BFD Sub-Second Failure Detection Advantage

Describe Bidirectional Forwarding Detection

Answer Correct answer: D — BFD has sub-second failure detection for layer 1 and layer 2 problems.

What is an advantage of using BFD?

It detects local link failure at layer 1 and updates the routing table.
It detects local link failure at layer 3 and updates the routing protocols.
It has sub-second failure detection for layer 1 and layer 3 problems.
It has sub-second failure detection for layer 1 and layer 2 problems. Correct Answer

Community Insight

This question tests the specific layer scope of BFD's native detection capabilities versus its integration with Layer 3 protocols.

Bidirectional Forwarding Detection (BFD) provides sub-second failure detection for both Layer 1 and Layer 2 problems, significantly reducing convergence time compared to traditional routing protocol timers.

Most candidates choose Option B because they associate BFD primarily with Layer 3 routing protocols like OSPF or BGP, overlooking that BFD itself operates at lower layers to detect physical/link failures.

Community Discussion (50 comments)

nabnet 👍 1 Selected: B

https://www.cisco.com/en/US/technologies/tk648/tk365/tk480/technologies_white_paper0900aecd80244005.html

8e4379b 👍 1 Selected: C

For Layer 2 failure detection, Cisco supports mechanisms such as: Layer 2 VPNs (L2VPNs) Ethernet Fault Detection (EFD) Xconnect as a Client of BFD So C is correct, Layer 1 & 3

TheNemesiz 👍 1 Selected: D

Technically it's a L3 protocol and is has no L1/L2 awareness. It just checks for BFD packets from it's neighbor. So in technical sense B would be more correct. That said, I believe Cisco would prefer you to say D as it is the whole reason why it's beneficial. It's the selling point if you will. Generally speaking this is how it's described. So the engineer would say B, the salesman will say D. Go with D, it's the safer answer

XxnightwingxX 👍 1 Selected: C

When we're talking BFD we have to be talking "sub-second failure detection". That's the whole point of BFD. So it has to be C since BFD adds link failure to the arsenal to notify the routing protocol that a routing change is needed. Per cisco book page 900. BFD is a detection protocol that works with "all media types..."

yasinii 👍 1 Selected: B

what i am thinking is b. bfd can colaborate with routing protocols so it can updates routing protocols.

Brahim90 👍 1 Selected: B

Per cisco book page 900. BFD is a “detection” protocol that works with all media types, routing protocols, topologies, and encapsulations. It is used to quickly detect reachability failures between two routers in the same Layer 3 network so that network issues can be identified as soon as possible, and convergence can occur at a far faster rate

Sammy3637 👍 1 Selected: C

C. It has sub-second failure detection for layer 1 and layer 3 problems. Explanation: BFD (Bidirectional Forwarding Detection) is a protocol used to detect link failures quickly in networking environments. It provides rapid failure detection for both Layer 1 (physical layer) and Layer 3 (network layer) problems, significantly improving network convergence times.

Sammy3637 👍 1

Sorry , changed mind to option B as it's primarily designed for layer 3 failure detection

XBfoundX 👍 1

I will go with D this is why at least in D we have layer 2 detection also in the answer: "Bidirectional Forwarding Detection provides a method for network administrators to configure sub-second Layer 2 failure detection between adjacent network nodes. Furthermore, they can configure their routing protocols to respond to BFD notifications, and begin Layer 3 route convergence almost immediately. Provided that the appropriate caveats mentioned in this document are followed, BFD can be a powerful tool and an important part of a network-wide availability plan."

XBfoundX 👍 1

https://www.cisco.com/en/US/technologies/tk648/tk365/tk207/technologies_white_paper0900aecd80243fe7.html

bf10690 👍 1 Selected: D

This feels like one of those questions written not to test your knowledge but rather to trick you by using weird wording that leaves things up for interpretation. In any case, my vote goes to D because it feels like it is the most correct.

jabal93 👍 1 Selected: D

BFD doesn't "update (influence) the routing table" add to that the important benefit for BFD that it was decreased the failure detection between routers to SUBSEONDS. Which is less than what the routing protocol hello messages which was measured by SECONDS

26307ae 👍 2 Selected: D

When using dynamic routing protocols with topologies where P2P links are not directly connected, when links go down, it takes long time (dead timers) until it is detected and until the topology starts to heal and converge. It can take up to 40 seconds which is a long time. BFD uses hellos in subsecond, we can make a dynamic routing protocol to use information from BFD and when BFD session goes down the dynamic protocol starts to converge immediately. The key word here is Peer to Peer Link and not local link. So D is correct.

GoodServant 👍 1 Selected: B

Based on both BFD white paper and CCNP ENARSI official book, BFD DOES interact with L3 routing protocols. And https://www.cisco.com/en/US/technologies/tk648/tk365/tk480/technologies_white_paper0900aecd80244005.html Quote from Enarsi book: "For example, if you wanted EIGRP to discover neighbor issues quickly, you could set the EIGRP hello and hold timers to 1 and 3, respectively. This would allow any EIGRP neighbor issues to be detected within 3 seconds, and convergence would occur... If instead you used BFD between the routers, you could leave the hello interval at 5 and hold time at 15 and use the lightweight BFD packets to keep track of the connection between the two routers. In this case, if anything happened to the connection between the two routers, BFD would notify its client (EIGRP in this case) so that EIGRP could converge as needed without waiting for the EIGRP hold timer to expire."

hennnn 👍 2

The correct Answer is D CONCLUSION Bidirectional Forwarding Detection provides a method for network administrators to configure sub-second Layer 2 failure detection between adjacent network nodes. https://www.cisco.com/en/US/technologies/tk648/tk365/tk207/technologies_white_paper0900aecd80243fe7.html

Defilet 👍 1 Selected: B

I believe it is B. The main advantage is to notify routing protocols that a link failure occur in a less than a second and for sure less time that the hello and hold timers require. There are several advantages to implementing BFD over reduced timer mechanisms for routing protocols: •Although reducing the EIGRP, IS-IS, and OSPF timers can result in minimum detection timer of one to two seconds, BFD can provide failure detection in less than one second. •Because BFD is not tied to any particular routing protocol, it can be used as a generic and consistent failure detection mechanism for EIGRP, IS-IS, and OSPF. •Because some parts of BFD can be distributed to the data plane, it can be less CPU-intensive than the reduced EIGRP, IS-IS, and OSPF timers, which exist wholly at the control plane.

Andryel 👍 2 Selected: B

As reported in the book: "It is used to quickly detect reachability failures between two routers in the same Layer 3 network so that network issues can be identified as soon as possible, and convergence can occur at a far faster rate." Plus I found this: "The protocol that can be configured to use BFDs notifications are BGP, EIGRP, OSPF, HSRP, MPLS, LDP and probably some more." So BFD is used in Layer 3, so the correct answer is B. It's implicit that if there's a problem on Layer 1 (Physical) or Layer 2 (Data-Link), Layer 3 is automatically involved in the issue.

eeze 👍 2

Correct answer is B RFC 5880: https://datatracker.ietf.org/doc/html/rfc5880#section-3.1 Cisco Doc: https://www.cisco.com/c/en/us/td/docs/ios/12_0s/feature/guide/fs_bfd.html "BFD is a “detection” protocol that works with all media types, routing protocols, topologies, and encapsulations. It is used to quickly detect reachability failures between two routers in the same Layer 3 network so that network issues can be identified as soon as possible, and convergence can occur at a far faster rate. BFD is a lightweight protocol (that is, it has small fixed-length packets), which means it is less CPU intensive than fast routing protocol hellos." Raymond, Lacoste; Edgeworth Brad. CCNP Enterprise Advanced Routing ENARSI 300-410 Official Cert Guide (p. 1773). Pearson Education. Kindle Edition.

louisvuitton12 👍 9 Selected: B

From Cisco U ENARSI Course: "Typically, BFD can be used at any protocol layer. However, the Cisco implementation of BFD supports only Layer 3 clients, in particular, the BGP, Enhanced Interior Gateway Routing Protocol (EIGRP), IS-IS, and OSPF routing protocols, as well as the high availability protocol HSRP and also static routing."

BTK0311 👍 1

The advantage of using BFD (Bidirectional Forwarding Detection) is described as: C. It has sub-second failure detection for layer 1 and layer 3 problems. BFD provides rapid detection of network failures at both Layer 1 (physical layer) and Layer 3 (network layer), and it can detect these problems within milliseconds (sub-second). This quick detection helps in minimizing network downtime and improving network reliability by promptly identifying and responding to issues at these layers. per ChatGPT

Fenix7 👍 1

It's B. Look at the text below from Cisco. "BFD treats routing protocols, such as OSPF, as clients for creating the BFD sessions. The routing protocol discovers the neighbor using its own detection mechanism and then uses this information to form the BFD session with the neighboring router. If a link failure is detected by BFD, the client routing protocol is notified. This allows OSPF to tear down the routing neighbor adjacency immediately, instead of waiting multiple seconds for the hold timers to expire."

Fenix7 👍 1

Sorry... It's D.

Youssefmetry 👍 2

BFD can be used at any protocol layer. It could, for example, detect Physical or Data Link layers failures. https://www.cisco.com/en/US/technologies/tk648/tk365/tk207/technologies_white_paper0900aecd80243fe7.html

JieW 👍 1 Selected: B

Voting B. BFD is designed for IP level failure detection. Read section 2. Design. It does make notice of physical links but not the layer itself. It only refers to layers 2 and 3 by name. https://datatracker.ietf.org/doc/html/rfc5880

mabus 👍 1 Selected: B

B is corect

mabus 👍 3 Selected: B

BFD works only for directly connected neighbors. BFD neighbors must be no more than one IP hop away. Multihop configurations are not supported -> “BFD detects local link failure” is correct. Typically, BFD can be used at any protocol layer. However, the Cisco implementation of BFD for Cisco IOS Releases 12.2(18)SXE, 12.0(31)S, and 12.4(4)T supports only Layer 3 clients, in particular, the BGP, EIGRP, IS-IS, and OSPF routing protocols. Reference: https://www.cisco.com/c/en/us/td/docs/ios/12_0s/feature/guide/fs_bfd.html According to the reference above, it is a bit weird but answer B is the best choice here.

adudeguy 👍 1

B. BFD can be used at any protocol layer. It could, for example, detect Physical or Data Link layers failures, if the existing mechanisms did not provide sufficiently speedy detection. However, in the first phase of Cisco BFD support, all BFD clients, particularly the Layer 3 routing protocols (OSPF, IS-IS, EIGRP, and BGP) are at the Network layer.

MasterMatt 👍 1 Selected: B

Typically, BFD can be used at any protocol layer. However, the Cisco implementation of BFD for Cisco IOS Releases 12.2(18)SXE, 12.0(31)S, and 12.4(4)T supports only Layer 3 clients, in particular, the BGP, EIGRP, IS-IS, and OSPF routing protocols. Source: https://www.cisco.com/en/US/docs/ios/12_4t/ip_route/configuration/guide/t_bfd.html

6dd4aa0 👍 2 Selected: B

Koume 👍 11 Selected: D

I Go for D just following reasion A. It detects local link failure at layer 1 and updates the routing table. WRONG - BFD detects local link failures, but BFD does not interact with the routing table WRONG B. It detects local link failure at layer 3 and updates the routing protocols. (ALMOST RIGHT BUT WRONG) - The question here is What is the advantage of using BFD. routing procols can detect local link failures, so this is not an advantage. C. It has sub-second failure detection for layer 1 and layer 3 problems. WRONG TRICKY' - sub second failure detection is an advange of BFD, but BFD only detects L1-L2 problems, ink itself, BFD can not detect L3 problem like address misconfig. D. It has sub-second failure detection for layer 1 and layer 2 problems. RIGHT!! - The advantage of BFD is it's sub-second failure detection, and just detect L1-L2 problems.

DaanB 👍 1

Conclusion here https://www.cisco.com/en/US/technologies/tk648/tk365/tk207/technologies_white_paper0900aecd80243fe7.html says: Bidirectional Forwarding Detection provides a method for network administrators to configure sub-second Layer 2 failure detection between adjacent network nodes. Furthermore, they can configure their routing protocols to respond to BFD notifications, and begin Layer 3 route convergence almost immediately. If I understand this correctly, you (network administrator) use the method provided by BFD to configure Layer 2 sub-second failure detection. Then you can configure your routing protocol to act upon BFD notifications and begin route recalculations.

Edwinmolinab 👍 3 Selected: D

From https://www.cisco.com/en/US/technologies/tk648/tk365/tk207/technologies_white_paper0900aecd80243fe7.html BFD can be used at any protocol layer. It could, for example, detect Physical or Data Link layers failures, if the existing mechanisms did not provide sufficiently speedy detection. However, in the first phase of Cisco BFD support, all BFD clients, particularly the Layer 3 routing protocols (OSPF, IS-IS, EIGRP, and BGP) are at the Network layer.

balari 👍 1

B https://www.cisco.com/en/US/docs/ios/12_4t/ip_route/configuration/guide/t_bfd.html

IceFireSoul 👍 1

Correct answer B: See https://www.cisco.com/c/en/us/td/docs/ios/12_0s/feature/guide/fs_bfd.html BFD provides fast BFD peer failure detection times independently of all media types, encapsulations, topologies, and routing protocols BGP, EIGRP, IS-IS, and OSPF. By sending rapid failure detection notices to the routing protocols in the local router to initiate the routing table recalculation process, BFD contributes to greatly reduced overall network convergence time. Figure 1 shows a simple network with two routers running OSPF and BFD. When OSPF discovers a neighbor (1) it sends a request to the local BFD process to initiate a BFD neighbor session with the OSPF neighbor router (2). The BFD neighbor session with the OSPF neighbor router is established (3).

Dacusai 👍 1

B is the correct answer. The book for this exam talk only about detect routing problems nothing about layer 1 or 2, if a layer 1 issue occur the int goes down, and so with layer 2 protocol down.

xziomal9 👍 2 Selected: B

The correct answer is: B

YaPet 👍 2 Selected: D

D is correct

RexChen 👍 2 Selected: D

Reduced EIGRP timers have an absolute minimum detection timer of 1-2 seconds; BFD can provide sub-second failure detection. https://www.cisco.com/en/US/technologies/tk648/tk365/tk207/technologies_white_paper0900aecd80243fe7.html

wts 👍 3

BFD detects all problems from level 1 to 4, as it is encapsulated in UDP. There is no correct answer here.

wts 👍 1

The advantage of using it is less convergence time.

Networkingguy 👍 3 Selected: D

D looks to be correct here

geek1992 👍 1

D this answer is d

gndrx78 👍 1

Shitty question. It seems B because BFD works notifying routing protocols but BFD works at layer 2 and can detect Layer 1 issues so I think it is 99% answer D

studybuddy10 👍 2

D makes the most sense, not A or B, BFD doesn't do any routing protocol interactions. Routing protocols can react to BFD. It focusses on link issues so is testing layer 1-2 by running hello's on layer 3. A layer 3 problem would be a routing issue, not a link issue. Given answer is correct -D.

HieuPham 👍 1

C is correct. We focus sub-second. BFD has on Layer 2.

error_909 👍 1

Answer is B based on the offical book: BFD to the rescue. BFD is a “detection” protocol that works with all media types, routing protocols, topologies and encapsulations. It is used to quickly detect reachability failures between two routers in the same L3 network so that network issues can be identified as soon as possible,

Surfside92 👍 2

I think you are misinterpreting the book. BFD does indeed "work with all media types, routing protocols, topologies and encapsulations" But it does not work AT all those levels - just WITH them.

error_909 👍 5

The given answer is correct "Bidirectional Forwarding Detection provides a method for network administrators to configure sub-second Layer 2 failure detection between adjacent network nodes. Furthermore, they can configure their routing protocols to respond to BFD notifications, and begin Layer 3 route convergence almost immediately. Provided that the appropriate caveats mentioned in this document are followed, BFD can be a powerful tool and an important part of a network-wide availability plan. " https://www.cisco.com/en/US/technologies/tk648/tk365/tk207/technologies_white_paper0900aecd80243fe7.html

yoyo_simon 👍 1

should be B

vdsdrs 👍 2

BFD detects L3 problems https://www.ciscolive.com/c/dam/r/ciscolive/emea/docs/2015/pdf/BRKDCT-2333.pdf

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Expert Analysis

Why the Answer Is Correct

Option D is correct because Bidirectional Forwarding Detection (BFD) is a generic failure detection protocol designed to provide sub-second failure detection. According to Cisco documentation, BFD can be used to detect failures at Layer 1 (Physical) and Layer 2 (Data Link). It sends rapid hello packets between adjacent nodes to monitor link health. When a failure is detected at these lower layers, BFD notifies the attached routing protocols or applications, allowing them to react much faster than waiting for their own dead timers.

Why the Other Options Are Wrong

Option A is incorrect because BFD does not directly update the routing table; it notifies the routing protocol, which then updates the table. Option B is incorrect because while BFD clients are often Layer 3 protocols, BFD itself detects failures at Layer 1 and Layer 2. The statement "detects local link failure at layer 3" mischaracterizes BFD's operational layer. Option C is incorrect because BFD does not typically detect Layer 3 problems natively in the same way; it focuses on the adjacency/link state (L1/L2), whereas Layer 3 reachability is usually handled by the routing protocol itself once the link is up.

Community Comment Notes

Community consensus strongly favors Option D. User 'Koume' explains that while routing protocols handle L3, BFD handles the underlying link failures. User 'louisvuitton12' cites the ENARSI course noting that while BFD clients are L3, the detection mechanism itself supports L1/L2 failure detection. User 'error_909' references a Cisco white paper stating BFD provides "sub-second Layer 2 failure detection," reinforcing the L1/L2 focus. User 'Andryel' notes that BFD is used to quickly detect reachability failures, but the key advantage is the speed derived from lower-layer monitoring.

Official Reference

https://www.cisco.com/en/US/technologies/tk648/tk365/tk207/technologies_white_paper0900aecd80243fe7.html

Exam Strategy

Remember that BFD is a 'generic' failure detection protocol. While it integrates with Layer 3 protocols (OSPF, BGP, EIGRP), its primary function is to detect connectivity issues at Layers 1 and 2 rapidly. Do not confuse the client (L3) with the detection mechanism (L1/L2).

Frequently Asked Questions

Does BFD detect Layer 3 failures?

BFD itself detects Layer 1 and Layer 2 failures. It notifies Layer 3 protocols (like OSPF or BGP) so they can react to the loss of adjacency.

Why is Option B wrong if BFD works with OSPF?

While OSPF is a Layer 3 client of BFD, BFD's native detection capability is focused on the physical and data link layers (L1/L2) to ensure rapid failure identification.

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