, referred to as a VID, to each frame. Each receiving switch examines this VID to determine the destination VLAN of the frame. VIDs are only associated with frames traversing a trunk link. When a frame enters or exits the switch on an access link, no VID is present. The ASIC on the switch port assigns the VID to a frame as it is placed on a trunk link, and also strips off the VID if the frame exits an access switch port. Trunk links should be managed so that they carry only traffic for intended VLANs. This practice keeps unwanted VLAN data traffic from traversing links unnecessarily. Trunk links are used between the access and distribution layers of the campus switch block. These are the trunk protocols used to carry multiple VLANs over a single link :

Inter-Switch Link (ISL): Cisco ISL
802.1Q: IEEE standard trunking protocol

Depending on the trunking protocol, data frames sent across a trunk link are either encapsulated or tagged. The purpose of encapsulating or tagging frames is to provide the receiving switch with a VID to identify the VLAN from which the frame originated. The trunking protocol ISL, a Cisco proprietary protocol, encapsulates frames, while IEEE 802.1Q inserts a tag into the original Layer 2 data frame. 802.1Q is not proprietary and can be deployed in any Ethernet standards-based Layer 2 device. It is specific to a single Layer 2 protocol (Ethernet) because it modifies the Layer 2 Ethernet frame by inserting a tag between two specific fields of the frame and therefore must be aware of the frame header details. ISL is Layer 2–protocol independent. Because the original Layer 2 frame is fully encapsulated and not altered, ISL can transport data frames from various Layer 2 media types.

Content 2.3 Implementing Trunks 2.3.2 Describing ISL Trunking ISL is a Cisco proprietary protocol option for configuring Layer 2 trunk links. It is the original standard for trunking between switches and predates IEEE trunking standards. ISL takes original Layer 2 frames and encapsulates them with a new ISL header and trailer. Because an entirely new header is appended to the original frame, the header offers some features not found in 802.1Q, an alternative trunking protocol. The following are some features of the ISL protocol:

Supports multiple Layer 2 protocols (Ethernet, Token Ring, FDDI, and ATM).
Supports PVST.
Does not use a native VLAN, so it encapsulates every frame.
Encapsulation process leaves original frames unmodified.

ISL Encapsulation Process When a switch port is configured as an ISL trunk port, the entire original Layer 2 frame, including the header and FCS trailer, is encapsulated before it traverses the trunk link. Encapsulation places an additional header in the front and a trailer at the end of the original Layer 2 frame. The ISL header contains the VID of the VLAN where the frame originated. At the receiving end, the VID is read, the header and trailer are removed, and the original frame is forwarded like any regular Layer 2 frame on that VLAN. Only ISL trunk ports can properly receive ISL encapsulated frames. A non-ISL port receiving an ISL frame may consider the frame size to be invalid or may not recognize the fields in the header. The frame is usually dropped and counted as a transmission error when received by a non-ISL port. ISL Header The ISL header contains various fields with values that define attributes of the original Layer 2 data within the encapsulated frame. This information is used for forwarding, media identification, and VLAN identification. The population of the fields within the ISL header varies, based on the type of VLAN and the media of the link. The ASIC on an Ethernet port encapsulates the frames with a 26-byte ISL header and a 4-byte FCS. This 30-byte ISL encapsulation overhead is consistent among the Layer 2 protocols supported on Cisco Catalyst switches, but the overall size of the frame varies and is limited by the maximum transmission unit (MTU) of the original Layer 2 protocol. The ISL Ethernet frame header contains these information fields:

DA (destination address): 40-bit destination address. This is a multicast address and is set at 0x01-00-0C-00-00 or 0x03-00-0c-00-00. The first 40 bits of the DA field signal to the receiver that the packet is in ISL format.
Type: 4-bit descriptor of the encapsulated frame types: Ethernet (0000), Token Ring (0001), FDDI (0010), and ATM (0011).
User: 4-bit descriptor used as the Type field extension or to define Ethernet priorities. It is a binary value from 0, the lowest priority, to 3, the highest priority. The default User field value is 0000. For Ethernet frames, the User field bits 0 and 1 indicate the priority of the packet as it passes through the switch.
SA (source address): 48-bit source MAC address of the transmitting Cisco Catalyst switch port.
LEN (length): 16-bit frame-length descriptor minus DA, Type, User, SA, LEN, and CRC.
AAAA03: Standard Subnetwork Access Protocol (SNAP) 802.2 logical link control (LLC) header.
HS (high bits of source address): First 3 bytes of the SA (manufacturer or unique organizational ID).
VID: 15-bit VID. Only the lower 10 bits are used for 1024 VLANs.
BPDU (bridge protocol data unit): 1-bit descriptor identifying whether the frame is a spanning tree BPDU. It also identifies if the encapsulated frame is a Cisco Discovery Protocol (CDP) or VLAN Trunk Protocol (VTP) frame and indicates if the frame should be sent to the control plane of the switch.
INDX (index): 16 bits to indicate the port index of the source of the packet as it exits the switch. It is used for diagnostic purposes only and may be set to any value by other devices. It is a 16-bit value and is ignored in received packets.
RES: 16 bits reserved for Token Ring and FDDI frames.
Encapsulated Ethernet Frame: Encapsulated data packet, including its own CRC value, completely unmodified. The internal frame must have a CRC value that is valid when the ISL encapsulation fields are removed. A receiving switch may strip off the ISL encapsulation fields and use this ENCAP FRAME field as the frame is received (associating the appropriate VLAN and other values with the received frame as indicated for switching purposes).

ISL Trailer The trailer portion of the ISL encapsulation is an FCS that carries a CRC value calculated on the original frame plus the ISL header as the ISL frame was placed onto the trunk link. The receiving ISL port recalculates this value. If the CRC values do not match, the frame is discarded. If the values match, the switch discards the FCS as a part of removing the ISL encapsulation so that the original frame can be processed. The ISL trailer consists of the 4-byte FCS field: This sequence contains a 32-bit CRC value, which is created by the sending MAC and is recalculated by the receiving MAC to check for damaged frames. The FCS is generated over the DA, SA, LEN, Type, and Data fields. When an ISL header is attached, a new FCS is calculated for the entire ISL packet and added to the end of the frame.

Content 2.3 Implementing Trunks 2.3.3 Describing 802.1Q Trunking Like ISL, 802.1Q is a protocol that allows a single physical link to carry traffic for multiple VLANs. It is the IEEE standard VLAN trunking protocol. Rather than encapsulating the original Layer 2 frame in its entirety, 802.1Q inserts a tag into the original Ethernet header, then recalculates and updates the FCS in the original frame and transmits the frame over the trunk link. The 802.1Q protocol, often referred to as “dot-1Q,” offers the clear benefit of