300-410 Mock Exam 85 — 25 Questions

Question 1

Examine the partial BFD configuration on a router:

interface GigabitEthernet0/0
 bfd interval 100 min_rx 100 multiplier 3
!
interface GigabitEthernet0/1
 bfd interval 200 min_rx 200 multiplier 3
!
router ospf 1
 bfd all-interfaces
!

The router has OSPF neighbors on both interfaces. Which statement is true?

Accepted Answer

The BFD session on GigabitEthernet0/0 will detect failures in 300 ms, and on GigabitEthernet0/1 in 600 ms.. BFD timers are configured per interface. Each BFD session independently uses the timers configured on its respective interface. The multiplier is applied per session.

Answer

Both BFD sessions will use the same timer values because OSPF is configured with 'bfd all-interfaces'.

Answer

The BFD session on GigabitEthernet0/1 will not form because the interval is too high.

Answer

The router will use the minimum interval across all interfaces for consistency.

Question 2

A router has the following BFD configuration for a static route:

ip route 10.0.0.0 255.255.255.0 192.168.1.2
bfd map 192.168.1.2 10.0.0.0 255.255.255.0
interface GigabitEthernet0/0
 ip address 192.168.1.1 255.255.255.0
 bfd interval 100 min_rx 100 multiplier 3
!

What is the purpose of the 'bfd map' command in this context?

Accepted Answer

It creates a BFD session to the next-hop 192.168.1.2 and associates it with the static route to 10.0.0.0/24.. For static routes, BFD must be explicitly mapped to the next-hop and destination prefix. The 'bfd map' command associates a BFD session with a static route so that if BFD detects a failure, the static route is removed from the routing table.

Answer

It maps the BFD session to the interface, enabling BFD for all static routes using that interface.

Answer

It maps the BFD session to the OSPF process, which is incorrect for static routes.

Answer

It is used to configure BFD for multiple static routes simultaneously.

Question 3

What is the default BFD interval and multiplier on Cisco IOS-XE devices when BFD is enabled under an interface without explicit timer configuration?

Accepted Answer

interval 150 ms, min_rx 150 ms, multiplier 3. If no BFD timers are configured under the interface, the default values are: interval = 150 ms, min_rx = 150 ms, multiplier = 3. These are the defaults used when 'bfd interval' is not specified.

Answer

interval 100 ms, min_rx 100 ms, multiplier 3

Answer

interval 200 ms, min_rx 200 ms, multiplier 5

Answer

interval 50 ms, min_rx 50 ms, multiplier 3

Question 4

Which of the following statements about BFD echo mode is true?

Accepted Answer

Echo mode allows the local router to detect failures without requiring the remote router to send BFD control packets.. BFD echo mode is an optional mode where the local router sends echo packets to the remote router, which loops them back. This reduces the processing load on the remote router because it does not need to generate BFD control packets. Echo mode is enabled by default on Cisco IOS-XE.

Answer

Echo mode increases the load on the remote router because it must process echo packets.

Answer

Echo mode is disabled by default and must be explicitly enabled.

Answer

Echo mode uses the same timer values as the BFD control packets.

Question 5

Which TWO configuration steps are required to enable BFD for OSPF on a Cisco IOS-XE router? (Choose TWO.)

Accepted Answer

Configure 'bfd' under the interface. To enable BFD for OSPF, you must first enable BFD on the interface using the 'bfd' interface command, and then enable BFD support for OSPF under the OSPF routing process with 'bfd all-interfaces' or per-neighbor. The other options are either incorrect or not required.

Answer

Configure 'ip ospf bfd' under the interface

Answer

Configure 'bfd interval 50 min_rx 50 multiplier 3' globally

Answer

Configure 'router bfd' globally

Question 6

An OSPF network has multiple areas and uses Flexible NetFlow to monitor inter-area traffic. After applying a flow monitor to the ABR's interface, OSPF neighbor relationships fail to form. Router R1 (ABR) shows: show ip ospf neighbor | include (FULL|DOWN) Neighbor 10.0.0.2, interface GigabitEthernet0/0, state DOWN. show flow monitor FLOW-MONITOR statistics | include (Packets|Errors) Packets exported: 1000, Errors: 0. What is the root cause?

Accepted Answer

The flow monitor is using a sampler that samples only 1 out of 100 packets, causing OSPF hello packets to be missed.. OSPF neighbor relationships can fail if the interface is configured with a flow monitor that uses a sampler or if the flow record includes fields that cause the router to process OSPF packets incorrectly. In this case, the flow monitor is exporting packets successfully, but the OSPF neighbor is down. The most likely cause is that the flow monitor is configured with a flow record that includes the 'ipv4 ttl' field, and the router is using that to modify the TTL of OSPF packets, causing them to be dropped. However, the correct answer is that the flow monitor is applied in the output direction and is using a flow record that includes the 'ipv4 dscp' field, but the OSPF packets have a specific DSCP value that is being changed by the flow monitor, causing the neighbor to reject them. Actually, the most plausible root cause is that the flow monitor is configured with a match statement that matches on OSPF packets (protocol 89) and applies an action to drop them, but Flexible NetFlow does not drop packets. The correct answer is that the flow monitor is using a sampler that samples only 1 out of 100 packets, causing OSPF hello packets to be missed, leading to neighbor down.

Answer

The flow monitor is configured with a flow record that includes the 'ipv4 ttl' field, causing the router to decrement the TTL of OSPF packets, making them invalid.

Answer

The flow exporter is configured to use the OSPF router ID as the source, causing a conflict.

Answer

The OSPF network type is misconfigured on the interface, but the flow monitor is not related.

Question 7

Which THREE statements about BFD multihop sessions are true? (Choose THREE.)

Accepted Answer

Multihop BFD is used when the neighbors are not directly connected.. BFD multihop sessions are used when the two routers are not directly connected, requiring multiple hops. They use a different destination UDP port (4784) compared to single-hop sessions (3784). Additionally, multihop sessions require explicit configuration of the BFD source and destination IP addresses. The other statements are false because multihop sessions do not require the same subnet, and they can operate over any routed path.

Answer

Multihop BFD requires both routers to be in the same subnet.

Answer

Multihop BFD can only be used with BGP.

Question 8

Router R1 and R2 are OSPF neighbors over a FastEthernet link with BFD enabled. R1 has 'ip ospf network point-to-point' configured. R2 does not. After a reload, BFD sessions fail to establish. R1#show bfd neighbors shows no sessions. R2#show bfd neighbors shows no sessions. What is the root cause?

Accepted Answer

OSPF network type mismatch between R1 and R2 prevents BFD session establishment.. BFD requires matching OSPF network types on the same link for proper session establishment. When R1 is configured as point-to-point and R2 remains as broadcast, OSPF forms a neighbor relationship but BFD sessions fail because BFD expects consistent interface parameters.

Answer

BFD is not supported on FastEthernet interfaces.

Answer

BFD timers are not configured globally.

Answer

OSPF hello/dead intervals must match for BFD to work.

Question 9

According to RFC 5880, what is the maximum number of BFD sessions that can be uniquely identified using the discriminator fields?

Accepted Answer

2^32 (4,294,967,296). The discriminator fields are 32-bit values, allowing up to 2^32 (approximately 4.29 billion) unique session identifiers.

Answer

2^16 (65536)

Answer

2^24 (16,777,216)

Answer

Unlimited

Question 10

In an MPLS network, routers R1 and R2 are LDP neighbors with BFD enabled. R1#show mpls ldp neighbor shows R2 as 'Oper Down' for the BFD session. R2#show bfd neighbors shows the session with R1 as 'Down'. R1 and R2 have matching 'mpls ldp router-id' loopback0. The link between them is up. What is the root cause?

Accepted Answer

The LDP router-id must be the IP address of the interface where BFD is enabled.. BFD for LDP requires that the LDP session be established over the same interface as the BFD session. If the LDP router-id is not the interface IP used for BFD, the BFD session may fail. Here, the loopback0 is used as LDP router-id, but BFD is configured on the physical link, causing mismatch.

Answer

BFD is not supported with LDP on this platform.

Answer

The BFD interval is too low for LDP.

Answer

LDP must be configured with 'no bfd' to work properly.

Question 11

R1 and R2 are EIGRP neighbors over a serial link with BFD enabled. R1#show ip eigrp neighbors shows R2 in state 'Pending' for BFD. R2#show bfd neighbors shows the session as 'Up'. R1 has 'bfd interval 50 min_rx 50 multiplier 3'. R2 has 'bfd interval 200 min_rx 200 multiplier 3'. What is the root cause?

Accepted Answer

R1's min_rx interval is lower than R2's min_tx interval, causing BFD session failure.. EIGRP BFD requires that the BFD session be established before EIGRP can form adjacency. If the BFD session is up on one side but not the other, it indicates a timer mismatch. However, BFD timers are negotiated; the issue is that R1's min_rx of 50 ms is lower than R2's min_rx of 200 ms, causing R2 to reject the session because it cannot transmit at that rate.

Answer

The BFD multiplier on R1 is too low.

Answer

EIGRP must be configured with 'bfd all-interfaces' to work.

Answer

The serial link requires 'no ip split-horizon' for BFD.

Question 12

R1 and R2 are BGP peers with BFD enabled. R1#show bgp neighbors 10.1.1.2 shows BGP state 'Active' and BFD session 'Down'. R2#show bfd neighbors shows the session as 'Up' with R1. R1 has 'neighbor 10.1.1.2 fall-over bfd' configured. R2 has 'neighbor 10.1.1.1 fall-over bfd' configured. The link between them is stable. What is the root cause?

Accepted Answer

R1 is missing 'bfd interval' configuration on the interface facing R2.. BFD for BGP fall-over requires that the BFD session be established before BGP can form. If the BFD session is up on R2 but down on R1, it indicates a unidirectional issue. This can happen if R1 has an ACL blocking BFD control packets from R2, or if R1's BFD configuration is missing. Here, R1 likely lacks 'bfd interval' configuration on the interface, causing BFD to not initiate.

Answer

BGP requires 'neighbor 10.1.1.2 ebgp-multihop' for BFD to work.

Answer

R2 has a higher BFD multiplier causing session failure.

Answer

The BGP update-source is not set to the interface IP.

Question 13

R1 and R2 are OSPF neighbors over a VLAN interface with BFD enabled. R1#show ip ospf interface vlan10 shows 'BFD is enabled' but R1#show bfd neighbors shows no sessions. R2#show bfd neighbors shows no sessions. R1 has 'bfd interval 100 min_rx 100 multiplier 3' under vlan10. R2 has same. The VLAN is up. What is the root cause?

Accepted Answer

BFD must be enabled on the physical interface as well as the VLAN interface.. BFD on VLAN interfaces requires that the underlying physical interface also support BFD. If the physical interface (e.g., GigabitEthernet0/1) does not have BFD enabled, the VLAN interface cannot establish BFD sessions. BFD is not automatically inherited from VLAN to physical.

Answer

The VLAN interface must have 'ip ospf bfd' disabled.

Answer

OSPF must be configured with 'bfd all-interfaces' globally.

Answer

The BFD timers must match exactly on both sides.

Question 14

R1 and R2 are IS-IS neighbors with BFD enabled. R1#show clns is-neighbors shows R2 as 'Up'. R1#show bfd neighbors shows the session as 'Down'. R2#show bfd neighbors shows the session as 'Up' with R1. R1 has 'bfd interval 50 min_rx 50 multiplier 3' on the interface. R2 has 'bfd interval 100 min_rx 100 multiplier 3'. The link is stable. What is the root cause?

Accepted Answer

R1 has an ACL blocking UDP port 3784 from R2.. IS-IS BFD requires that the BFD session be established between the same IP addresses used for IS-IS. If R1's BFD session is down but R2's is up, it indicates a unidirectional issue. This can be due to an MTU mismatch where R1's BFD packets are fragmented and dropped, or a firewall blocking inbound BFD packets on R1. Here, the most likely cause is that R1 has an ACL denying UDP port 3784 from R2.

Answer

IS-IS requires 'bfd all-interfaces' to work with BFD.

Answer

The BFD multiplier on R1 is too low.

Answer

The IS-IS metric must be set to 1 for BFD.

Question 15

R1 and R2 are EIGRP neighbors with BFD enabled. R1#show ip eigrp neighbors shows R2 in state 'Init' for BFD. R1#show bfd neighbors shows the session as 'Up'. R2#show bfd neighbors shows the session as 'Up'. R1 has 'eigrp stub' configured. R2 does not. What is the root cause?

Accepted Answer

The EIGRP stub configuration on R1 prevents full adjacency, but BFD is unaffected.. EIGRP stub routers can form BFD sessions but the EIGRP adjacency may remain in Init state if the stub router is not configured correctly. However, BFD session is up, so the issue is with EIGRP itself. The stub router may be blocking queries, but that does not affect BFD. The real issue is that R1 has 'eigrp stub' but is missing the 'receive-only' keyword, causing it to not advertise routes, but BFD is fine. The BFD session is up, so the problem is EIGRP, not BFD.

Answer

BFD requires 'eigrp stub' to be removed.

Answer

The BFD interval on R1 is too low for EIGRP.

Answer

EIGRP must be configured with 'no eigrp stub' for BFD to work.

Question 16

R1 and R2 are OSPF neighbors over a tunnel interface with BFD enabled. R1#show ip ospf interface tunnel0 shows 'BFD is enabled' but R1#show bfd neighbors shows the session as 'Down'. R2#show bfd neighbors shows the session as 'Down'. The tunnel is up and OSPF adjacency is full. R1 has 'bfd interval 100 min_rx 100 multiplier 3' under tunnel0. R2 has same. What is the root cause?

Accepted Answer

BFD is not supported over tunnel interfaces; it requires physical interfaces.. BFD over tunnel interfaces requires that the underlying transport (e.g., GRE) supports BFD. BFD cannot be established over a tunnel if the tunnel endpoint IP addresses are not directly connected; BFD requires physical connectivity. Since OSPF is full, the tunnel is working, but BFD fails because the tunnel is considered a virtual interface and BFD expects a direct physical link.

Answer

The tunnel mode must be changed to 'ipsec' for BFD to work.

Answer

OSPF must be configured with 'bfd all-interfaces' to work over tunnels.

Answer

The BFD timers must be increased for tunnel interfaces.

Question 17

R1 and R2 are OSPF neighbors with BFD enabled. R1#show ip ospf neighbor shows R2 as 'FULL/DR'. R1#show bfd neighbors shows the session as 'Up' with R2. R2#show bfd neighbors shows the session as 'Up' with R1. However, R1#show ip route shows that the route to 10.1.1.0/24 via R2 is missing. R1 has 'summary-address 10.1.1.0 255.255.255.0' configured under OSPF. What is the root cause?

Accepted Answer

The OSPF summary-address on R1 is suppressing the more specific route from R2.. OSPF summary-address on an ABR or ASBR suppresses more specific routes. If R1 is an ABR and has a summary-address for 10.1.1.0/24, it will not install the specific route from R2, even though BFD and OSPF adjacency are fine. The summary route may be advertised instead, but the specific route is missing from the routing table.

Answer

BFD is causing the route to be removed due to fast detection.

Answer

R2 is not advertising the route due to a filtering policy.

Answer

The OSPF cost on R1 is too high, causing the route to be ignored.

Question 18

A network engineer runs the following command to troubleshoot a BFD issue:

R1# debug bfd event

*Mar  1 00:12:34.567: BFD: [R1-to-R2] state DOWN -> UP (async)
*Mar  1 00:12:34.568: BFD: [R1-to-R2] echo mode enabled, min-echo-rx-interval 50 ms
*Mar  1 00:12:34.569: BFD: [R1-to-R2] starting echo timer, interval 50 ms
*Mar  1 00:12:34.570: BFD: [R1-to-R2] sending async packet, state UP, interval 300 ms

What does this output indicate?

Accepted Answer

BFD session is established and echo mode is active.. The debug output shows BFD session transitioning from DOWN to UP, with echo mode enabled and the async interval set to 300 ms. This indicates a successful BFD session establishment.

Answer

BFD session is flapping between DOWN and UP states.

Answer

BFD session is down due to echo failure.

Answer

BFD async interval is misconfigured at 300 ms.

Question 19

Drag and drop the steps to establish a BFD session with OSPF into the correct order, from first to last.

Accepted Answer

Enable BFD globally using the bfd command. The correct order starts with enabling BFD globally, then configuring BFD on the interface, enabling OSPF BFD support, verifying the BFD session, and finally verifying OSPF neighbor state reflects BFD integration.

Question 20

A network engineer runs the following command to verify BFD operation:

R1# show bfd neighbors detail

IPv4 Sessions
NeighAddr      LD/RD         RH/RS     State     Int
10.1.1.2      1/2           Up        Up        Gi0/0
Session state is UP and not using echo function.
Session type: single-hop
Local Diag: 0, Demand mode: 0, Poll bit: 0
MinTxInt: 1000000, MinRxInt: 1000000, Multiplier: 3
Received MinRxInt: 1000000, Received Multiplier: 3
Holddown (hits): 0 (0), Hello (hits): 1000/5
Rx Count: 1000, Rx Interval (ms) min/max/avg: 900/1100/1000
Tx Count: 1000, Tx Interval (ms) min/max/avg: 900/1100/1000

What does this output indicate?

Accepted Answer

BFD session is operating with async mode, intervals at 1 second.. The output shows a BFD session in UP state with consistent transmit and receive intervals around 1000 ms, no echo function, and a multiplier of 3. The session is healthy.

Answer

BFD session is using echo mode for fast failure detection.

Answer

BFD session is down and not sending hellos.

Answer

BFD session has a holddown timer of 3 seconds.