IEEE 802.1Q Protocol
As with Cisco ISL, IEEE 802.1Q can be used for VLAN identification with Ethernet trunks. However, instead of encapsulating each frame with a VLAN ID header and trailer, 802.1Q embeds its tagging information within the Layer 2 frame. This method is referred to as single tagging or internal tagging.
802.1Q also introduces the concept of a native VLAN on a trunk. Frames belonging to this VLAN are not encapsulated with any tagging information. If an end station is connected to an 802.1Q trunk link, the end station can receive and understand only the native VLAN frames. This provides a simple way to offer full trunk encapsulation to the devices that can understand it, while giving normal-access stations some inherent connectivity over the trunk. In an Ethernet frame, 802.1Q adds a 4-byte tag just after the source Address field, as
shown in Figure 4-4.
The first two bytes are used as a Tag Protocol Identifier (TPID) and always have a value of 0x8100 to signify an 802.1Q tag. The remaining two bytes are used as a Tag Control Information (TCI) field. The TCI information contains a three-bit Priority field, which is used to implement class-of-service (CoS) functions in the accompanying 802.1Q/802.1p prioritization standard. One bit of the TCI is a Canonical Format Indicator (CFI), flagging whether
the MAC addresses are in Ethernet or Token Ring format. (This also is known as canonical format, or little-endian or big-endian format.)
The last 12 bits are used as a VLAN identifier (VID) to indicate the source VLAN for the frame. The VID can have values from 0 to 4095, but VLANs 0, 1, and 4095 are reserved.
Note that both ISL and 802.1Q tagging methods have one implication—they add to the ength of an existing Ethernet frame. ISL adds a total of 30 bytes to each frame, whereas 802.1Q adds 4 bytes. Because Ethernet frames cannot exceed 1518 bytes, the additional VLAN tagging information can cause the frame to become too large. Frames that barely exceed the MTU size are called baby giant frames. Switches usually report these frames
as Ethernet errors or oversize frames.
Note: Baby giant, or oversize, frames can exceed the frame size set in various standards.
To properly handle and forward them anyway, Catalyst switches use proprietary hardware with the ISL encapsulation method. In the case of 802.1Q encapsulation, switches can comply with the IEEE 802.3ac standard, which extends the maximum frame length to 1522 bytes.
