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How does ARP work when a SonicWall appliance is configured in Transparent Mode?
03/26/2020 
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Description
How does ARP work when a SonicWall appliance is configured in Transparent Mode?
Resolution
While Transparent Mode allows a security appliance running SonicOS Enhanced to be introduced into an existing network without the need for re-addressing, it presents a certain level of disruptiveness, particularly with regard to ARP, VLAN support, multiple subnets, and non-IPv4 traffic types.
ARP – Address Resolution Protocol (the mechanism by which unique hardware addresses on network interface cards are associated to IP addresses) is proxied in Transparent Mode. If the Workstation on Server on the left had previously resolved the Router (192.168.0.1) to its MAC address 00:99:10:10:10:10, this cached ARP entry would have to be cleared before these hosts could communicate through the SonicWall. This is because the SonicWall proxies (or answers on behalf of) the gateway’s IP (192.168.0.1) for hosts connected to interfaces operating in Transparent Mode. So when the Workstation at the left attempts to resolve 192.168.0.1, the ARP request it sends is responded to by the SonicWall with its own X0 MAC address (00:06:B1:10:10:10).
The SonicWall also proxy ARPs the IP addresses specified in the Transparent Range (192.168.0.100 to 192.168.0.250) assigned to an interface in Transparent Mode for ARP requests received on the X1 (Primary WAN) interface. If the Router had previously resolved the Server (192.168.0.100) to its MAC address 00:AA:BB:CC:DD:EE, this cached ARP entry would have to be cleared before the router could communicate with the host through the SonicWall. This typically requires a flushing of the router’s ARP cache either from its
management interface or through a reboot. Once the router’s ARP cache is cleared, it can then send a new ARP request for 192.168.0.100, to which the SonicWall will respond with its X1 MAC 00:06:B1:10:10:11.
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