CCNA DC FAQ: IPv4 Routing Protocol Concepts

CCNA  FAQ: IPv4 Routing Protocol Concepts

Figure: EIGRP Choosing the Longer but Better Route to Subnet

Q1. Which of the following distance vector features prevents routing loops by causing the routing protocol to advertise only a subset of known routes, as opposed to the full routing table, under normal stable conditions?
a. Route poisoning
b. Dijkstra SPF
c. Hello
d. Split horizon

Answer: D. Split horizon causes a router to not advertise a route out of the same interface on which the router was learned. It also causes the router to not advertise about the connected route on an interface in updates sent out that interface. Route poisoning also helps prevent loops, although it does not affect how many routes a router advertises about on an interface. The other two answers are unrelated to loop prevention.

Q2. Which of the following distance vector features prevents routing loops by advertising an infinite metric route when a route fails?
a. Dijkstra SPF
b. Hello
c. Split horizon
d. Route poisoning

Answer: D. Route poisoning means advertising the failed route with an “infinite” metric, as opposed to simply ceasing to advertise the route. Of the incorrect answers, SPF defines how link-state protocols calculate and choose routes; hello refers to the messages some routing protocols use to discover and monitor neighboring routers; and split horizon limits what routes a router advertises to help avoid routing loops.

Q3. Which of the following is true about both RIPv1 and RIPv2? (Choose two answers.)
a. Uses a hop-count metric
b. Sends update messages to multicast address
c. Supports authentication
d. Uses split horizon

Answer: A and D. RIPv2 continues to use several features like RIPv1 in an effort to allow easier migration from RIPv1 to RIPv2. The features include using the same hop-count metric and using split horizon as one of the loop prevention mechanisms. The other two answers are true about RIPv2, but not RIPv1.

Q4. Router R1 uses RIPv1 and learns one possible route to reach subnet That route would have a metric of 15 from R1’s perspective. Which of the following is true?
a. R1 cannot use the route, because metric 15 is considered to be infinity.
b. R1 will add the route to its routing table.
c. The cumulative bandwidth between R1 and subnet is 15 Mbps.
d. The slowest bandwidth of the links between R1 and subnet is 15 Kbps.

Answer: B. RIPv1 and RIPv2 both use hop-count as the metric, so the two answers referencing the bandwidth are incorrect. Both RIPv1 and RIPv2 consider a route with metric 16 to be a poisoned (bad) route, and the metric value 16 to be infinite, but a route with metric 15 is a usable route. So, R1 adds the route for to its IP routing table.

Q5. Routers A and B use EIGRP. How does router A watch for the status of router B so that router A can react if router B fails?
a. By using EIGRP hello messages, with A needing to receive periodic hello messages to believe B is still working.
b. By using EIGRP update messages, with A needing to receive periodic update messages to believe B is still working.
c. Using a periodic ping of B’s IP address based on the EIGRP neighbor timer.
d. None of the other answers is correct.

Answer: A. EIGRP separates the function of monitoring neighbor state into the hello message process, relying on the receipt of a hello message. If a router does not receive an EIGRP hello within the configured EIGRP hold time, the local router believes the neighbor has failed.

Q6. Which of the following affect the calculation of EIGRP metrics when all possible default values are used? (Choose two answers.)
a. Bandwidth
b. Delay
c. Load
d. Reliability
e. Hop count

Answer: A and B. EIGRP uses bandwidth and delay by default. Load and reliability can be added to the mix with configuration, but Cisco recommends against adding these to the metric calculation.

Q7. Which of the following routing protocols is considered to use link-state logic?
a. RIPv1
b. RIPv2

Answer: D. Both versions of RIP use distance vector logic, and EIGRP uses a different kind of logic, characterized either as advanced distance vector or a balanced hybrid.

Q8. Which of the following is true about how a router using a link-state routing protocol chooses the best route to reach a subnet?
a. The router finds the best route in the link-state database.
b. The router calculates the best route by running the SPF algorithm against the information in the link-state database.
c. The router compares the metrics listed for that subnet in the updates received from each neighbor and picks the best (lowest) metric route.
d. The router uses the path that has the lowest hop count.

Answer: B. Link-state protocols do not exchange data that lists routes. They do list metric information, but it is per-interface information, and it is not tied to a subnet. Linkstate protocols do require the SPF algorithm to take the varied pieces of information and create routes based on that information.

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James Palmer

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