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Question is asking which protocol inserts a four byte tag into the ethernet frame and recalculates CRC value?

the question is written incorrectly.

802.1q inserts a 4 byte tag into the ethernet frame but recalculates the FCS not the CRC which is a different error detection algorithim,
ISL adds a ISL header to the start of the frame and then adds a CRC at the end of the frame by the means of double tagging.

This is a 802.1Q frame converted[Image: 741_4-1.gif]

that CRC in the question should be changed to FCS.

I am getting my information from.

CCNP BCMSN Official Certification Guide Fouth Edition Year 2006 Press
Chapter 5 Vlan Trunks Page: 121 ISL Protocol Paragraph 2 Figure 5-3
Chapter 5 Vlan Trunks Page: 122 IEEE 802.1Q Protocol Paragraph 3 - 6 Figure 5-4
Frame check sequence (FCS)- 4 bytes. This sequence contains a 32-bit cyclic redundancy check (CRC) value, which is created by the sending MAC and is recalculated by the receiving MAC to check for damaged frames.

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