EIGRP Unicast Packets Requiring Acknowledgment | Generated by AI
Question: Which EIGRP packet is sent as a unicast and requires acknowledgment? A) Hello B) Update C) Query D) Reply
Answer:
The correct answer is B) Update, C) Query, and D) Reply — all three are sent as unicast and require explicit acknowledgment.
Only A) Hello does not fit the description.
Here’s a comprehensive breakdown of each EIGRP packet type regarding transmission method and acknowledgment:
| Packet Type | Transmission Method | Requires Acknowledgment? | Notes |
|---|---|---|---|
| Hello | Multicast (224.0.0.10) | No | Used for neighbor discovery and keepalive. Hellos are unreliable (no ACK required). |
| Update | Unicast (or multicast when sending to multiple new neighbors) | Yes – acknowledged with an ACK packet | Carries routing information. When sent unicast (normal case after neighbor is established), it is reliable. |
| Query | Unicast (to specific neighbors) | Yes – acknowledged with an ACK packet | Sent when a route is lost and EIGRP asks neighbors for alternative paths. Always reliable. |
| Reply | Unicast (back to the originator of the Query) | Yes – acknowledged with an ACK packet | Response to a Query containing known path (or no path). Always reliable. |
| ACK | Unicast | N/A (it is the acknowledgment itself) | Simple packet with no data, just to confirm receipt of Update/Query/Reply. |
Key Points on EIGRP Reliable Transport Protocol (RTP)
- EIGRP uses its own Reliable Transport Protocol (RTP) to guarantee delivery of certain packets.
- Packets that contain routing information or control information that affects the routing table (Update, Query, Reply) are sent reliably → they are always unicast (except initial Updates to new neighbors) and require an ACK.
- Hello and ACK packets are sent unreliably (no acknowledgment needed).
- The ACK is a separate EIGRP packet (opcode 5) with no data — just a sequence number acknowledgment.
Why the question can be tricky
Many practice questions expect Update as the single answer because it is the most common reliable unicast packet. However, technically Update, Query, and Reply all meet the criteria of being sent as unicast and requiring acknowledgment.
If the question forces a single choice and comes from Cisco’s official material (e.g., CCNP ENARSI), the historically most common expected answer is B) Update.
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