GRE Keyword

The GRE keyword, as defined in RFC 2890, is a four octet number inserted by the encapsulator. It may be used by the receiver to authenticate the source of the packet. If a GRE keyword is configured at either end of the tunnel, the keyword configuration must match at both ends of the tunnel. If a mis-match occurs, packets are dropped and an asterisk (*) is displayed to the left of the show tunnel command tunnel entry.

Use the tunnel keyword command, in tunnel configuration mode, to specify a GRE keyword for the tunnel.