Prerequisites for IP Security
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For the IPsec and NAT/PAT to work on the ASR 901S router a physical loopback connection is required from the management port to any available Gigabit port before issuing the following command in configuration mode:
platform mgmt loopback interface GigabitEthernet0/4.
In this case, the physical connection is between the management port and Gigabit port 0/4.