350-401 EIGRP — 30 Questions

Question 1

A network engineer is troubleshooting an EIGRP issue in a large enterprise network. Two routers, R1 and R2, are connected via a T1 link. R1 is learning a route to 10.0.0.0/8 from R2 with a metric of 28160, but the same route is also learned from another neighbor with a metric of 26880. The engineer notices that the route from R2 is not being installed in the routing table. What is the most likely cause?

Accepted Answer

The route with metric 28160 is not installed because EIGRP selects the route with the lowest metric.. C is correct because EIGRP installs only the route with the best (lowest) metric into the routing table. The route from R2 has a metric of 28160, while the other neighbor advertises the same route with a metric of 26880. Since 26880 is lower, R1 selects that route as the successor and does not install the higher-metric route from R2.

Answer

The route from R2 is a feasible successor, so it is not installed in the routing table.

Answer

EIGRP is using unequal-cost load balancing, so the higher metric route is not used.

Answer

The route from R2 is a summary route, so it is not installed in the routing table.

Question 2

An engineer configures EIGRP on a new router in a DMVPN network. The router has a single physical interface with two subinterfaces: one for the DMVPN tunnel and one for a direct point-to-point link to a hub router. The engineer notices that EIGRP adjacencies form only on the point-to-point link, not on the DMVPN tunnel. The tunnel interface is configured with ip nhrp network-id 1 and ip nhrp nhs 10.1.1.1. What is the most likely reason?

Accepted Answer

The tunnel interface is not configured with the 'ip eigrp' command under the interface configuration.. Option B is correct because the most common reason for EIGRP not forming an adjacency over a tunnel interface is the absence of the 'ip eigrp' command under that interface. Without this command, EIGRP is not enabled on the tunnel, so it will not send or process Hello packets, preventing adjacency formation even if NHRP and tunnel parameters are correctly configured.

Answer

The tunnel interface is not configured with the 'ip nhrp map' command for the hub router.

Answer

The DMVPN tunnel is using a different autonomous system number than the point-to-point link.

Answer

The tunnel interface is in a different VRF than the point-to-point link.

Question 3

A network engineer is designing an EIGRP network with multiple routers. The network has a core layer where all routers are fully meshed. The engineer wants to ensure that if a link fails, EIGRP converges quickly without relying on route redistribution or static routes. The engineer configures EIGRP with default timers. However, during a failure simulation, convergence takes over 15 seconds. What is the most likely reason?

Accepted Answer

The failed link was the only feasible successor for the affected routes, causing EIGRP to go into active state and query neighbors.. When the only feasible successor (FS) for a route fails, EIGRP cannot perform a local recomputation and must transition the route to the active state. It then sends query packets to all neighbors to find an alternative path, which introduces significant delay due to the need to wait for replies from every neighbor in a fully meshed core. With default timers, this query/reply process can easily exceed 15 seconds, especially if any neighbor is slow to respond.

Answer

EIGRP is using passive interfaces on the core routers, preventing rapid updates.

Answer

EIGRP hold timers are set to 180 seconds by default, causing slow detection.

Answer

The engineer configured 'eigrp stub' on the core routers, which prevents query propagation.

Question 4

An engineer is troubleshooting an EIGRP issue where a router is not learning a specific route from a neighbor. The engineer runs 'show ip eigrp topology all-links' and sees the route in the topology table with a feasible distance of 100 and a reported distance of 120. The neighbor's advertised distance is 80. The router's own computed distance to the network is 150. The route is not in the routing table. What is the most likely cause?

Accepted Answer

The route is not installed because the feasible distance (100) is not the best metric; the router has another route with a lower metric.. Option C is correct because EIGRP installs the route with the best feasible distance (FD) into the routing table. The router's computed distance of 150 is lower than the feasible distance of 100, meaning the router has a better path (FD 100) already in the topology table. Since the route is not in the routing table, the router must have another route with a lower metric (FD 100) that is already installed, and the route with FD 150 is not selected as the successor.

Answer

The route is a feasible successor, but the successor route is not present.

Answer

The route is not installed because the reported distance (80) from the neighbor is less than the feasible distance (100), but the router's computed distance (150) is higher.

Answer

The route is not installed because EIGRP is configured for stub routing, which prevents learning routes.

Question 5

A network engineer is configuring EIGRP on a router that connects to multiple remote sites via Frame Relay. The engineer wants to ensure that EIGRP does not form adjacencies over the Frame Relay interfaces to reduce overhead, but still wants to advertise the connected networks. The engineer applies the 'passive-interface' command to the Frame Relay interfaces. However, the remote sites stop receiving the routes. What is the most likely reason?

Accepted Answer

The 'passive-interface' command also prevents EIGRP from sending routing updates on that interface.. The 'passive-interface' command in EIGRP prevents both hello and routing updates from being sent on the specified interface. Since EIGRP relies on hello packets to form and maintain neighbor adjacencies, applying this command to the Frame Relay interfaces stops adjacency formation. Without an adjacency, no routes are exchanged, so the remote sites stop receiving the advertised networks.

Answer

The 'passive-interface' command only affects hello packets, not updates, but the remote sites are not configured correctly.

Answer

The engineer should use the 'neighbor' command under the EIGRP process to specify the remote routers.

Answer

The remote sites are using a different EIGRP autonomous system number.

Question 6

An engineer is troubleshooting an EIGRP convergence issue in a network with redundant links. The engineer notices that when a primary link fails, the backup link takes over immediately, but the routing table shows the route with a higher metric. The engineer wants to ensure that the backup link is used only when the primary fails, and that traffic is not load-balanced. The engineer has configured 'variance 2' on all routers. What is the most likely effect of this configuration?

Accepted Answer

The variance 2 command causes EIGRP to install both the primary and backup routes, resulting in unequal-cost load balancing.. The 'variance 2' command in EIGRP allows the router to install multiple routes to the same destination network in the routing table, even if their metrics are not equal, as long as the metric of the alternate route is within the variance multiplier (2x) of the best metric (the feasible distance). Since the backup link has a higher metric but is within the variance, EIGRP installs both routes, causing unequal-cost load balancing. This explains why the backup link is actively used for traffic, contrary to the engineer's desire to use it only as a failover.

Answer

The variance 2 command causes EIGRP to install only the best metric route, so the backup link is not used.

Answer

The variance 2 command has no effect on route installation; it only affects the feasible successor selection.

Answer

The variance 2 command is used for equal-cost load balancing only.

Question 7

A network engineer is configuring EIGRP on a router that connects to a service provider network. The engineer wants to advertise a default route to internal routers. The engineer configures 'ip default-network 0.0.0.0' and redistributes a static default route into EIGRP. However, internal routers are not receiving the default route. The engineer checks the EIGRP topology table and sees the default route with a metric of 1. What is the most likely reason?

Accepted Answer

The engineer used 'ip default-network' which is not supported in EIGRP; instead, 'default-information originate' should be used.. Option A is correct because EIGRP does not support the 'ip default-network' command to originate a default route; this command is used with IGRP. To advertise a default route in EIGRP, the engineer must use the 'default-information originate' command under the EIGRP process, which redistributes a static default route (0.0.0.0/0) into EIGRP. The presence of the default route in the topology table with a metric of 1 indicates it was redistributed, but without 'default-information originate', EIGRP will not advertise it to neighbors.

Answer

The static default route is not configured correctly; the engineer should use 'ip route 0.0.0.0 0.0.0.0 <next-hop>'.

Answer

The internal routers have a route to the default network with a better metric from another source.

Answer

The engineer needs to configure 'eigrp stub' on the router to allow default route advertisement.

Question 8

A network engineer is designing an EIGRP network with multiple routers in a hub-and-spoke topology. The engineer wants to ensure that the spoke routers do not become transit routers for traffic between other spokes. The engineer configures 'eigrp stub' on the spoke routers. However, after configuration, the spoke routers stop learning some routes from the hub. What is the most likely reason?

Accepted Answer

The spoke routers are configured with 'eigrp stub receive-only', which prevents them from advertising any routes, so the hub does not have routes to the spoke's networks.. The 'eigrp stub receive-only' command configures the spoke router to only receive routes and not advertise any of its own networks. This causes the hub router to lack routes to the spoke's directly connected or summarized networks, breaking reachability from the hub to the spoke. The correct answer is A because this specific stub mode prevents the spoke from advertising any routes, which is the most likely reason the spoke stops learning some routes from the hub (since the hub may not have a route back to the spoke's networks).

Answer

The 'eigrp stub' command prevents the spoke from learning routes from the hub.

Answer

The hub router is also configured as stub, which prevents route propagation.

Answer

The spoke routers have a different EIGRP autonomous system number than the hub.

Question 9

A network engineer is troubleshooting an EIGRP issue where two routers, R1 and R2, are directly connected. R1 shows an EIGRP adjacency with R2, but R2 does not show an adjacency with R1. The engineer checks the interface configurations and finds that R1 has 'ip authentication mode eigrp 1 md5' and 'ip authentication key-chain eigrp 1 MYKEY' configured, while R2 has no authentication configured. What is the most likely cause?

Accepted Answer

R1 has authentication configured, but R2 does not, so R1 will reject R2's hello packets, and no adjacency forms.. EIGRP authentication is configured per interface and must match on both sides for an adjacency to form. R1 has MD5 authentication enabled, so it will include the authentication data in its hello packets and will reject any hello packets received from R2 that lack the correct authentication. Since R2 has no authentication configured, its hello packets are sent without authentication data, causing R1 to discard them. Consequently, R1 sees R2 as a neighbor (because it receives R2's unauthenticated hellos and may still attempt to form adjacency depending on implementation), but R2 never receives a valid hello from R1 (because R1's hellos are authenticated and R2 does not process the authentication field), so R2 does not establish an adjacency with R1.

Answer

R2 will automatically learn the authentication key from R1 and form an adjacency.

Answer

R1 will form an adjacency with R2 because authentication is optional.

Answer

The adjacency will form but only for routes that are not authenticated.

Question 10

A network engineer runs the following command on Router R1:

R1# show ip eigrp neighbors
EIGRP-IPv4 Neighbors for AS(100)
H   Address                 Interface       Hold Uptime   SRTT   RTO  Q  Seq
                                            (sec)         (ms)       Cnt Num
0   192.168.1.2             Gi0/0            13 00:12:34   12   100  0  45
1   10.1.1.2                Gi0/1            12 00:10:20   15   120  0  32

Based on this output, what can be concluded?

Accepted Answer

Both neighbors are fully established and exchanging routing information.. The 'show ip eigrp neighbors' output displays two neighbors in a stable state, indicated by the 'Q Cnt' (Queue Count) of 0 for both, meaning no packets are waiting to be sent. The 'Seq Num' (Sequence Number) values (45 and 32) show that R1 has received and processed EIGRP updates from each neighbor, confirming the adjacency is fully established and routing information is being exchanged. This output does not provide any metric or authentication details, so only the conclusion that both neighbors are fully operational is valid.

Answer

Both neighbors are using MD5 authentication.

Answer

Neighbor 192.168.1.2 has a higher metric than 10.1.1.2.

Answer

Router R1 is using EIGRP named mode.

Question 11

A network engineer runs the following command on Router R1:

R1# show ip eigrp topology
EIGRP-IPv4 Topology Table for AS(100)/ID(192.168.1.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status

P 10.1.1.0/24, 1 successors, FD is 1310720
        via 192.168.1.2 (1310720/1310720), GigabitEthernet0/0
P 10.2.2.0/24, 1 successors, FD is 1310720
        via 192.168.1.2 (1310720/1310720), GigabitEthernet0/0
P 10.3.3.0/24, 1 successors, FD is 1310720
        via 192.168.1.2 (1310720/1310720), GigabitEthernet0/0

Based on this output, what can be concluded?

Accepted Answer

Each route has exactly one successor and no feasible successor.. The output shows each route with a code 'P' (Passive) and exactly one successor, with no feasible successor listed. In EIGRP, a feasible successor is only present if there is a backup route that satisfies the feasibility condition (reported distance < feasible distance). Since only one next-hop is shown per route and no additional entries exist, there is no feasible successor. Option C correctly identifies this.

Answer

All routes have a feasible successor.

Answer

The routes are in Active state, meaning the router is querying for alternate paths.

Answer

The router is using EIGRP stub routing.

Question 12

A network engineer runs the following command on Router R1:

R1# show ip eigrp interfaces detail
EIGRP-IPv4 Interfaces for AS(100)

Interface: GigabitEthernet0/0
  Mtu: 1500, Bandwidth: 1000000 Kbit, Delay: 100 microseconds
  Reliability: 255/255, Load: 1/255, Min MTU: 1500
  Hello interval: 5 sec, Hold time: 15 sec
  Next hello in: 3 secs
  Passive interface: No
  Split horizon: Enabled
  Authentication: None
  Neighbor count: 1

Interface: GigabitEthernet0/1
  Mtu: 1500, Bandwidth: 100000 Kbit, Delay: 1000 microseconds
  Reliability: 255/255, Load: 1/255, Min MTU: 1500
  Hello interval: 5 sec, Hold time: 15 sec
  Next hello in: 1 secs
  Passive interface: No
  Split horizon: Enabled
  Authentication: None
  Neighbor count: 1

Based on this output, what can be concluded?

Accepted Answer

GigabitEthernet0/1 will have a higher EIGRP metric than GigabitEthernet0/0 for the same prefix.. Option B is correct because EIGRP uses the composite metric formula: metric = (K1 * bandwidth + (K2 * bandwidth) / (256 - load) + K3 * delay) * (K5 / (reliability + K4)). With default K values (K1=K3=1, others=0), the metric simplifies to bandwidth + delay. Gi0/0 has bandwidth 1,000,000 Kbit and delay 100 microseconds, while Gi0/1 has bandwidth 100,000 Kbit and delay 1000 microseconds. The lower bandwidth and higher delay on Gi0/1 result in a higher metric for the same prefix.

Answer

Both interfaces have the same metric weight for bandwidth and delay.

Answer

Interface Gi0/1 is configured as a passive interface.

Answer

EIGRP authentication is enabled on both interfaces.

Question 13

A network engineer runs the following command on Router R1:

R1# show ip eigrp traffic
EIGRP-IPv4 Traffic Statistics for AS(100)
  Hellos sent/received: 1000/950
  Updates sent/received: 45/40
  Queries sent/received: 2/3
  Replies sent/received: 3/2
  Acks sent/received: 50/48
  Input queue high water mark: 1
  Input queue depth: 0
  Total packets sent: 1100
  Total packets received: 1043

Based on this output, what can be concluded?

Accepted Answer

The network is stable with few topology changes.. The output shows a very low number of EIGRP Queries (2 sent, 3 received) and Replies (3 sent, 2 received), which indicates that the network has experienced very few topology changes. A stable EIGRP network with minimal route flaps will have a high ratio of Hellos to Updates/Queries, as seen here (1000 Hellos vs. 45 Updates). Therefore, the network is stable with few topology changes, making option B correct.

Answer

The router has experienced many route flaps.

Answer

The router is using EIGRP stub to suppress queries.

Answer

There is a high packet loss on the network.

Question 14

A network engineer runs the following command on Router R1:

R1# show ip eigrp topology 10.1.1.0/24
EIGRP-IPv4 Topology Entry for AS(100)/ID(192.168.1.1) for 10.1.1.0/24
  State: Passive, Reply status: 0, Originating router: 192.168.1.1
  Routing Descriptor Blocks:
  0.0.0.0 (Null0), from 0.0.0.0, Send flag: 0x0
      Composite metric: (128256/0), Route is Internal
      Vector metric:
        Minimum bandwidth: 1000000 Kbit
        Total delay: 100 microseconds
        Reliability: 255/255
        Load: 1/255
        Minimum MTU: 1500
        Hop count: 0
  192.168.1.2 (GigabitEthernet0/0), from 192.168.1.2, Send flag: 0x0
      Composite metric: (1310720/128256), Route is Internal
      Vector metric:
        Minimum bandwidth: 1000000 Kbit
        Total delay: 1100 microseconds
        Reliability: 255/255
        Load: 1/255
        Minimum MTU: 1500
        Hop count: 1

Based on this output, what can be concluded?

Accepted Answer

The route is directly connected on Router R1.. The presence of a Null0 route in the EIGRP topology table with a composite metric of (128256/0) and a hop count of 0 indicates that Router R1 has a directly connected interface on the 10.1.1.0/24 network. EIGRP automatically installs a summary or connected route to Null0 to prevent routing loops, and the metric (128256/0) with hop count 0 confirms it is locally originated, not learned from a neighbor.

Answer

The route 10.1.1.0/24 is learned from neighbor 192.168.1.2 and is the successor.

Answer

The router has a feasible successor for this route.

Answer

The router is in Active state for this route.

Question 15

A network engineer runs the following command on Router R1:

R1# debug eigrp packets hello
EIGRP: Received HELLO on Gi0/0 nbr 192.168.1.2
  AS 100, Flags 0x0, Seq 0/0 interfaceQ 0/0
EIGRP: Sending HELLO on Gi0/0
  AS 100, Flags 0x0, Seq 0/0 interfaceQ 0/0
EIGRP: Received HELLO on Gi0/1 nbr 10.1.1.2
  AS 100, Flags 0x0, Seq 0/0 interfaceQ 0/0
EIGRP: Sending HELLO on Gi0/1
  AS 100, Flags 0x0, Seq 0/0 interfaceQ 0/0

Based on this output, what can be concluded?

Accepted Answer

The hello packets are being exchanged normally without any issues.. The debug output shows EIGRP hello packets being sent and received on both interfaces without any errors, sequence mismatches, or authentication failures. The 'Flags 0x0' and 'Seq 0/0' indicate normal operation, and the neighbor IPs are present, confirming that the EIGRP adjacency is forming correctly. Therefore, the hello packets are being exchanged normally without any issues.

Answer

EIGRP is using MD5 authentication between neighbors.

Answer

There is a K-value mismatch between R1 and its neighbors.

Answer

The router is using EIGRP named mode.

Question 16

A network engineer runs the following command on Router R1:

R1# show ip eigrp neighbors detail
EIGRP-IPv4 Neighbors for AS(100)
H   Address                 Interface       Hold Uptime   SRTT   RTO  Q  Seq
                                            (sec)         (ms)       Cnt Num
0   192.168.1.2             Gi0/0            13 00:12:34   12   100  0  45
  Version 2.0/2.0, Retrans: 0, Retry: 0, Maxseq: 0
  Prefixes: 3
  Topology ids: 0
  Authentication: None
  Topology: base (0x0)

Based on this output, what can be concluded?

Accepted Answer

The neighbor has advertised 3 prefixes to R1.. The output shows 'Prefixes: 3' under the neighbor details, which indicates that the neighbor has advertised exactly three prefixes to R1. This is a direct interpretation of the 'show ip eigrp neighbors detail' command, where the 'Prefixes' field lists the number of routes learned from that neighbor.

Answer

The neighbor is using EIGRP version 1.

Answer

There is a high number of retransmissions indicating packet loss.

Answer

The neighbor is using MD5 authentication.

Question 17

A network engineer runs the following command on Router R1:

R1# show ip eigrp interfaces
EIGRP-IPv4 Interfaces for AS(100)

Interface                   Peers    Xmit Queue   Mean   Pacing Time   Multicast    Pending
                           Un/Reliable  SRTT   Un/Reliable   Flow Timer   Routes
Gi0/0                         1       0/0       12       0/10        50           0
Gi0/1                         1       0/0       15       0/10        55           0

Based on this output, what can be concluded?

Accepted Answer

Interface Gi0/1 has higher latency than Gi0/0.. The 'Mean SRTT' (Smooth Round-Trip Time) column shows the average time in milliseconds for EIGRP packets to reach a neighbor and receive an acknowledgment. Gi0/1 has an SRTT of 15 ms, while Gi0/0 has an SRTT of 12 ms, indicating higher latency on Gi0/1. This directly correlates to the path's delay, which EIGRP uses in its composite metric calculation.

Answer

Both interfaces have pending routes to send.

Answer

The router is using EIGRP named mode.

Answer

There is a queue backlog on Gi0/0.

Question 18

A network engineer runs the following command on Router R1:

R1# show ip eigrp topology all-links
EIGRP-IPv4 Topology Table for AS(100)/ID(192.168.1.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status

P 10.1.1.0/24, 1 successors, FD is 1310720, serno 5
        via 192.168.1.2 (1310720/1310720), GigabitEthernet0/0
        via 10.2.2.2 (1587200/1310720), GigabitEthernet0/1
P 10.2.2.0/24, 1 successors, FD is 1310720, serno 6
        via 192.168.1.2 (1310720/1310720), GigabitEthernet0/0
        via 10.2.2.2 (1587200/1310720), GigabitEthernet0/1

Based on this output, what can be concluded?

Accepted Answer

The router has two paths to 10.1.1.0/24, but only one is in the routing table.. The output shows that for 10.1.1.0/24, the feasible distance (FD) is 1310720 and there is only one successor (the route via 192.168.1.2). The alternate path via 10.2.2.2 has a reported distance (RD) of 1310720, which equals the FD, so it does not satisfy the feasibility condition (RD < FD) and therefore is not a feasible successor. Only the successor route is installed in the routing table.

Answer

Both routes have a feasible successor via 10.2.2.2.

Answer

The route 10.1.1.0/24 has two successors.

Answer

The alternate path via 10.2.2.2 will be used immediately if the successor fails.

Question 19

Consider the following EIGRP configuration on a Cisco IOS router:

router eigrp 100
 network 10.0.0.0
 passive-interface default
 no passive-interface GigabitEthernet0/1

What is the effect of this configuration?

Accepted Answer

All interfaces except GigabitEthernet0/1 are passive and will not form EIGRP adjacencies.. The 'passive-interface default' command sets all interfaces to passive by default, preventing them from sending EIGRP hellos and forming adjacencies. The 'no passive-interface GigabitEthernet0/1' command then overrides this default for that specific interface, allowing it to send hellos and form adjacencies. Therefore, only GigabitEthernet0/1 is active for EIGRP neighbor discovery, while all other interfaces remain passive.

Answer

EIGRP hellos are sent on all interfaces, but GigabitEthernet0/1 is prevented from forming adjacencies.

Answer

Only the network 10.0.0.0 is advertised, and all interfaces are passive.

Answer

EIGRP will only form adjacencies on interfaces with an IP address in the 10.0.0.0/8 range.

Question 20

Given the following EIGRP configuration on a router:

router eigrp 200
 network 192.168.1.0 0.0.0.255
 network 10.0.0.0

Which statement about this configuration is true?

Accepted Answer

EIGRP will be enabled on any interface with an IP address in the 10.0.0.0/8 range, and only on interfaces with an IP address in the 192.168.1.0/24 range.. Option A is correct because the first network statement uses a wildcard mask (0.0.0.255) to enable EIGRP only on interfaces in the 192.168.1.0/24 subnet, while the second network statement (10.0.0.0) is classful and enables EIGRP on all interfaces in the 10.0.0.0/8 range. EIGRP network statements can include a wildcard mask to specify a subnet; without one, the router assumes the classful boundary.

Answer

Both network statements are classful and will enable EIGRP on all interfaces with IP addresses in the 192.168.0.0/16 and 10.0.0.0/8 ranges.

Answer

EIGRP will only be enabled on interfaces with an IP address in the 192.168.1.0/24 range.

Answer

This configuration will cause an error because wildcard masks are not allowed in EIGRP network statements.