SonicOS: How does ARP work when a SonicWall appliance is configured in L2 Bridge Mode?
ARP in L2 Bridge Mode
L2 Bridge Mode employs a learning bridge design where it will dynamically determine which hosts are on which interface of an L2 Bridge (referred to as a Bridge-Pair). ARP is passed through natively, meaning that a host communicating across an L2 Bridge will see the actual host MAC addresses of their peers. For example, the Workstation communicating with the Router (192.168.0.1) will see the router as 00:99:10:10:10:10, and the Router will see the Workstation (192.168.0.100) as 00:AA:BB:CC:DD:EE.
This behavior allows for a SonicWall operating in L2 Bridge Mode to be introduced into an existing network with no disruption to most network communications other than that caused by the momentary discontinuity of the physical insertion.
It should be noted that stream-based TCP protocols communications (for example, an FTP session between a client and a server) will need to be re-established upon the insertion of an L2 Bridge Mode SonicWall. This is by design so as to maintain the security afforded by stateful packet inspection (SPI); since the SPI engine can not have knowledge of the TCP connections which pre-existed it, it will drop these established packets with a log event such as TCP packet received on non-existent/closed connection; TCP packet dropped.
Source: L2 Bridge Mode and Transparent Mode Deployment Guide