200-901 Network Fundamentals Practice Question
Which TCP flag is set in the second step of the three-way handshake?
⚠ Common exam trap
Cisco often tests the misconception that the second step uses only an ACK flag, confusing it with the third step where the client sends an ACK to complete the handshake.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
SYN and ACK
The TCP three-way handshake begins with the client sending a SYN segment to initiate a connection. In the second step, the server responds with a SYN-ACK segment, which both acknowledges the client's SYN (using the ACK flag) and synchronizes its own sequence number (using the SYN flag). This combined flag is essential for establishing a reliable, bidirectional connection.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
ACK
Why it's wrong here
ACK alone is not the second step; the server's reply sets both SYN and ACK, acknowledging the client's sequence number while opening its own. Pure ACK appears in the third step and in ordinary data transfer, so choosing it ignores the synchronisation half of the server's response.
- ✓
SYN and ACK
Why this is correct
The second handshake step has the server responding to the client's initial SYN with its own sequence number while acknowledging the client's, so both SYN and ACK flags are set. This synchronises sequence numbers in both directions.
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SYN
Why it's wrong here
SYN alone describes the first step, where the client requests synchronisation. The server's second-step reply carries SYN to synchronise its own sequence number plus ACK to confirm the client's, so selecting SYN alone omits the acknowledgement that distinguishes the second step.
- ✗
FIN
Why it's wrong here
FIN tears down an established connection and never appears during handshake setup. It is the correct flag when describing graceful connection termination, but the second handshake step requires synchronisation and acknowledgement flags, so FIN is unrelated to this phase.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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