Follow the Frame Across a Trunk

About this interactive

From the lesson: - **Without trunking,** every VLAN needs its own cable between switches. A trunk carries them all on one. - **IEEE 802.1Q** slips a 4-byte tag between the source MAC address and the type field: a 2-byte tag protocol identifier, 3 bits of priority code point, 1 drop eligible bit and the 12-bit VLAN ID. - **Tagged on trunks, plain on access ports.** The switch adds the tag before a frame crosses a trunk and removes it before it leaves an access port. - **A broadcast stays in its VLAN,** crossing every trunk that carries that VLAN and leaving only through that VLAN's ports. - **12 bits give 4,096 IDs.** 0 and 4,095 are reserved, so usable VLANs run 1 to 4,094. (The captions garble the ranges: normal is 1 to 1,005, of which 1,002 to 1,005 are reserved, and extended is 1,006 to 4,094.)

What you'll learn

Predict how a frame crosses an 802.1Q trunk: tagged on the trunk, untagged on access ports, and delivered only to ports in its own VLAN
Explain why a trunk replaces one cable per VLAN, and what the 4-byte tag and its 12-bit VLAN ID carry

Aligned to

Cisco CCNA

2.2 Configure and verify interswitch connectivity

Key terms

VLAN Trunking: A method of carrying multiple VLANs across a single inter-switch link, eliminating the need for a separate physical connection per VLAN.
IEEE 802.1Q: The standard for VLAN tagging on Ethernet, which inserts a four-byte tag containing a 12-bit VLAN ID and a 3-bit priority field into the frame header so that a single link can carry traffic for many VLANs.
802.1Q Tag: An optional four-octet field inserted into an Ethernet frame header to identify which VLAN the frame belongs to.
Trunk Port: A switch port that carries traffic for multiple VLANs over a single link by tagging frames with their VLAN ID, used between switches or between a switch and a router. All VLANs are tagged except the native VLAN.
Access Port: A switch port that belongs to a single data VLAN and sends and receives untagged frames, used for connecting end devices such as PCs and printers. An access port may additionally carry a tagged voice VLAN for an IP phone without becoming a trunk.
VLAN ID: A numerical identifier assigned to a VLAN that distinguishes it from other VLANs on the network.
Allowed VLAN List: The configured set of VLANs permitted to pass traffic across a trunk port, managed using add, remove, or none keywords.

Topics

Vlan Trunking 802 1q Frame Tagging Vlans Interactive Predict

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