Which two of these are characteristics of the 802.1Q protocol? (Choose two.)
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A. B. C. D. E.AD
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).
The correct answers are C and E.
802.1Q is a protocol used for tagging VLAN frames in Ethernet networks. It adds a 4-byte tag to the original Ethernet frame, which allows the network to carry multiple VLANs on a single physical link.
A. This statement is incorrect. The 802.1Q protocol does not modify the 802.3 frame header. Instead, it adds a 4-byte tag to the original Ethernet frame.
B. This statement is incorrect. The 802.1Q protocol is used for tagging VLAN frames, but it also addresses network reconvergence following switched network topology changes. When a switch receives a frame with an unknown VLAN tag, it floods the frame to all ports in the VLAN, except for the port on which it was received. This helps to ensure that all switches in the VLAN learn the MAC addresses of the hosts connected to the network.
C. This statement is correct. The 802.1Q protocol is a Layer 2 messaging protocol which maintains VLAN configurations across networks. It allows multiple VLANs to be carried over a single physical link, which can help to reduce network complexity and improve network efficiency.
D. This statement is incorrect. The 802.1Q protocol is a trunking protocol, but it cannot carry untagged frames. All frames sent over an 802.1Q trunk link must be tagged with a VLAN ID.
E. This statement is correct. The 802.1Q protocol includes an 8-bit field which specifies the priority of a frame. This field is used to implement Quality of Service (QoS) features in the network, allowing higher-priority traffic to be given preferential treatment over lower-priority traffic.