Exam 400-007 topic 1 question 117 discussion

B. BFD asynchronous mode with and without echo packets use control packets C. BFD asynchronous mode with echo packets combines the control packets and echo packets into a single packet

a confusing question echo mode doesn't send control packets, instead echo packets are used. control packets are unidirectional

How to Configure Bidirectional Forwarding Detection You start a BFD process by configuring BFD on the interface. When the BFD process is started, no entries are created in the adjacency database, in other words, no BFD control packets are sent or received. BFD echo mode, which is supported in BFD Version 1 for Cisco IOS 12.4(9)T, is enabled by default. BFD echo packets are sent and received in addition to BFD control packets.

D is incorrect, because Control packet is sent periodically, it is not a responding to previous control packet. Answer is A B

Control packet are unidirectional They are always transmitted: in echo mode or not

Answer is AD. Echo packets—Encapsulated into UDP packets with port number 3785. Control packets—Encapsulated into UDP packets with port number 3784 for single-hop detection or port number 4784 for multihop detection. Asynchronous mode—The device periodically sends BFD control packets. The device considers that the session is down if it does not receive any BFD control packets within a specific interval. https://techhub.hpe.com/eginfolib/networking/docs/switches/5930/5200-4571_hi-avail_cg/content/491355164.htm BFD echo mode works with asynchronous BFD. Echo packets are sent by the forwarding engine and forwarded back along the same path in order to perform detection https://www.cisco.com/c/en/us/td/docs/ios/12_0s/feature/guide/fs_bfd.html