Hello Wei Ping
When the echo mode feature is enabled, both control packets and echo packets are sent out at the same time. However, the echo function allows for a reduction in the number of BFD control packets. So yes, both control packets and echo packets are sent out. As stated in the Cisco documentation linked below:
“The echo function and the forwarding engine are responsible for the detection process, therefore the number of BFD control packets that are sent out between two BFD neighbors is reduced.”
A detailed description of this operation can be found at the following Cisco documentation:
I hope this has been helpful!
Laz