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Technical Documentation > SonicOS 7 System > Interfaces > Interface Settings IPv4 > Layer 2 Bridged Mode > Key Concepts to Configuring L2 Bridged Mode and Transparent Mode

Key Concepts to Configuring L2 Bridged Mode and Transparent Mode

The following terms are used when referring to the operation and configuration of L2 Bridged Mode:

L2 Bridged Mode – A method of configuring a SonicWall Security Appliance, which enables it to be inserted inline into an existing network with absolute transparency, beyond even that provided by Transparent Mode. Layer 2 Bridged Mode also refers to the IP Assignment configuration that is selected for Secondary Bridge Interfaces that are placed into a Bridge-Pair.

Transparent Mode – A method of configuring a SonicWall Security Appliance that allows it to be inserted into an existing network without the need for IP reconfiguration by spanning a single IP subnet across two or more interfaces through the use of automatically applied ARP and routing logic.

IP Assignment – When configuring a Trusted (LAN) or Public (DMZ) interface, the IP Assignment for the interface can either be:

Static – The IP address for the interface is manually entered.

Transparent Mode – The IP address(es) for the interface is assigned using an Address Object (Host, Range, or Group) that falls within the WAN Primary IP subnet, effectively spanning the subnet from the WAN interface to the assigned interface.

Layer 2 Bridged Mode – An interface placed in this mode becomes the Secondary Bridge Interface to the Primary Bridge Interface to which it is paired. The resulting Bridge-Pair then behaves like a two-port learning bridge with full L2 transparency, and all IP traffic that passes through is subjected to full stateful failover and deep packet inspection.

Bridge-Pair – The logical interface set composed of a Primary Bridge Interface and a Secondary Bridge Interface. The terms primary and secondary do not imply any inherent level of operational dominance or subordination; both interfaces continue to be treated according to their zone type, and to pass IP traffic according to their configured Access Rules. Non-IPv4 traffic across the Bridge-Pair is controlled by the Block all non-IPv4 traffic setting on the Secondary Bridge Interface. A system might support as many Bridge Pairs as it has interface pairs available. In other words, the maximum number of Bridge-Pairs is equal to ½ the number of physical interfaces on the platform. Membership in a Bridge-Pair does not preclude an interface from conventional behavior; for example, if X1 is configured as a Primary Bridge Interface paired to X3 as a Secondary Bridge Interface, X1 can simultaneously operate in its traditional role as the Primary WAN, performing NAT for Internet-bound traffic through the Auto-added X1 Default NAT Policy.

Primary Bridge Interface – A designation that is assigned to an interface after a Secondary Bridge Interface has been paired to it. A Primary Bridge Interface can belong to an Untrusted (WAN), Trusted (LAN), or Public (DMZ) zone.

Secondary Bridge Interface – A designation that is assigned to an interface whose IP Assignment has been configured for Layer 2 Bridged Mode. A Secondary Bridge Interface can belong to a Trusted (LAN), or Public (DMZ) zone.

Bridge Management Address – The address of the Primary Bridge Interface is shared by both interfaces of the Bridge-Pair. If the Primary Bridge Interface also happens to be the Primary WAN interface, it is this address that is used for outbound communications by the appliance, such as NTP, and License Manager updates. Hosts that are connected to either segment of the Bridge-Pair may also use the Bridge Management Address as their gateway, as is common in Mixed-Mode deployments.

Bridge-Partner – The term used to refer to the other member of a Bridge-Pair.

Non-IPv4 Traffic – SonicOS supports the following IP protocol types: ICMP (1), IGMP (2), TCP (6), UDP (17), GRE (47), ESP (50), AH (51), EIGRP (88), OSPF (89), PIM-SM (103), L2TP (115). More esoteric IP types, such as Combat Radio Transport Protocol (126), are not natively handled by the appliance, nor are non-IPv4 traffic types such as IPX or (currently) IPv6. L2 Bridged Mode can be configured to either pass or drop Non-IPv4 traffic.

Captive-Bridged Mode – This optional mode of L2 Bridge operation prevents traffic that has entered an L2 bridge from being forwarded to a non-Bridge-Pair interface. By default, L2 Bridge logic forwards traffic that has entered the L2 Bridge to its destination along the most optimal path as determined by ARP and routing tables. In some cases, the most optimal path might involve routing or NATing to a non-Bridge-Pair interface. Activating Captive-Bridged Mode ensures that traffic that enters an L2 Bridge exits the L2 Bridge rather than taking its most logically optimal path. In general, this mode of operation is only required in complex networks with redundant paths, where strict path adherence is required.

Pure L2 Bridge Topology – Refers to deployments where the Security Appliance is used strictly in L2 Bridged Mode for the purposes of providing in-line security to a network. This means that all traffic entering one side of the Bridge-Pair is bound for the other side, and is not routed/NATed through a different interface. This is common in cases where there is an existing perimeter appliance, or where in-line security is desired along some path (for example, inter-departmentally, or on a trunked link between two switches) of an existing network. Pure L2 Bridge Topology is not a functional limitation, but rather a topological description of a common deployment in heterogeneous environments.

Mixed-Mode Topology – Refers to deployments where the Bridge-Pair are not the only point of ingress/egress through the appliance. This means that traffic entering one side of the Bridge-Pair may be destined to be routed/NATed through a different interface. This is common when the appliance is simultaneously used to provide security to one or more Bridge-Pair while also providing:

Perimeter security, such as WAN connectivity, to hosts on the Bridge-Pair or on other interfaces.
Firewall and Security services to additional segments, such as Trusted (LAN) or Public (DMZ) interface, where communications occur between hosts on those segments and hosts on the Bridge-Pair.
Wireless services with SonicPoints, where communications occur between wireless clients and hosts on the Bridge-Pair.

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