What does the passive interface command do in EIGRP?

The passive interface command in EIGRP is designed to suppress the exchange of routing updates on a specified interface. When this command is applied to an interface, the router will no longer send or receive EIGRP routing updates on that interface while still allowing EIGRP to maintain neighbor relationships on other active interfaces. This is particularly useful in scenarios where you want to prevent sending routing updates on interfaces connected to end devices or external networks where routing updates are unnecessary or could pose a security risk.

For instance, if a router interface connects to a segment that has hosts rather than other routers, using the passive interface command ensures that the router does not mistakenly send routing advertisements on that link. This helps in optimizing network performance and maintaining security, as it limits the routing information shared in the network.