With two routers connected by two Ethernet switches, OSPF will not detect a failure between the two switches until the $\_\_\_\_\_$ timer has expired $(\_\_\_$ seconds by default$).$ It is possible to decrease the $\_\_\_\_$ interval for OSPF sessions, but the processing of these messages increases the overhead within the OSPF process on the $\_\_\_.\_\_\_\_$ provides a lightweight network protocol that operates at the level of the physical interface to detect the loss of packet forwarding capabilities on the link.