If 'check-gateway' is enabled for an ECMP route and one of the gateways is unreachable,...

When multiple gateways are used in an ECMP (Equal Cost Multi-Path) configuration, the check-gateway option ensures that RouterOS will actively monitor the health of each gateway using ping (or ARP). If a gateway becomes unreachable, RouterOS temporarily removes it from the active ECMP gateway list.

A.✘Incorrect – Unreachable gateways are excluded from packet forwarding.

B.✔Correct – Only reachable gateways are used in the ECMP round robin logic.

C.✘Incorrect – The entire ECMP route remains active; only the failed gateway is excluded.

Extract from MTCNA Course Material – ECMP Routing:

“With check-gateway enabled, RouterOS will exclude unreachable gateways from ECMP rotation.”

Extract from MikroTik Wiki – Check-Gateway Option:

“When a gateway is unreachable, it is skipped in ECMP logic until it becomes reachable again.”

Extract from René Meneses Study Guide – ECMP and Gateway Monitoring:

“Check-gateway helps prevent blackholing by skipping dead gateways. The route remains active.”

==================================