CCNA 2.5.e: 802.1Q


802.1Q is the standard used for switches to communicate Layer 2 VLAN information between each other


Study Notes:


  • 802.1Q, or dot1q for short, is a way to tag vlans on an Ethernet frame
  • It is an open standard and more popularly used than Cisco's proprietary VLAN encapsulation protocol called Inter-Switch Link (ISL)
  • ISL is not even mentioned on the CCNA exam topics so focus on 802.1Q!
  • Network devices that are vlan aware can add a vlan tag to the Ethernet frame that represents the vlan membership of the frame (Remember the Data and Padding portion of an Ethernet frame?)
  • Each frame must belong to only one vlan
  • The vlan tag is 4 bytes in length (32 bits)
    • Tag protocol identifier (TPID) - 16 bits - set to 0x8100 to identify the frame as IEEE 802.1Q-tagged frame
    • Tag control information (TCI) - 16 bits
      • Priority code point (PCP) - 3 bits - 802.1p class of service stating the frame's priority
      • Drop eligible indicator (DEI) - 1 bit - indicate frames eligible to be dropped
      • VLAN identifier (VID) - 12 bits - 4,094 vlans available from 0x001 to 0xFFE



PacketTracer Lab: CCNA-2.5.e-How-to-configure-802.1Q-trunk-links.pkt

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  1. joseph yang
    joseph yang
    July 4, 2019 at 2:22 am

    i dont understand what the PCP is. something about Quality of Service? and wiki has the code chart, but no explanation of what they mean.

  2. Joe Barger (CCNP/CCDP)
    Joe Barger (CCNP/CCDP) • Post Author •
    July 4, 2019 at 8:40 am

    Yes it’s for QoS. PCP is a class of service (CoS) marking that uses 3 bits of the 802.1q 4-byte portion of an Ethernet frame. So it’s a layer 2 QoS solution.
    Compare CoS to DSCP, which uses 8 bits of the IP header. DSCP is a layer 3 QoS feature.
    To have your switch respect the CoS markings you would enter the command:

    mls qos trust cos

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