Actually, if you look at the information I provided, you can see that each hello contains its own response (a GRE message in a GRE message).
If I send a keepalive from 184.108.40.206 to 220.127.116.11 the internal GRE message is from 18.104.22.168 to 22.214.171.124, which is king of a bidirectional link testing. Once the entire packet is reach by the peer, the external IP and GRE headers are removed and the internal IP and GRE are forwarded back to the sender. This allows for the keepalive to be supported from one side only if required.
In this case the keepalive is 48 bytes and the response is 24 bytes.
Bear in mind that keepalives might be sent from both peers.
Hope this helps,
Harold Ritter Sr. Technical Leader CCIE 4168 (R&S, SP) email@example.com México móvil: +52 1 55 8312 4915 Cisco México Paseo de la Reforma 222 Piso 19 Cuauhtémoc, Juárez Ciudad de México, 06600 México
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