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GRE tunnel does not work over NAT

Publication Date:  2016-09-30 Views:  186 Downloads:  0
Issue Description
GRE traffic from private subnet is not translated over the Internet using NAPT NAT
Handling Process

Solution 1: configure a nat server specifically for the the GRE protocol

acl number 3100
rule 5 permit ip source 0 destination x.x.x.x
nat server protocol gre global interface GigabitEthernet1/0/1 inside

Solution 2: configure detect pptp on the interface connecting to the Internet

Root Cause
GRE is a Layer 3 protocol and does not use ports for communication, therefore NAPT NAT cannot be used