Vlan Trunks


What is a Trunk?
It is hard to describe VLANs without mentioning VLAN trunks. You learned about controlling network broadcasts with VLAN segmentation, and you saw how VLAN trunks transmitted traffic to different parts of the network configured in one VLAN
Definition of a VLAN Trunk
A trunk is a point-to-point link between one or more Ethernet switch interfaces and another networking device, such as a router or a switch. Ethernet trunks carry the traffic of multiple VLANs over a single link. A VLAN trunk allows you to extend the VLANs across an entire network. Cisco supports IEEE 802.1Q for coordinating trunks on Fast Ethernet and Gigabit Ethernet interfaces. You will learn about 802.1Q later in this section.
A VLAN trunk does not belong to a specific VLAN, rather it is a conduit for VLANs between switches and routers.
802.1Q Frame Tagging
Remember that switches are layer 2 devices. They only use the Ethernet frame header information to forward packets. The frame header does not contain information about which VLAN the frame should belong to. Subsequently, when Ethernet frames are placed on a trunk they need additional information about the VLANs they belong to. This is accomplished by using the 802.1q encapsulation header. This header adds a tag to the original Ethernet frame specifying the VLAN for which the frame belongs to.
Frame tagging has been mentioned a number of times. The first time was in reference to the voice mode configuration on a switch port. There you learned that once configured, a Cisco phone (which includes a small switch) tags voice frames with a VLAN ID. You also learned that VLAN IDs can be in a normal range, 1-1005, and an extended range, 1006-4094. How do VLAN IDs get inserted into a frame?
VLAN Frame Tagging Overview
Before exploring the details of an 802.1Q frame, it is helpful to understand what a switch does when it forwards a frame out a trunk link. When the switch receives a frame on a port configured in access mode with a static VLAN, the switch takes apart the frame and inserts a VLAN tag, recalculates the FCS and sends the tagged frame out a trunk port.
Note: An animation of the trunking operation is presented later in this section.
VLAN Tag Field Details
The VLAN tag field consists of an EtherType field, a tag control information field,and the FCS field.
EtherType field
Set to the hexadecimal value of 0x8100. This value is called the tag protocol ID (TPID) value. With the EtherType field set to the TPID value, the switch receiving the frame knows to look for information in the tag control information field.
Tag control information field
The tag control information field contains:
3 bits of user priority - Used by the 802.1p standard, which specifies how to provide expedited transmission of Layer 2 frames. A description of the IEEE 802.1p is beyond the scope of this course; however, you learned a little about it earlier in the discussion on voice VLANs.
1 bit of Canonical Format Identifier (CFI) - Enables Token Ring frames to be carried across Ethernet links easily.
12 bits of VLAN ID (VID) - VLAN identification numbers; supports up to 4096 VLAN IDs.
FCS field
After the switch inserts the EtherType and tag control information fields, it recalculates the FCS values and inserts it into the frame.
Native VLANs and 802.1Q Trunking
Now that you know more about how a switch tags a frame with the correct VLAN, it is time to explore how the native VLAN supports the switch in handling tagged and untagged frames that arrive on an 802.1Q trunk port.
Tagged Frames on the Native VLAN
Some devices that support trunking tag native VLAN traffic as a default behavior. Control traffic sent on the native VLAN should be untagged. If an 802.1Q trunk port receives a tagged frame on the native VLAN, it drops the frame. Consequently, when configuring a switch port on a Cisco switch, you need to identify these devices and configure them so that they do not send tagged frames on the native VLAN. Devices from other vendors that support tagged frames on the native VLAN include IP phones, servers, routers, and non-Cisco switches.
Untagged Frames on the Native VLAN
When a Cisco switch trunk port receives untagged frames it forwards those frames to the native VLAN. As you may recall, the default native VLAN is VLAN 1. When you configure an 802.1Q trunk port, a default Port VLAN ID (PVID) is assigned the value of the native VLAN ID. All untagged traffic coming in or out of the 802.1Q port is forwarded based on the PVID value. For example, if VLAN 99 is configured as the native VLAN, the PVID is 99 and all untagged traffic is forward to VLAN 99. If the native VLAN has not been reconfigured, the PVID value is set to VLAN 1.

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