Network Traffic Through the Firewall Module

By default, no traffic can pass through the FWSM to access the network. On PIX and ASA appliance software, traffic flow from higher-level interfaces (Inside) to lower-level interfaces (Outside) will pass unrestricted. However, the FWSM software does not allow any traffic to flow between the interfaces unless explicitly permitted with an ACL. The security level does not provide explicit permission for traffic from a high-security interface to a low-security interface. This applies to all types of FWSM implementation (routed and transparent mode). To control network traffic, access lists are applied to FWSM interfaces. ACLs determine which IP addresses and traffic can pass through the interfaces to access other networks.