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The VPN tunnel does not come up on the ASA 5520 with Proxy ARP enabled on the outside interface

Core issue

There are a few reasons that a VPN tunnel may not to come up on Adaptive Security Appliance (ASA). One reason might be the Proxy Address Resolution Protocol (ARP).

When a host sends IP traffic to another device on the same Ethernet network, the host needs to know the MAC address of the device. ARP is a Layer 2 (L2) protocol that resolves an IP address to a MAC address. A host sends an ARP request asking  Who is this IP address?

The device owning the IP address replies,  I own that IP address; here is my MAC address.

Proxy ARP is when a device responds to an ARP request with its own MAC address, even though the device does not own the IP address. The security appliance uses proxy ARP when you configure Network Address Translation (NAT) and specify a global address that is on the same network as the security appliance interface. The only way traffic can reach the hosts is if the security appliance uses proxy ARP to claim that the security appliance MAC address is assigned to destination global addresses.

Resolution

If there is a router sitting in front of ASA, disable Proxy ARP on the outside interface of ASA. It interferes with the ARP table on router.

To disable Proxy ARP, issue the no sysopt noproxyarp interface_name command.

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