Ch 4 - Fundamental EIGRP Concepts

Question 1

A router has been configured with the commands router eigrp 9 and network 172.16.1.0 0.0.0.255. No other EIGRP-related commands have been configured. The answers list the IP addresses that could be assigned to this router’s Fa0/0 interface. Which answers list an IP address/prefix length that would cause the router to enable EIGRP on Fa0/0? (Choose two answers.)

a. 172.16.0.1/23
b. 172.16.1.1/26
c. 172.16.1.1/24
d. 172.16.0.255/23
e. None of the other answers are correct.

Answer 1

B and C. The network 172.16.1.0 0.0.0.255 command tells Cisco IOS to match the first three octets when comparing the interface IP addresses to the configured “172.16.1.0” value. Only two answers match in the first three octets. The other two answers have a 0 in the third octet, making the addresses not match the network command.

Question 2

Router R1 has working interfaces with IP address/prefix combinations of:

• S0/0 - 10.10.10.1/24
• S0/1 - 10.10.11.2/24
• S0/2 - 10.10.12.3/22

R1’s configuration includes the commands router eigrp 9 and network 10.0.0.0. The show ip eigrp interfaces command lists S0/0 and S0/1 in the command output, but not S0/2. Which answer gives a possible reason for the omission?

a. R1 has EIGRP neighbors reachable through S0/0 and S0/1, but not through S0/2, so it is not included.
b. S0/2 might currently be in a state other than up/up.
c. The network 10.0.0.0 command requires the use of mask 255.0.0.0 because of EIGRP being classful by default.
d. S0/2 might be configured as a passive interface.

Answer 2

D. The show ip eigrp interfaces command displays interfaces on which EIGRP has been enabled but omits passive interfaces. Making the interface passive would omit the interface from the output of this command.

Question 3

Routers R1 and R2 are EIGRP neighbors using their Fa0/0 interfaces, respectively. An engineer adds the ip hello-interval eigrp 9 6 command to R1’s Fa0/0 configuration. Which of the following is true regarding the results from this change?

a. The show ip eigrp neighbors command on R1 lists the revised Hello timer.
b. The show ip eigrp interfaces command on R1 lists the revised Hello timer.
c. The R1-R2 neighborship fails because of a Hello timer mismatch.
d. The show ip eigrp interfaces detail command on R1 lists the revised Hello timer.

Answer 3

D. The show ip eigrp interfaces detail command does display a router’s EIGRP Hello timer setting for each enabled interface. The other listed commands do not display the timer. Also, EIGRP routers do not have to have matching Hello timers to become neighbors.

Question 4

Router R1 has been configured with the commands router eigrp 9 and network 172.16.2.0 0.0.0.255, with no other current EIGRP configuration. R1’s (working) Fa0/0 interface has been configured with IP address 172.16.2.2/26. R1 has found three EIGRP neighbors reachable through interface Fa0/0, including the router with IP address 172.16.2.20. When the engineer attempts to add the neighbor 172.16.2.20 fa0/0 command in EIGRP configuration mode, which of the following occurs? a. Fa0/0 fails. b. The command is rejected. c. The existing three neighbors fail. d. The neighborship with 172.16.2.20 fails and then reestablishes. e. None of the other answers is correct.

Answer 4

C. The neighbor 172.16.2.20 fa0/0 command would only be rejected if the IP address (172.16.2.20) is not inside the range of addresses in the subnet (172.16.2.0/26, range 172.16.2.0–172.16.2.63). This command does not impact the interface state. The command does disable all EIGRP multicasts, and because the three dynamically discovered neighbors require the EIGRP multicasts, all three neighbors fail. Although 172.16.2.20 is a valid potential neighbor, both routers must be configured with static neighbor commands, and we know that 172.16.2.20 was not previously configured with a static neighbor command; otherwise, it could not have been a neighbor with R1.

Question 5

Which of the following settings could prevent two potential EIGRP neighbors from becoming neighbors? (Choose two answers.) a. The interface used by one router to connect to the other router is passive in the EIGRP process. b. Duplicate EIGRP router IDs. c. Mismatched Hold Timers. d. IP addresses of 10.1.1.1/24 and 10.2.2.2/24, respectively.

Answer 5

A and D. Table 4-4 lists the issues. For EIGRP, Router IDs do not have to be unique for EIGRP routers to become neighbors, and the hold timer does not have to match between the two neighbors. However, making an interface passive disables the processing of all EIGRP messages on the interface, preventing all neighborships. Mismatched IP subnets also prevent neighborships from forming.

Question 6

A company has a Frame Relay WAN with one central-site router and 100 branch office routers. A partial mesh of PVCs exists: one PVC between the central site and each of the 100 branch routers. Which of the following could be true about the number of EIGRP neighborships? a. A partial mesh totaling 100: one between the central-site router and each of the 100 branches. b. A full mesh — (101 * 100) / 2 = 5050 — One neighborship between each pair of routers. c. 101 — One between each router (including the central site) and its nearby PE router. d. None of the answers is correct.

Answer 6

A. EIGRP forms neighborships only when two routers can communicate directly over a data link. As a result, with Frame Relay, EIGRP neighborships occur only between routers on the ends of a PVC, so in this case, 100 neighborships exist.

Question 7

T/F: ASNs must match for an EIGRP neighborship to form.

Answer 7

True.

Question 8

What network(s) are enabled with EIGRP if this command is issued? R1(config-router)# network 10.10.100.72

Answer 8

All interfaces that fall in the range 10.0.0.0 - 10.255.255.255. With no wild card mask configured, a classful network is matched.

Question 9

T/F: If a wildcard mask is configured, ACL logic is applied to the network id.

Answer 9

True.

Question 10

What range of interface address will match with this wildcard mask? R1(config-router)# network 10.1.0.0 0.0.31.255

Answer 10

10.0.0.0 - 10.1.31.255 The low end is the network id itself. The high end is determined easily by adding the wildcard mask to the network id.

Question 11

What command lists all interfaces enabled with EIGRP, but omits passives?

Answer 11

show ip eigrp interfaces

Question 12

What command lists the contents of the net config commands for each routing process, lists neighbors, explicitly lists passive interfaces and shows interfaces enabled with EIGRP?

Answer 12

Show ip protocols

Question 13

What command lists known and working neighbors?

Answer 13

Show ip eigrp neighbors

Question 14

What command shows all the successor and feasible routes?

Answer 14

show ip eigrp topology

Question 15

Name 3 tables that EIGRP uses.

Answer 15

Neighbor table, topology table and routing table.

Question 16

What table holds the candidates for the best IP routes?

Answer 16

Routing table

Question 17

What address does EIGRP send update messages to?

Answer 17

224.0.0.10 and a neighbor unicast address.

Question 18

What is the protocol used to send EIGRP update messages?

Answer 18

RTP - Reliable Transport Protocol. (Different than Real Time Transport protocol that is used for video/voice)

Question 19

T/F: RTP provides a guarantee of message delivery?

Answer 19

True.

Question 20

T/F: EIGRP routers calculate an integer metric using the EIGRP composite metric components and share these metrics with their neighbors.

Answer 20

False. Each router does it’s own calculations for route metrics independent of the others.

Question 21

What two EIGRP metrics are used by default?

Answer 21

Bandwidth and delay.

Question 22

T/F: the other three EIGRP metric components are load, reliability and MTU size.

Answer 22

True.

Question 23

T/F: MTU size is not actually used in the route metric calculation.

Answer 23

True. MTU size is only used in case of an equal cost metric as a tie breaker. It is not directly used in the metric calculations.

Question 24

T/F: EIGRP metric = [(10^7 / least BW) + cumulative delay] * 256 , where least BW is the lowest bandwidth link in the route in Kbs and cumulative delay = the sum of all egress delay values for all links in the route in units of 10s of microseconds.

Answer 24

True.

Question 25

How do you find the cumulative delay in tens of microseconds?

Answer 25

Show interface will show the delay in microseconds. Add up all the egresses and divide by 10, this will give you the right units, 10s of microseconds.

Question 26

T/F: EIGRP only supports a 128-bit hash value produced by the hashing method MD5 for authentication.

Answer 26

True.

Question 27

T/F: EIGRP sends the subnet mask along with the updates for each route. Because of this discontiguous networks can be supported.

Answer 27

True. VLSM works because of this too.

Question 28

T/F: EIGRP allows manual summarization at any point.

Answer 28

True.

Question 29

T/F: EIGRP supports automatic summarization at classfull boundaries but does not default to using this.

Answer 29

False. It actually does default to this.

Question 30

T/F: Neighborships require a common subnet.

Answer 30

True.

Question 31

T/F: One of the slowest parts of the convergence chain of events is the timer used to recognize that a route has gone down,.

Answer 31

True. If an interface is not up/up, EIGRP knows immediately. But what if the int is up and the link is bad? … Hold timer. This has a 15 second default on LANs, with a 5 second default Hello timer. This is much slower on T1 or slower or Frame Relay, on those the Hold Timer defaults to 180 seconds.

Question 32

T/F: Hello and Hold timers do not have to match for a neighborship to form.

Answer 32

True.

Question 33

T/F: Hello timer is a local setting and the Hold timer is actually sent to the neighbor routers and sets the hold timer on them.

Answer 33

True.

Question 34

What command will show the EIGRP timers?

Answer 34

Show ip eigrp interfaces detail OR show run.

Question 35

What if you want to advertise a subnet, but disallow neighborships on that interface? Which of these will work? A. Enable EIGRP on that interface with the network command and make the interface passive. B. Enable EIGRP on that interface with the network command but put it in a different ASN. C. Do not enable EIGRP on that interface and advertise the connected route by using the ‘redistribute connected’ D. Do not enable EIGRP on that interface and advertise the connected route by using the‘default-information originate’ command.

Answer 35

A, C

Question 36

Which of the following is true about passive interfaces? A. EIGRP sends no messages on that interface, multicast or unicast B. EIGRP does not advertise connected subnets that match the network statement C. EIGRP ignores any messages received on that interface D. EIGRP still advertises connected subnets that match the network statement E. EIGRP sends only unicast messages on that interface, no multicast updates are allowed.

Answer 36

A, C, D

Question 37

What is the AD on a route that has been redistributed with EIGRP?

Answer 37

170. Redistributed routes show as external routes.

Question 38

T/F: EIGRP supports static neighbor definitions.

Answer 38

True. Rarely used but useful in a scenario like Frame Relay which supports neither multicast nor broadcast.

Question 39

What is the command to define a static EIGRP neighbor?

Answer 39

R1(config-router)# neighbor Note: IP must be on same subnet as outgoing interface. Note2: network command is still required.

Question 40

What command can you use to show if a neighbor is defined statically?

Answer 40

Show ip eigrp neighbors detail. (sh ip eigrp neigh does not show this, you have to use detail)

Question 41

T/F: Cisco disables all EIGRP multicast processing on static neighbor interfaces. This means no dynamic discovery and if they are already discovered, they will be forgotten.

Answer 41

True.

Question 42

T/F: IP MTU must match for an EIGRP neighborship to form.

Answer 42

False. True for OSPF, but not for EIGRP.

Question 43

T/F: Router IDs must be unique for an EIGRP neighborship to form.

Answer 43

False. True for OSPF, but not for EIGRP. It is good practice to keep unique RIDs, but it is usually unimportant in day to day use of EIGRP. A RID value is required though.

Question 44

T/F: The “ metric weights” command can be used to change the K values.

Answer 44

True. The only value that cannot be changed is the first value, the TOS value is always 0.

Question 45

T/F: If the EIGRP RID is not set with “eigrp router-id ” command, then next option two options are 1. Use the highest IPv4 address on an up/up non-loopback 2. Use the highest IPv4 address on a loopback interface

Answer 45

False. These are correct, but the order is reversed.

Question 46

T/F: EIGRP neighborships can form over Frame Relay PVCs.

Answer 46

True. FR is a L2 WAN service and the PVCs provide a logical path for the routers to form neighborships over. A PVC must exist between two routers in order for this to occur.

Question 47

T/F: MPLS service can connect to routers with either serial, ethernet or Frame Relay PVCs.

Answer 47

True.

Question 48

T/F: CE routers can form neighborships with each other through an MPLS cloud.

Answer 48

False. The neighborship is actually formed to the PE router only. Routes are exchanged between the CE routers via the PE routers, the provider cloud and the magic of MP-BGP.

Question 49

T/F: In MetroE, all the CE routers can become neighbors because they appear to all be on the same L2 VLAN.

Answer 49

True. CPE sees the service handoff as a VLAN.