Cisco 200-125 Exam

Which two of these are characteristics of the 802.1Q protocol? (Choose two.)

  • A. It modifies the 802.3 frame header, and thus requires that the FCS be recomputed.
  • B. It is used exclusively for tagging VLAN frames and does not address network reconvergence following switched network topology changes.
  • C. It is a Layer 2 messaging protocol which maintains VLAN configurations across networks.
  • D. It is a trunking protocol capable of carrying untagged frames.
  • E. It includes an 8-bit field which specifies the priority of a frame.
Answer: Option A. D.

802.1q inserts a new section into the frame header which holds the 12-bit VLAN id for the frame and a 3-bit priority value (thus, not E). Thus, the frame’s contents are changed, and the FCS checksum must be recalculated (A). It also includes MSTP to perform Spanning Tree across multiple VLANs (rules out B). Frames belonging to the native VLAN do not carry VLAN tags when sent over the trunk. Conversely, if an untagged frame is received on a trunk port, the frame is associated with the native VLAN configured on that port (D is true).

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