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CCNP Route FAQ: EIGRP Topology, Routes, and Convergence

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CCNP Route FAQ: EIGRP Topology, Routes, and Convergence

Q1. Which of the following are methods EIGRP uses to initially populate (seed) its EIGRP topology table, before learning topology data from neighbors? (Choose two.)
a. By adding all subnets listed by the show ip route connected command
b. By adding the subnets of working interfaces over which static neighbors have been defined
c. By adding subnets redistributed on the local router from another routing source
d. By adding all subnets listed by the show ip route static command

Answer: B and C. Other than the two listed correct answers, the local router also adds connected routes for which the network command matches the corresponding interfaces, so it may not add all connected routes. Also, EIGRP does not add static routes to the EIGRP topology table, unless those routes are redistributed.

Q2. Which of the following are both advertised by EIGRP in the Update message and included in the formula for calculating the integer EIGRP metric? (Choose two.)
a. Jitter
b. Delay
c. MTU
d. Reliability

Answer: B and D. EIGRP sends bandwidth, delay, reliability, load, MTU, and hop-count in the message. The formula to calculate the metric includes bandwidth, delay, reliability, and load.

Q3. Router R1 uses S0/0 to connect via a T/1 to the Frame Relay service. Five PVCs terminate on the serial link. Three PVCs (101, 102, and 103) are configured on subinterface S0/0.1, and one each (104 and 105) are on S0/0.2 and S0/0.3. The configuration shows no configuration related to EIGRP WAN bandwidth control, and the bandwidth command is not configured at all. Which of the following is true about how IOS tries to limit EIGRP’s use of bandwidth on S0/0?
a. R1 limits EIGRP to around 250Kbps on DLCI 102.
b. R1 limits EIGRP to around 250Kbps on DLCI 104.
c. R1 limits EIGRP to around 150Kbps on every DLCI.
d. R1 does not limit EIGRP because no WAN bandwidth control has been configured.

Answer: A. EIGRP performs WAN bandwidth control without any explicit configuration, using default settings. Because no bandwidth commands have been configured, each subinterface uses the default 1544 Kbps setting. For S0/0.1, WAN bandwidth control divides the 1544 by 3 (515 Kbps), and then takes the (default) WAN bandwidth of 50 percent, meaning about 250 Kbps for each of the three DLCIs. For the two subinterfaces with one PVC, the default 1544 is multiplied by the 50 percent default WAN bandwidth, meaning that each could use about 750 Kbps.

Q4. The output of show ip eigrp topology on Router R1 shows the following output, which is all the output related to subnet 10.11.1.0/24. How many feasible successor routes does R1 have for 10.11.1.0/24?
P 10.11.1.0/24, 2 successors, FD is 2172419
via 10.1.1.2 (2172423/28167), Serial0/0/0.1
via 10.1.1.6 (2172423/28167), Serial0/0/0.2
a. 0
b. 1
c. 2
d. 3

Answer: A. This command lists all successor and feasible successor routes. The output states that two successors exist, and only two routes (listed with the “via…” text) exist. So, no feasible successor routes exist.


Figure: Successors and Feasible Successors with EIGRP

Q5. A network design shows that R1 has four different possible paths from itself to the Data Center subnets. Which of the following can influence which of those routes become feasible successor routes, assuming that you follow the Cisco recommended practice of not changing metric weights? (Choose two.)
a. The configuration of EIGRP offset lists
b. Current link loads
c. Changing interface delay settings
d. Configuration of variance

Answer: A and C. By default, the metric weights cause EIGRP to consider bandwidth and delay in the metric calculation, so changing either bandwidth or delay impacts the calculation of the feasible distance and reported distance, and impacts choice of feasible successor routes. Offset lists also change the metric, which in turn can change whether a route is an FS route. Link loading would impact the metrics, but not without changing the metric weights to nonrecommended values. Finally, variance impacts which routes end up in the IP routing table, but it is not considered by EIGRP when determining which routes are FS routes.

Q6. Router R1 is three router hops away from subnet 10.1.1.0/24. According to various show interfaces commands, all three links between R1 and 10.1.1.0/24 use the following settings: bandwidth: 1000, 500, 100000 and delay: 12000, 8000, 100. Which of the following answers correctly identifies a value that feeds into the EIGRP metric calculation? (Choose two correct answers.)
a. Bandwidth of 101,500
b. Bandwidth of about 34,000
c. Bandwidth of 500
d. Delay of 1200
e. Delay of 2010
f. Delay of 20100

Answer: C and E. The EIGRP metric calculation treats bandwidth and delay differently. For bandwidth, EIGRP takes the lowest bandwidth, in Kbps, which is in this case 500 Kbps. For delay, EIGRP takes the cumulative delay, which is 20100 per the various show interfaces commands. However, the show interfaces command uses a unit of microseconds, and the interface delay command, and the EIGRP metric formula uses a unit of tens-of-microseconds, making the delay that feeds into the formula be 2010.

Q7. Routers R1 and R2 are EIGRP neighbors. R1 has been configured with the eigrp stub connected command. Which of the following is true as a result? (Choose two correct answers.)
a. R1 can learn EIGRP routes from R2, but R2 cannot learn EIGRP routes from R1.
b. R1 can send IP packets to R2, but R2 cannot send IP packets to R1.
c. R2 no longer learns EIGRP routes from R1 for routes not connected to R1.
d. R1 no longer replies to R2’s Query messages.
e. R2 no longer sends to R1 Query messages.

Answer: C and E. R1, as a stub router with the connected option, still advertises routes, but only routes for connected subnets. R1 announces its stub attribute to R2, so R2 chooses to not send Query messages to R1, knowing that R1 cannot be a transit router for other subnets anyway.

Q8. A network design shows that R1 has four different possible paths from itself to the Data Center subnets. Which one of the following commands is most likely to show you all the possible next-hop IP addresses for these four possible routes?
a. show ip eigrp topology
b. show ip eigrp topology all-links
c. show ip route eigrp
d. show ip route eigrp all-links
e. show ip eigrp topology all-learned

Answer: B. Of the five options, the show ip route eigrp all-links and show ip eigrp topology all-learned are not valid commands. Both show ip eigrp topology and show ip route eigrp can show at most successor and feasible successor routes. However, show ip eigrp topology all-links shows also nonfeasible successor routes, making it more likely to show all possible neighbors.

Q9. Router R1 lists 4 routes for subnet 10.1.1.0/24 in the output of the show ip eigrp topology all-links command. The variance 200 command is configured, but no other related commands are configured. Which of the following rules are true regarding R1’s decision of what routes to add to the IP routing table? Note that RD refers to reported distance and FD to feasible distance.
a. Adds all routes for which the metric is <= 200 * the best metric among all routes
b. Adds all routes because of the ridiculously high variance setting
c. Adds all successor and feasible successor routes
d. Adds all successor and feasible successor routes for which the metric is <= 200 * the best metric among all routes

Answer: D. EIGRP considers only successor and feasible successor routes. Each of those routes must have metrics such that variance * metric is less than the best route’s metric; the best route’s metric is called the feasible distance (FD).