The 8021Q Tagging Process

Token Ring Encapsulation Flag t_

Token Ring Encapsulation Flag

To identify a frame with a given VLAN, the 802.1Q protocol adds a tag, or a field, to the standard Layer 2 Ethernet data frame. The components of this tag are shown in the figure. Because inserting the tag alters the original frame, the switch must recalculate and alter the FCS value for the original frame before sending it out the 802.1Q trunk port. In contrast, ISL does not modify the original frame at all.

The new 802.1Q Tag field has these components:

■ EtherType: Uses EtherType 0x8100 to indicate that this is an 802.1Q frame.

■ PRI: 3 bits; carries priority information for the frame.

■ Token Ring Encapsulation Flag: Indicates the canonical interpretation of the frame if it is passed from Ethernet to Token Ring. This value is always set to zero for Ethernet switches.

■ VID: VLAN association of the frame. By default, all normal and extended-range VLANs are supported.

If a non-802.1Q-enabled device or an access port receives an 802.1Q frame, the tag data is ignored, and the packet is switched at Layer 2 as a standard Ethernet frame. This allows for the placement of Layer 2 intermediate devices, such as other switches or bridges, along the 802.1Q trunk path. To process an 802.1Q tagged frame, a device must allow an MTU of 1522 or higher.

Note An Ethernet frame that has a larger MTU than expected (1518 by default for Ethernet), but no larger than 1600 bytes, will register as a Layer 2 error frame called a "baby giant." For ISL, the original frame plus ISL encapsulation can generate a frame as large as 1548 bytes, and 1522 bytes for an 802.1 Q tagged frame.

2-48 Building Cisco Multilayer Switched Networks (BCMSN) v3.0 © 2006 Cisco Systems, Inc.

Explaining 802.1Q Native VLANs

This topic defines an 802.1Q native VLAN.

802.1Q Native VLAN

Native VLAN frames are carried over the trunk link untagged.

When configuring an 802.1Q trunk, a matching native VLAN must be defined on each end of the trunk link. A trunk link is inherently associated with tagging each frame with a VID. The purpose of the native VLAN is to allow frames that are not tagged with a VID to traverse the trunk link. An 802.1Q native VLAN is defined as one of these:

■ The VLAN that a port is associated with when not in trunking operational mode

■ The VLAN that is associated with untagged frames that are received on a switch port

■ The VLAN to which Layer 2 frames will be forwarded if received untagged on an 802.1Q trunk port

Compare this to ISL, in which no frame may be transported on the trunk link without encapsulation, and any unencapsulated frames received on a trunk port are immediately dropped.

Each physical port has a parameter called a port VLAN ID (PVID). Every 802.1Q port is assigned a PVID value equal to the native VID. When a port receives a tagged frame that is to traverse the trunk link, the tag is respected. For all untagged frames, the PVID is considered the tag. This allows the frames to traverse devices that may be unable to read VLAN tag information.

© 2006 Cisco Systems, Inc. Defining VLANs 2-49

Native VLANs have these attributes:

■ A trunk port will support only one native active VLAN per operational mode. The modes are access and trunk.

■ By default, on Cisco Catalyst switches, all switch ports and native VLANs for 802.1Q are assigned to VLAN1.

■ The 802.1Q trunk ports connected to each other via physical or logical segments must all have the same native VLAN configured to operate correctly.

■ If the native VLAN is misconfigured for trunk ports on the same trunk link, Layer 2 loops can occur due to diverting Spanning Tree Protocol (STP) BPDUs from their correct VLAN.

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Continue reading here: VLAN Ranges

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