300-410 Mock Exam 84 — 25 Questions

Question 1

A network engineer is troubleshooting an OSPF adjacency that fails to come up. Both routers are directly connected via a serial link. BFD is enabled on the interface. The engineer sees that the BFD session is down. The OSPF configuration shows 'ip ospf bfd' under the interface. The serial interface is up/up. What should the engineer check first?

Accepted Answer

Verify that the 'bfd' command is configured under the OSPF process.. BFD on serial interfaces often requires the 'bfd interval' command to be explicitly configured, as the default may be disabled. Also, serial interfaces may need the 'bfd' command under the routing protocol as well.

Answer

Check if the serial interface is configured with 'encapsulation ppp'.

Answer

Ensure that the 'ip ospf bfd' command is also configured on the neighbor router.

Answer

Verify that the serial interface clock rate is set correctly.

Question 2

A network engineer is troubleshooting a BGP session that is not establishing. The routers are connected via a Layer 3 switch. BFD is configured for BGP. The engineer checks the BFD session and sees it is 'Down'. The BGP configuration appears correct. The interface between the routers is up/up. What is the most likely cause?

Accepted Answer

The BGP neighbor is not directly connected; BFD requires a directly connected interface or a static route pointing to the neighbor's IP.. BFD sessions require that the destination IP address be reachable via a directly connected interface or a static route. If the BGP neighbor is not directly connected (e.g., via a loopback), BFD may fail if the next hop is not directly connected or if there is a routing issue.

Answer

The Layer 3 switch is not configured for BFD, causing it to drop BFD packets.

Answer

The BGP session is using EBGP multihop, and the TTL is set to 1.

Answer

The interface is configured with 'bfd interval 50 min_rx 50 multiplier 3' but the neighbor is not configured for BFD.

Question 3

A network engineer is troubleshooting an OSPF adjacency that is not forming. Both routers are running OSPF with BFD enabled. The engineer checks the BFD session and sees it is 'Up'. However, the OSPF neighbor state is stuck in 'INIT'. What is the most likely cause?

Accepted Answer

The OSPF area ID is mismatched between the two routers.. OSPF stuck in INIT usually indicates that the router is not receiving OSPF hello packets from the neighbor. BFD being up suggests Layer 2 connectivity is fine, so the issue is likely OSPF configuration, such as a mismatch in area ID or authentication.

Answer

The OSPF hello interval is mismatched between the two routers.

Answer

The OSPF network type is mismatched (e.g., broadcast vs point-to-point).

Answer

The interface is configured with 'ip ospf bfd' but the OSPF process is not configured with 'bfd all-interfaces'.

Question 4

A network engineer is troubleshooting a scenario where BFD sessions are not forming between two routers running IS-IS. Both routers have BFD configured under the IS-IS process and on the interfaces. The engineer checks the BFD session and sees it is 'Down'. The IS-IS adjacency is up and operational. What is the most likely cause?

Accepted Answer

The BFD session is using the wrong source IP address; IS-IS BFD requires the interface IP to be used.. IS-IS BFD requires that the BFD session be established on the same interface as the IS-IS adjacency. If the IS-IS adjacency is up but BFD is down, the issue is often that the BFD configuration is not applied to the correct interface or that the BFD timers are not compatible.

Answer

The IS-IS process is configured with 'bfd all-interfaces' but the interface is not configured with 'isis bfd'.

Answer

The IS-IS metric is set to a high value, causing BFD to be ignored.

Answer

The interface is configured with 'bfd interval 50 min_rx 50 multiplier 3' but the neighbor is not configured for BFD.

Question 5

A VRF-aware network uses route leaking between VRF A and VRF B. After configuring Flexible NetFlow to monitor traffic in VRF A, some routes that were previously leaked to VRF B disappear. Router R1 has: ip route vrf A 10.0.0.0 255.0.0.0 Null0. route-map LEAK permit 10 match ip address prefix-list GLOBAL. The prefix-list GLOBAL permits 10.0.0.0/8. The flow monitor is applied to the VRF A interface. What is the root cause?

Accepted Answer

The route-map LEAK is also applied to the flow monitor as a filter, and the flow monitor's match statement is misconfigured to deny the route, preventing the route from being leaked.. Flexible NetFlow can be applied per VRF, but if the flow monitor is configured with a match statement that includes the VRF ID or if the flow record includes the 'vrf' field, it might cause the router to process packets differently. However, the issue is that the route-map used for leaking is also being used by the flow monitor, or the flow monitor is inadvertently matching on the route-map. The correct answer is that the flow monitor is configured with a match ip address prefix-list statement that references the same prefix-list used in the route-map, but the flow monitor is applied in the input direction, causing the router to evaluate the prefix-list for every packet, which may interfere with the route leaking process. More specifically, if the flow monitor uses a match ip address prefix-list that includes a deny statement, it could affect the route-map's operation. But the most likely root cause is that the flow monitor is configured with a flow record that includes the 'ipv4 destination prefix' field, and the router is using the same prefix-list for both purposes, causing a conflict. Actually, the correct answer is that the route-map LEAK is also used by the flow monitor as a filter, and the flow monitor's match statement is inadvertently denying the route.

Answer

The flow monitor is configured with a match ip address prefix-list that references the same prefix-list as the route-map, but the flow monitor's action is to drop packets matching that prefix, causing the route to be withdrawn.

Answer

The VRF A interface has a flow monitor applied that uses a flow record with the 'vrf' field, causing the router to ignore the route leaking configuration.

Answer

The route leaking configuration requires a next-hop that is not reachable due to the flow monitor consuming bandwidth.

Question 6

A network engineer is troubleshooting a BGP session that is flapping. The routers are connected via a direct Ethernet cable. BFD is configured for the BGP session. The engineer checks the BFD session and sees it is 'Up'. However, the BGP session goes down every 30 seconds. The BGP configuration includes 'neighbor 10.0.0.2 fall-over bfd'. What is the most likely cause?

Accepted Answer

The BGP hold timer is set to 30 seconds on one router and 90 seconds on the other.. The BGP fall-over bfd command causes BGP to monitor the BFD session. If the BFD session is up but BGP is flapping, the issue is likely a BGP configuration problem, such as a mismatch in hold timer or update-source.

Answer

The BFD session is using echo mode, which is not supported for BGP fall-over.

Answer

The interface is configured with 'bfd interval 50 min_rx 50 multiplier 3' but the neighbor is configured with 'bfd interval 100 min_rx 100 multiplier 3'.

Answer

The BGP session is using EBGP multihop, and the TTL is set to 2.

Question 7

A network engineer is troubleshooting an EIGRP adjacency that is not forming. Both routers are configured for EIGRP with BFD. The engineer checks the BFD session and sees it is 'Down'. The interface between the routers is up/up. The EIGRP configuration appears correct. What is the most likely cause?

Accepted Answer

The BFD timers are configured with 'bfd interval 50 min_rx 50 multiplier 3' on one router and 'bfd interval 100 min_rx 100 multiplier 3' on the other, causing a negotiation failure.. EIGRP BFD requires that the BFD session be established before the EIGRP adjacency can form. If the BFD session is down, the issue is often a mismatch in BFD parameters or a lack of BFD configuration on one side.

Answer

The EIGRP process is configured with 'bfd all-interfaces' but the interface is not configured with 'eigrp bfd'.

Answer

The interface is configured with 'ip address 10.0.0.1 255.255.255.252' but the neighbor is using a different subnet.

Answer

The EIGRP autonomous system number is mismatched between the two routers.

Question 8

A network engineer runs the following command on Router R1:

R1# show bfd neighbors detail

IPv4 Sessions
NeighborAddr      LD/RD         Int        State         Holdown(mult)  Intf
10.1.1.2          1/3           Gi0/0      Up            1500(3)        Gi0/0

Session state is UP and not using echo function.
OurAddr: 10.1.1.1
Handle: 1
Local Diag: 0, Demand mode: 0, Poll bit: 0
MinTxInt: 1000000, MinRxInt: 1000000, Multiplier: 3
Received MinRxInt: 1000000, Received Multiplier: 3
Holddown (hits): 1500(0)
Rx Count: 120, Tx Count: 150

Based on this output, which statement is correct?

Accepted Answer

The BFD session is up and the holddown timer is 1500 ms, which is correct based on the configured multiplier and MinRxInt.. The output shows BFD session details. The Holddown value of 1500 ms is calculated as MinRxInt * Multiplier (1000 ms * 3 = 3000 ms), but the output shows 1500 ms. This discrepancy indicates a misconfiguration or a bug, but the key point is that the session is UP and the holddown timer is 1500 ms, which is half of the expected value. However, the correct interpretation is that the BFD session is established and operating, and the holddown timer is 1500 ms as shown.

Answer

The BFD session is down because the holddown timer is incorrect.

Answer

The BFD session is using echo mode, which is why the holddown timer is 1500 ms.

Answer

The BFD session is up but the multiplier is set to 1, causing the holddown timer to be 1500 ms.

Question 9

A network engineer runs the following command on Router R1:

R1# show bfd neighbors detail

IPv4 Sessions
NeighborAddr      LD/RD         Int        State         Holdown(mult)  Intf
10.1.1.2          1/3           Gi0/0      Down          0(0)          Gi0/0

Session state is DOWN
OurAddr: 10.1.1.1
Handle: 1
Local Diag: 1, Demand mode: 0, Poll bit: 0
MinTxInt: 1000000, MinRxInt: 1000000, Multiplier: 3
Received MinRxInt: 1000000, Received Multiplier: 3
Holddown (hits): 0(0)
Rx Count: 0, Tx Count: 50

Based on this output, what is the most likely cause of the BFD session being down?

Accepted Answer

The BFD session is down because the neighbor is not responding to BFD packets, likely due to a network connectivity issue or neighbor misconfiguration.. The Local Diag field shows 1, which indicates 'Control Detection Time Expired'. This means the local router did not receive BFD control packets from the neighbor within the expected time. The Rx Count is 0, confirming no packets were received. Tx Count is 50, so the local router is sending packets but not receiving any.

Answer

The neighbor router is not configured for BFD.

Answer

The BFD session is down because the local router is not sending BFD packets.

Answer

The BFD session is down because the multiplier is set too low.

Question 10

According to RFC 5880, what is the maximum number of BFD sessions that can be supported by a single interface on a Cisco router?

Accepted Answer

Unlimited, but limited by platform resources. RFC 5880 does not specify a maximum number of BFD sessions per interface; it is platform-dependent. Cisco routers can support multiple BFD sessions per interface, limited only by hardware resources.

Answer

1 session per interface

Answer

256 sessions per interface

Answer

16 sessions per interface

Question 11

A network engineer runs the following command on Router R1:

R1# show bfd neighbors detail

IPv4 Sessions
NeighborAddr      LD/RD         Int        State         Holdown(mult)  Intf
10.1.1.2          1/3           Gi0/0      Up            3000(3)        Gi0/0

Session state is UP and using echo function.
OurAddr: 10.1.1.1
Handle: 1
Local Diag: 0, Demand mode: 0, Poll bit: 0
MinTxInt: 1000000, MinRxInt: 1000000, Multiplier: 3
Received MinRxInt: 1000000, Received Multiplier: 3
Holddown (hits): 3000(0)
Rx Count: 150, Tx Count: 150
Echo function: Active

Based on this output, which statement is correct?

Accepted Answer

The BFD session is using echo mode, which reduces the load on the control plane because echo packets are processed in hardware.. The output shows that the BFD session is up and using echo function. The echo function allows faster failure detection by sending echo packets that are looped back by the neighbor. The holddown timer is 3000 ms, which is consistent with MinRxInt * Multiplier = 3000 ms. The Rx and Tx counts are equal, indicating bidirectional communication.

Answer

The BFD session is using echo mode, but the holddown timer is incorrect because echo mode should halve the timer.

Answer

The BFD session is up, but the echo function is not working because Rx and Tx counts are equal.

Answer

The BFD session is up and using echo function, but the neighbor must also have echo mode enabled for it to work.

Question 12

A network engineer runs the following command on Router R1:

R1# show bfd neighbors detail

IPv4 Sessions
NeighborAddr      LD/RD         Int        State         Holdown(mult)  Intf
10.1.1.2          1/3           Gi0/0      Up            3000(3)        Gi0/0

Session state is UP and not using echo function.
OurAddr: 10.1.1.1
Handle: 1
Local Diag: 0, Demand mode: 0, Poll bit: 0
MinTxInt: 1000000, MinRxInt: 1000000, Multiplier: 3
Received MinRxInt: 500000, Received Multiplier: 3
Holddown (hits): 1500(0)
Rx Count: 200, Tx Count: 200

Based on this output, what is the holddown timer value in milliseconds and why?

Accepted Answer

The holddown timer is 3000 ms, but the output shows 1500 ms because the holddown timer displayed is half of the actual holddown time.. The holddown timer is calculated as the maximum of the local MinRxInt and the received MinRxInt, multiplied by the multiplier. Local MinRxInt is 1000000 microseconds, received MinRxInt is 500000 microseconds. The maximum is 1000000 microseconds. Multiplied by 3 gives 3000000 microseconds = 3000 ms. However, the output shows 1500 ms. This is because the holddown timer displayed is actually half of the calculated value due to a Cisco implementation detail where the holddown timer is divided by 2 for display purposes. The actual holddown time is 3000 ms.

Answer

The holddown timer is 1500 ms, which is the received MinRxInt (500 ms) multiplied by the multiplier (3).

Answer

The holddown timer is 1500 ms, which is the local MinRxInt (1000 ms) multiplied by the multiplier (3) divided by 2.

Answer

The holddown timer is 3000 ms, and the output is correct as is.

Question 13

A network engineer runs the following command on Router R1:

R1# show bfd neighbors detail

IPv4 Sessions
NeighborAddr      LD/RD         Int        State         Holdown(mult)  Intf
10.1.1.2          1/3           Gi0/0      Up            3000(3)        Gi0/0

Session state is UP and not using echo function.
OurAddr: 10.1.1.1
Handle: 1
Local Diag: 0, Demand mode: 0, Poll bit: 0
MinTxInt: 1000000, MinRxInt: 1000000, Multiplier: 3
Received MinRxInt: 1000000, Received Multiplier: 3
Holddown (hits): 3000(0)
Rx Count: 100, Tx Count: 100

Based on this output, what is the BFD session's negotiated transmit interval?

Accepted Answer

The negotiated transmit interval is 1000 ms.. The negotiated transmit interval is the maximum of the local MinTxInt and the received MinRxInt. Local MinTxInt is 1000000 microseconds, received MinRxInt is 1000000 microseconds. The maximum is 1000000 microseconds, which is 1000 ms. The BFD session will transmit control packets every 1000 ms.

Answer

The negotiated transmit interval is 500 ms.

Answer

The negotiated transmit interval is 3000 ms.

Answer

The negotiated transmit interval is 100 ms.

Question 14

A network engineer runs the following command on Router R1:

R1# show bfd neighbors detail

IPv4 Sessions
NeighborAddr      LD/RD         Int        State         Holdown(mult)  Intf
10.1.1.2          1/3           Gi0/0      Up            3000(3)        Gi0/0

Session state is UP and not using echo function.
OurAddr: 10.1.1.1
Handle: 1
Local Diag: 0, Demand mode: 0, Poll bit: 0
MinTxInt: 1000000, MinRxInt: 1000000, Multiplier: 3
Received MinRxInt: 1000000, Received Multiplier: 3
Holddown (hits): 3000(0)
Rx Count: 100, Tx Count: 100

Based on this output, what is the BFD session's negotiated receive interval?

Accepted Answer

The negotiated receive interval is 1000 ms.. The negotiated receive interval is the maximum of the local MinRxInt and the received MinTxInt. However, the output does not show the received MinTxInt directly. The received MinRxInt is shown, but the receive interval is determined by the local MinRxInt. The local MinRxInt is 1000000 microseconds, so the router can receive BFD packets at intervals of 1000 ms or slower. The negotiated receive interval is typically the local MinRxInt, which is 1000 ms.

Answer

The negotiated receive interval is 500 ms.

Answer

The negotiated receive interval is 3000 ms.

Answer

The negotiated receive interval is 100 ms.

Question 15

A network engineer runs the following command on Router R1:

R1# show bfd neighbors detail

IPv4 Sessions
NeighborAddr      LD/RD         Int        State         Holdown(mult)  Intf
10.1.1.2          1/3           Gi0/0      Up            3000(3)        Gi0/0

Session state is UP and not using echo function.
OurAddr: 10.1.1.1
Handle: 1
Local Diag: 0, Demand mode: 0, Poll bit: 0
MinTxInt: 1000000, MinRxInt: 1000000, Multiplier: 3
Received MinRxInt: 1000000, Received Multiplier: 3
Holddown (hits): 3000(0)
Rx Count: 100, Tx Count: 100

Based on this output, what is the BFD session's detection time?

Accepted Answer

The detection time is 3000 ms.. The detection time is the holddown timer, which is the negotiated transmit interval multiplied by the multiplier. The negotiated transmit interval is 1000 ms (maximum of local MinTxInt and received MinRxInt), and the multiplier is 3, so the detection time is 3000 ms.

Answer

The detection time is 1000 ms.

Answer

The detection time is 1500 ms.

Answer

The detection time is 9000 ms.

Question 16

A network engineer runs the following command on Router R1:

R1# show bfd neighbors detail

IPv4 Sessions
NeighborAddr      LD/RD         Int        State         Holdown(mult)  Intf
10.1.1.2          1/3           Gi0/0      Up            3000(3)        Gi0/0

Session state is UP and not using echo function.
OurAddr: 10.1.1.1
Handle: 1
Local Diag: 0, Demand mode: 0, Poll bit: 0
MinTxInt: 1000000, MinRxInt: 1000000, Multiplier: 3
Received MinRxInt: 1000000, Received Multiplier: 3
Holddown (hits): 3000(0)
Rx Count: 100, Tx Count: 100

Based on this output, what is the BFD session's local discriminator?

Accepted Answer

The local discriminator is 1.. The local discriminator is the LD value shown in the output. In this case, it is 1.

Answer

The local discriminator is 3.

Answer

The local discriminator is 10.1.1.1.

Answer

The local discriminator is 10.1.1.2.

Question 17

A network engineer runs the following command on Router R1:

R1# show bfd neighbors detail

IPv4 Sessions
NeighborAddr      LD/RD         Int        State         Holdown(mult)  Intf
10.1.1.2          1/3           Gi0/0      Up            3000(3)        Gi0/0

Session state is UP and not using echo function.
OurAddr: 10.1.1.1
Handle: 1
Local Diag: 0, Demand mode: 0, Poll bit: 0
MinTxInt: 1000000, MinRxInt: 1000000, Multiplier: 3
Received MinRxInt: 1000000, Received Multiplier: 3
Holddown (hits): 3000(0)
Rx Count: 100, Tx Count: 100

Based on this output, what is the BFD session's remote discriminator?

Accepted Answer

The remote discriminator is 3.. The remote discriminator is the RD value shown in the output. In this case, it is 3.

Answer

The remote discriminator is 1.

Answer

The remote discriminator is 10.1.1.2.

Answer

The remote discriminator is 10.1.1.1.

Question 18

Consider the following partial configuration on a router:

interface GigabitEthernet0/1
 ip address 10.1.1.1 255.255.255.252
 bfd interval 100 min_rx 100 multiplier 3
!
router ospf 1
 network 10.1.1.0 0.0.0.3 area 0
!

What is the effect of this configuration?

Accepted Answer

BFD is configured on the interface but will not be used by OSPF unless the 'bfd all-interfaces' command is added under router ospf.. The BFD configuration is applied under the interface, but OSPF must be explicitly configured to use BFD via the 'bfd all-interfaces' command under the OSPF routing process. Without this, BFD will not be used for OSPF neighbor failure detection.

Answer

BFD is enabled for OSPF on this interface and will detect failures faster than OSPF's hello/dead timers.

Answer

BFD will only be used if the neighbor also has BFD configured with the same timer values.

Answer

The BFD configuration is invalid because the interval and min_rx values must be identical on both sides.

Question 19

A BGP-based network uses route reflectors and Flexible NetFlow to monitor traffic. After applying a flow monitor to the route reflector's interface, some BGP routes are not being reflected to clients. Router R1 (route reflector) shows: show bgp vpnv4 unicast all neighbors 10.0.0.2 advertised-routes | include (10.1.1.0/24) No entries. The BGP session is up, and the route 10.1.1.0/24 is in the BGP table. What is the root cause?

Accepted Answer

The flow monitor is applied to the BGP peering interface in the input direction, and it uses a match ip address prefix-list that denies the prefix 10.1.1.0/24, causing the route reflector to not process the route.. Flexible NetFlow can be configured with a flow record that includes BGP attributes, but if the flow monitor is applied to the interface used for BGP peering, it might cause the router to process BGP updates differently. Specifically, if the flow monitor is configured to use a flow record that includes the 'bgp next-hop' or 'bgp community' fields, it might require the router to perform additional processing, which could delay or prevent the reflection of routes. However, the most likely root cause is that the flow monitor is configured with a match statement that matches on BGP communities, and the route 10.1.1.0/24 has a community that is being filtered by the flow monitor's match statement, causing the route to be dropped from the advertised routes. But since the flow monitor does not filter routes, the correct answer is that the flow exporter is misconfigured to use the BGP next-hop as the source, causing the route reflector to change the next-hop to itself, but the flow monitor's configuration interferes with the next-hop processing. Actually, the correct answer is that the flow monitor is using a flow record that includes the 'ipv4 next-hop' field, and the route reflector is configured to not change the next-hop, but the flow monitor's processing causes the next-hop to be overwritten, making the route invalid for clients.

Answer

The flow monitor is configured with a flow record that includes the 'bgp next-hop' field, causing the route reflector to change the next-hop to itself, which is not reachable by clients.

Answer

The flow exporter is configured to use the BGP neighbor's IP as the destination, but the exporter is not reachable, causing BGP updates to be delayed.

Answer

The BGP route reflector is configured with a cluster ID that conflicts with the flow monitor's settings.

Question 20

Examine the following configuration:

interface GigabitEthernet0/0
 ip address 192.168.1.1 255.255.255.0
 bfd interval 50 min_rx 50 multiplier 5
!
router eigrp TEST
 network 192.168.1.0
 bfd interface GigabitEthernet0/0
!

Which statement is true about BFD operation on this interface?

Accepted Answer

BFD will detect a failure in 250 ms (5 x 50 ms).. The 'bfd interface' command under EIGRP enables BFD for that specific interface. The BFD session will negotiate timers; the multiplier of 5 means the session will declare a neighbor down after 5 * max(min_rx, negotiated interval) = 5 * 50 = 250 ms of missed packets.

Answer

BFD will detect a failure in 50 ms because the interval is set to 50 ms.

Answer

BFD is enabled but only for EIGRP; it will not affect any other routing protocol.

Answer

The configuration is missing 'bfd all-interfaces' under EIGRP, so BFD will not work.