Which three statements are true regarding the EIGRP update message? (Choose three.)
EIGRP (Enhanced Interior Gateway Routing Protocol) is a routing protocol that is widely used in enterprise networks. EIGRP uses update messages to communicate routing information between routers. These update messages are periodically sent by routers to their neighbors to advertise changes in network topology.
Here are the explanations for each statement:
A. Updates require an acknowledgement with an ACK message. This statement is false. EIGRP update messages are sent using Reliable Transport Protocol (RTP), which is a proprietary protocol developed by Cisco. RTP provides reliability to EIGRP updates, meaning that routers do not require an acknowledgment (ACK) message for every update they send. Instead, routers use a reliable multicast mechanism, where they send updates to a multicast address and receive acknowledgments from all neighbors that received the update.
B. Updates can be sent to the multicast address 220.127.116.11. This statement is true. EIGRP update messages are sent to the multicast address 18.104.22.168, which is the well-known multicast address used by EIGRP routers to exchange routing information.
C. Updates are sent as unicasts when they are retransmitted. This statement is true. When a router detects that an EIGRP update message has been lost, it retransmits the update message as a unicast to the neighbor that did not receive it. This unicast is sent using RTP, the same reliable transport protocol used for the initial multicast update.
D. Updates always include all routes known by the router with partial updates sent in the Reply message. This statement is false. EIGRP updates do not always include all routes known by the router. Instead, EIGRP uses a Diffusing Update Algorithm (DUAL) to determine which routes need to be advertised in an update message. When a change in the network topology occurs, EIGRP sends only the updates for the affected routes. If a neighbor does not have a route that has changed, it will not receive an update for that route.
E. ACKs for updates are handled by TCP mechanisms. This statement is false. EIGRP does not use TCP to handle acknowledgments. As mentioned earlier, EIGRP uses its own Reliable Transport Protocol (RTP) to provide reliability to update messages. RTP uses its own mechanism for acknowledgments and does not rely on TCP.