疑问 关于802.1Q (v3.22 E-23题)

genish

Which two of these are **acteristics 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 a layer 2 messaging protocol which maintains VLSN configurations across networks.
C-
It includes an 8-bit field which specifies the priority of a frame.
D-
It is a trunking protocol capable of carrying untagged frames.

Correct A,D
Official explanation:
802.1Q protocol, or Virtual Bridged Local Area Networks protocol, mainly stipulates the realization of the VLAN 802.1Q is a standardized relay method that **s 4 bytes field into the original Ethernet frame and re-calculate the FCS 802.1Q frame relay supports two types of frame: marked and non-marked. Non-marked frame carries no VLAN identification information.

According to text book:
ISL
ISL functions at layer 2 by encapsulating a data frame with a new header and cyclic redundancy check (CRC).
IEEE 802.1Q
IEEE 802.1Q actually **s a field into the frame to identify the VLAN

所以无论从解释还是从教科书的描述， 我都觉得A选项比较符合ISL而不是本题所指的802.1Q, 那为什么还选A呢？

bookpig

不懂就问，做的很好，鼓励一下！……

fsseocn2009

在下对你的景仰之情有如滔滔长江之水，连绵不绝.