A is seems to be a correct answer and it is possible to do it that way. but if we think deeper about the questions there are 2 steps involved. it asked "first" steps to do, which is to enable "bfd all-interfaces" in the routing process and then only disable isis bfd on unnecessary interfaces. So I vote for D answer.
Going for D specifically because question asked to activate BFD in IS-IS environment , D will do that activate BFD in all interface that is running ISIS.
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
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
Differences in BFD in Cisco IOS XR Software and Cisco IOS Software
https://www.cisco.com/c/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/c/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
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 potential 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.
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.
A is correct:
https://www.cisco.com/c/en/us/td/docs/iosxr/ncs5500/routing/63x/b-routing-cg-ncs5500-63x/b-routing-cg-ncs5500-63x_chapter_0111.html#concept_CD8762104EE84F1795CE48058A19D9ED
I hate this question.
No where in this question does it specifically mentions just one interface, 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
upvoted 2 times
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