Good question. A router detects when an interface goes down by constantly monitoring the link status on that interface, which is essentially a signal indicating whether a physical connection and data transmission is happening on the line; if the signal drops below a certain threshold, the router interprets it as a link failure and marks the interface as "down." The router's hardware directly monitors the physical signals on the interface, such as voltage levels, to determine whether a connection is active. Beyond physical connectivity, routers also use protocol-level checks like sending "hello" packets to neighboring devices to verify if communication is possible on the link. If a neighbor receives no response within a set timeframe on a specific interface, the router assumes the link is down.
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How the first (active) router knows when the link is not present with switch ? How the active router monitors the link status to the switch ?
Good question. A router detects when an interface goes down by constantly monitoring the link status on that interface, which is essentially a signal indicating whether a physical connection and data transmission is happening on the line; if the signal drops below a certain threshold, the router interprets it as a link failure and marks the interface as "down." The router's hardware directly monitors the physical signals on the interface, such as voltage levels, to determine whether a connection is active.
Beyond physical connectivity, routers also use protocol-level checks like sending "hello" packets to neighboring devices to verify if communication is possible on the link. If a neighbor receives no response within a set timeframe on a specific interface, the router assumes the link is down.