Bidirectional Forwarding Detection (BFD)

A network engineer is troubleshooting an OSPF adjacency that is flapping between two routers. The adjacency forms and then drops repeatedly. Both routers are configured for BFD on the OSPF interface. The engineer checks the BFD session and sees it is up, but the OSPF neighbor state transitions from FULL to DOWN every few seconds. What is the most likely cause of this issue?

A network engineer is troubleshooting a scenario where two routers running EIGRP are not forming an adjacency. Both routers have BFD configured under the EIGRP process and on the interfaces. The BFD session is up and operational. However, the EIGRP neighbor status shows 'Pending' and never transitions to 'Up'. What is the most likely cause?

A network engineer is troubleshooting a BGP session that is dropping intermittently. The routers are connected via a Layer 2 switch. BFD is configured for the BGP session. The engineer notices that the BFD session goes down briefly, causing the BGP session to reset. The BFD timers are set to 100 ms interval with a multiplier of 3. The switch is not configured for BFD. What is the most likely cause?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

A network engineer has configured BFD for OSPF as shown: interface GigabitEthernet0/0 bfd interval 300 min_rx 300 multiplier 3 ! router ospf 1 bfd all-interfaces ! However, BFD sessions are not coming up. What is the most likely missing configuration?

Consider the following BGP configuration with BFD: router bgp 65000 neighbor 10.1.1.2 remote-as 65001 neighbor 10.1.1.2 fall-over bfd ! interface GigabitEthernet0/0 ip address 10.1.1.1 255.255.255.252 bfd interval 200 min_rx 200 multiplier 4 ! What is the effect of the 'neighbor fall-over bfd' command?

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?

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?

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

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

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

Which TWO commands can be used to verify the operational state of BFD sessions on a Cisco IOS-XE router? (Choose TWO.)

Which TWO statements about BFD echo mode are true? (Choose TWO.)

Which THREE symptoms indicate a BFD session failure? (Choose THREE.)

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

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

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

A network engineer runs the following command to troubleshoot BFD with OSPF: R1# show ip ospf interface gigabitethernet 0/0 GigabitEthernet0/0 is up, line protocol is up Internet Address 10.1.1.1/24, Area 0 Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1 Transmit Delay is 1 sec, State DR, Priority 1 Designated Router (ID) 1.1.1.1, Interface address 10.1.1.1 Backup Designated router (ID) 2.2.2.2, Interface address 10.1.1.2 Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 oob-resync timeout 40 Hello due in 00:00:03 Supports Link-local Signaling (LLS) Cisco NSF helper support enabled IETF NSF helper support enabled Index 1/1/1, flood queue length 0 Next 0x0(0)/0x0(0)/0x0(0) Last flood scan length is 1, maximum is 25 Last flood scan time is 0 msec, maximum is 0 msec Neighbor Count is 1, Adjacent neighbor count is 1 Adjacent with neighbor 2.2.2.2 (Backup Designated Router) Suppress hello for 0 neighbor(s) BFD enabled, BFD interval 100 msec, BFD multiplier 3 What does this output indicate?

A network engineer runs the following command to verify BFD with EIGRP: R1# show ip eigrp 100 topology 10.2.2.0/24 EIGRP-IPv4 Topology Entry for AS(100)/ID(10.2.2.0/24) State: Passive, Query origin flag: 1, 1 Successor(s), FD is 131072 Descriptor Blocks: 10.1.1.2 (GigabitEthernet0/0), from 10.1.1.2, Send flag: 0x0 Composite metric: (131072/130816), Route is Internal Vector metric: Minimum bandwidth is 100000 Kbit Total delay is 100 microseconds Reliability is 255/255 Load is 1/255 Minimum MTU is 1500 Hop count is 1 Originating router is 2.2.2.2 BFD enabled, BFD state: UP What does this output indicate?

A network engineer runs the following command to troubleshoot BFD with BGP: R1# show bgp ipv4 unicast 10.3.3.0/24 BGP routing table entry for 10.3.3.0/24, version 2 Paths: (1 available, best #1, table default) Advertised to update-groups: 1 Refresh Epoch 1 Local 10.1.1.2 from 10.1.1.2 (2.2.2.2) Origin IGP, metric 0, localpref 100, valid, external, best rx pathid: 0, tx pathid: 0x0 BFD enabled, BFD state: UP What does this output indicate?

A network engineer runs the following command to verify BFD session details: R1# show bfd neighbors detail | include (NeighAddr|LD/RD|State|Int|MinTxInt|Multiplier) NeighAddr LD/RD RH/RS State Int 10.4.4.2 100/200 Up Up Gi0/1 MinTxInt: 500000, MinRxInt: 500000, Multiplier: 5 What does this output indicate?

A network engineer runs the following command to troubleshoot BFD session flapping: R1# debug bfd packet *Mar 1 00:15:23.456: BFD: [R1-to-R3] received async packet from 10.5.5.2, state UP, diag 0 *Mar 1 00:15:23.457: BFD: [R1-to-R3] sending async packet, state UP *Mar 1 00:15:23.458: BFD: [R1-to-R3] received echo packet from 10.5.5.2, state UP *Mar 1 00:15:23.459: BFD: [R1-to-R3] echo packet lost, no echo received for 300 ms *Mar 1 00:15:23.460: BFD: [R1-to-R3] state UP -> DOWN (echo failure) What does this output indicate?

A network engineer runs the following command to verify BFD with MPLS LDP: R1# show mpls ldp neighbor 10.6.6.2 detail Peer LDP Ident: 10.6.6.2:0, Local LDP Ident: 10.6.6.1:0 TCP connection: 10.6.6.2.646 - 10.6.6.1.53456 State: Oper; Msgs sent/rcvd: 100/100; Downstream Up time: 00:10:00 LDP discovery sources: GigabitEthernet0/2, hello interval: 5 s, hello hold: 15 s Addresses bound to peer LDP ident: 10.6.6.2 10.7.7.2 BFD enabled, BFD state: UP What does this output indicate?

A network engineer runs the following command to troubleshoot BFD with static routes: R1# show ip route 10.8.8.0/24 Routing entry for 10.8.8.0/24 Known via "static", distance 1, metric 0 Routing Descriptor Blocks: * 10.9.9.2, via GigabitEthernet0/3 Route metric is 0, traffic share count is 1 BFD enabled, BFD state: UP What does this output indicate?

What is the default BFD hello timer interval on Cisco IOS-XE when BFD is configured on an interface without any explicit timer settings?

Which BFD packet type is used for initial session establishment and carries the discriminator values?

How does BFD prevent loops in a network where multiple BFD sessions might exist between the same pair of routers?

Which authentication type is the default when BFD authentication is enabled on Cisco IOS-XE?

What is the default BFD multiplier (detection time multiplier) on Cisco IOS-XE?

In BFD multihop sessions, what is the default value for the TTL (or hop limit) in outgoing BFD Control packets on Cisco IOS-XE?

Which BFD session state indicates that the session is fully established and operational?

What is the default behavior of Cisco IOS-XE regarding BFD echo mode when BFD is enabled on an interface?

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

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

Drag and drop the steps to troubleshoot a BFD adjacency or connectivity failure into the correct order, from first to last.

Drag and drop the steps to verify and validate BFD operational state into the correct order, from first to last.

Which TWO statements about BFD echo mode are true? (Choose TWO.)

Which TWO actions will prevent a BFD session from flapping when a routing protocol (e.g., OSPF or EIGRP) is configured to use BFD? (Choose TWO.)

An engineer must enable BFD for an OSPF single-hop session between two directly connected routers. Which TWO configuration changes are required on each router? (Choose TWO.)

Which THREE commands can be used to verify the operational state of a BFD session? (Choose THREE.)

Which TWO statements about BFD authentication are true? (Choose TWO.)

An engineer configures BFD on an OSPF link between two routers. The BFD session comes up, but OSPF remains stuck in EXSTART state. The engineer verifies that IP connectivity is fine and OSPF hello packets are exchanged. What is the most likely cause?

A network engineer configures BFD for EIGRP on a point-to-point link. The BFD session is up, but EIGRP neighbors are stuck in INIT state. The engineer checks that EIGRP hello packets are sent and received. Which of the following is the most likely explanation?

An engineer configures BFD for BGP between two directly connected eBGP peers. The BFD session is up, but BGP remains in the Idle state. The engineer verifies that the BGP configuration is correct and that TCP port 179 is reachable. What is the most likely cause?

An engineer configures BFD on a DMVPN Phase 2 spoke-to-spoke tunnel. The BFD session between two spokes comes up, but the spoke-to-spoke dynamic IPsec tunnel fails to establish. What is the most likely explanation?

An engineer configures BFD on an OSPF link and also applies a distribute-list outbound under the OSPF process. The BFD session is up, but OSPF routes are not being advertised to the neighbor. The engineer verifies that the distribute-list is correctly configured. What is the most likely cause?

An engineer configures BFD for EIGRP in named mode. The BFD session is up, but the EIGRP neighbor relationship is not forming. The engineer checks that the K values match and that the interfaces are up. What is the most likely explanation?

An engineer configures BFD for BGP on a route reflector client. The BFD session between the client and the route reflector is up, but the client does not receive routes from the route reflector. The engineer verifies that the BGP configuration is correct and that the client is sending routes. What is the most likely cause?

An engineer configures BFD on an OSPF link that is part of a virtual link. The BFD session between the two endpoints of the virtual link is up, but OSPF adjacency over the virtual link is not forming. What is the most likely explanation?

An engineer configures BFD for EIGRP and also applies an offset-list to increase the metric of a route. The BFD session is up, but the route with the offset-list is not being installed in the routing table. The engineer verifies that the offset-list is correctly configured. What is the most likely explanation?