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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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