A does seem to be the correct answer, and it is possible to do it that way. But if we look a bit deeper at the question, there are 2 steps involved. It asks for the "first" step to do, which is to enable "bfd all-interfaces" in the routing process and only then disable isis bfd on unnecessary interfaces. So I vote for D answer.

A is correct because you can enable it on a per-interface basis instead of in the routing process for all interfaces

I hate this question too... IOS XR - router isis config.. IOS XE - Interface config

https://www.cisco.com/c/en/us/td/docs/routers/asr9000/software/asr9k-r6-3/routing/configuration/guide/b-routing-cg-asr9000-63x/b-routing-cg-asr9000-63x_chapter_0100.html#ID310

For IOS: Step 1. Disable sending ICMP redirect messages Step 2. Enable BFD on an interface. Step 3. Configure the routing protocol client. For IOS XR all bfd configuration is done under the routing protocol. I guess the IOS type is clearly mentioned on the exam.

yup, A is very likely possibly surely definitely correct!

A is correct: https://www.cisco.com/en/us/td/docs/iosxr/ncs5500/routing/63x/b-routing-cg-ncs5500-63x/b-routing-cg-ncs5500-63x_chapter_0111.html#concept_CD8762104EE84F1795CE48058A19D9ED

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_isis/configuration/15-s/irs-15-s-book/irs-fscdt.html#GUID-D02CE67A-0F2F-4E48-84B6-FB21D9A538A2

Correct Answer is D. fd all-interfaces Example: Device(config-router)# bfd all-interfaces https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_isis/configuration/15-s/irs-15-s-book/irs-fscdt.html

I hate this question. Nowhere in this question does it specifically say just one interface, it just says enable BFD. and that first step is D. https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_isis/configuration/15-s/irs-15-s-book/irs-fscdt.html

Prerequisites IS-IS must be running on all participating routers. The baseline parameters for BFD sessions on the interfaces that you want to run BFD sessions to BFD neighbors over MUST be configured. See the Configuring BFD Session Parameters on the Interface for more information. .... it says MUST... so answer is A

Agreed

The question is about implementing BFD in an IS-IS environment, not on an interface. Then it says "first", so there are at least 2 possible steps. In XR you just configure bfd fast-detect under config-isis-if configuration mode. In IOS you can configure bfd all-interfaces under config-router and then you may also disable specific interfaces from the config-if mode.

Differences in BFD in Cisco IOS XR Software and Cisco IOS Software https://www.cisco.com/en-us/td/docs/routers/asr9000/software/asr9k-r6-5/routing/configuration/guide/b-routing-cg-asr9000-65x/b-routing-cg-asr9000-65x_chapter_0100.html#ID310 BFD Peers on Routers Running Cisco IOS and Cisco IOS XR Software: Example https://www.cisco.com/en-us/td/docs/routers/asr9000/software/asr9k-r6-5/routing/configuration/guide/b-routing-cg-asr9000-65x/b-routing-cg-asr9000-65x_chapter_0100.html#ID3293

Selected Answer: D Obviously D is correct. https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_isis/configuration/15-s/irs-15-s-book/irs-fscdt.html#GUID-D02CE67A-0F2F-4E48-84B6-FB21D9A538A2

D enable configure terminal router isis area-tag bfd all-interfaces exit

I am going with D specifically because the question asked to activate BFD in an IS-IS environment, and D will activate BFD on all interfaces where ISIS is running.

Selected Answer: A https://www.cisco.com/en/US/docs/ios/12_4t/ip_route/configuration/guide/t_bfd.html#:~:text=or%20More%20Interfaces-,Prerequisites,Interface%22%20section%20for%20more%20information. Configuring BFD Support for IS-IS Prerequisites - IS-IS must be running on all participating routers. - The baseline parameters for BFD sessions on the interfaces that you want to run BFD sessions to BFD neighbors over must be configured. See the "Configuring BFD Session Parameters on the Interface" section for more information.