I've read quite afew articles about GRE and a 47 numeric value associated to it.. Has anyone seen this? I've never setup GRE on a router for encapsulation purposes but will soon and want to clarify this number's meaning..
47 is the protocol number assigned in the IPv4 header. Most used protocols are:
1-ICMP
6-TCP
17-UDP
You may have a full list at RFC1700:
ftp://ftp.rfc-editor.org/in-notes/rfc1700.txt
This value could be used when defining access-lists to permit/deny traffic based on protocol value (in Cisco routers you may use the protocol name in most cases).
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