Keepalive Detection
The Keepalive detection function monitors tunnel status to check whether the remote end is reachable. If the tunnel is unreachable, the source end tears down the tunnel immediately. GRE alone does not provide the link status detection function. Therefore, if the remote interface is unreachable, the tunnel would otherwise not be immediately torn down. As a result, the source continuously forwards packets to the remote end which cannot receive the packets, generating data black holes. Using Keepalive detection prevents data black holes and ensures reliable data transmission.
After being enabled on the source end of a GRE tunnel, the source end starts a timer to periodically send and count Keepalive probes. Every time a Keepalive probe is sent, the value goes up by 1.
The remote end sends a reply packet to the source end after receiving a probe.
If the source end receives a reply packet before the counter value reaches the preset value, it considers the remote end reachable. If the source end does not receive any reply packet when the counter reaches the preset value (retry times), the source end considers the remote end unreachable and tears down the tunnel. In this case, the source interface still sends Keepalive probes to the remote interface. When the remote interface changes to Up, the source interface does as well, and sets up a tunnel with the remote interface.
The Keepalive detection function takes effect on one end of a tunnel, regardless of whether it is configured on the other end. For example, if the remote end receives a Keepalive probe, it sends a reply packet to the source end, regardless of whether it is configured with the Keepalive detection function.