yes indeed you re right PCAP stands for packet capture,thanx
i think the most of tools over there in the field such as ethereal do have filters i.e you can choose weither to capture TCP or IPX but not sure for GRE... so do a check .
other thing if you are in multilayer environment think of using SPAN, this feature works for bothe switched ports and routed ports your case i think is routed ports since you are dealing with GRE tunnels, so capture the whole traffic to an analyser tool and filter out only your GRE.
do rate if it does help
I think I can help you.
I have successfully used ethereal to capture BGP, OSPF, LDP, etc. I don't think it's a problem to capture the GRE packets.
Please wait a day or two so I can prepare for it.
I captured the GRE packets in my home lab yesterday. It was captured from a simple topology with 4 router in series: r1,r2,r3,and r4. All of them were running OSPF and the GRE tunnel was created between r1 and r4. Several type of packets were created to run over the GRE tunnel : EIGRP, ICMP, and TELNET.I hope you can enjoy analyzing these packets.Let me know if you have any problem.