thinQmaster Virtual testing partner Adaptive practice exams that find your weak spots and drill them. Every answer explained. From $69
Enroll

thinQmaster

Find your exam

Five questions free, before you decide.

Step 1 · choose a vendor

In one of our camps? Your practice exams come with the tuition — they are on your study page already.

Routing & Switching · Foundational

IEEE 802.1Q

The IEEE standard for VLAN tagging that inserts a 4-byte tag with a 12-bit VLAN ID into Ethernet frames.

Definition

The 802.1Q tag contains the TPID 0x8100, a 3-bit Priority Code Point (CoS), a 1-bit DEI and a 12-bit VLAN ID. It increases the maximum frame size by 4 bytes. Untagged frames on a trunk belong to the native VLAN. It replaced Cisco's proprietary ISL.

Example

switchport trunk encapsulation dot1q on switches that also supported ISL

Exam tip

802.1Q adds a 4-byte tag; ISL encapsulated the whole frame with 30 bytes. 802.1Q is the only one current switches support.

Related terms