WEBVTT

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Three Cisco routers form a triangle with two paths from router one to router three.

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Both OSPF neighbors start in the Full state.

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The route to the remote loopback uses the direct link because it has the lower cost.

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A ping from the local loopback confirms the starting path.

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Change the area on one end of the direct link to create a mismatch.

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The direct adjacency drops, while the neighbor on the backup path stays Full.

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The direct neighbor still replies to a ping, so the link itself is up.

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The route to the remote loopback now points through router two.

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Ping the remote loopback again to verify reachability over the backup path.

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Set the interface back to area zero and watch both neighbors return to Full.

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The lower cost route returns, and the remote loopback still replies.