Layer 2 Transparent Mode Overview


A device operates in transparent mode when there are interfaces defined as Layer 2 interfaces. The device operates in route mode (the default mode) if there are no physical interfaces configured as Layer 2 interfaces.

For SRX Series devices, transparent mode provides full security services for Layer 2 switching capabilities. On these SRX Series devices, you can configure one or more VLANs to perform Layer 2 switching. A VLAN is a set of logical interfaces that share the same flooding or broadcast characteristics. Like a virtual LAN (VLAN), a VLAN spans one or more ports of multiple devices. Thus, the SRX Series device can function as a Layer 2 switch with multiple VLANs that participate in the same Layer 2 network.

In transparent mode, the SRX Series device filters packets that traverse the device without modifying any of the source or destination information in the IP packet headers. Transparent mode is useful for protecting servers that mainly receive traffic from untrusted sources because there is no need to reconfigure the IP settings of routers or protected servers.

In transparent mode, all physical ports on the device are assigned to Layer 2 interfaces. Do not route Layer 3 traffic through the device. Layer 2 zones can be configured to host Layer 2 interfaces, and security policies can be defined between Layer 2 zones. When packets travel between Layer 2 zones, security policies can be enforced on these packets.

Table 1 lists the security features that are supported and are not supported in transparent mode for Layer 2 switching.

Table 1: Security Features Supported in Transparent Mode

Mode Type


Not Supported

Transparent mode

  • Application Layer Gateways (ALGs)

  • Firewall User Authentication (FWAUTH)

  • Intrusion Detection and Prevention (IDP)

  • Screen

  • AppSecure

  • Unified Threat Management (UTM)

  • Network Address Translation (NAT)

  • VPN

  • Starting in Junos OS Release 12.3X48-D10 and Junos OS Release 17.3R1, mixed mode is the default mode, and you can configure an SRX Series device using both transparent mode (Layer 2) and route mode (Layer 3) simultaneously, with no reboot required.

  • On all SRX Series devices, transparent mode is not supported on mPIMs.

  • On SRX300, SRX320, SRX340, SRX345, and SRX550M devices, the DHCP server propagation is not supported in Layer 2 transparent mode.

Layer 2 Switching Exceptions on SRX Series Devices

The switching functions on the SRX Series devices are similar to the switching features on Juniper Networks MX Series routers. However, the following Layer 2 networking features on MX Series routers are not supported on SRX Series devices:

  • Layer 2 control protocols—These protocols are used on MX Series routers for Rapid Spanning Tree Protocol (RSTP) or Multiple Spanning Tree Protocol (MSTP) in customer edge interfaces of a VPLS routing instance.

  • Virtual switch routing instance—The virtual switching routing instance is used on MX Series routers to group one or more VLANs.

  • Virtual private LAN services (VPLS) routing instance—The VPLS routing instance is used on MX Series routers for point-to-multipoint LAN implementations between a set of sites in a VPN.

In addition, the SRX Series devices do not support the following Layer 2 features:

  • Spanning Tree Protocol (STP), RSTP, or MSTP—It is the user’s responsibility to ensure that no flooding loops exist in the network topology.

  • Internet Group Management Protocol (IGMP) snooping—Host-to-router signaling protocol for IPv4 used to report their multicast group memberships to neighboring routers and determine whether group members are present during IP multicasting.

  • Double-tagged VLANs or IEEE 802.1Q VLAN identifiers encapsulated within 802.1Q packets (also called “Q in Q” VLAN tagging)—Only untagged or single-tagged VLAN identifiers are supported on SRX Series devices.

  • Nonqualified VLAN learning, where only the MAC address is used for learning within the VLAN—VLAN learning on SRX Series devices is qualified; that is, both the VLAN identifier and MAC address are used.

Also, on SRX100, SRX110, SRX210, SRX220, SRX240, SRX300, SRX320, SRX340, SRX345, SRX550, or SRX650 devices, some features are not supported. (Platform support depends on the Junos OS release in your installation.) The following features are not supported for Layer 2 transparent mode on the mentioned devices:

  • G-ARP on the Layer 2 interface

  • IP address monitoring on any interface

  • Transit traffic through IRB

  • IRB interface in a routing instance

  • IRB interface handling of Layer 3 traffic


    The IRB interface is a pseudointerface and does not belong to the reth interface and redundancy group.

Layer 2 Transparent Mode on the SRX5000 Line Module Port Concentrator

The SRX5000 line Module Port Concentrator (SRX5K-MPC) supports Layer 2 transparent mode and processes the traffic when the SRX Series device is configured in Layer 2 transparent mode.

When the SRX5K-MPC is operating in Layer 2 mode, you can configure all interfaces on the SRX5K-MPC as Layer 2 switching ports to support Layer 2 traffic.

The security processing unit (SPU) supports all security services for Layer 2 switching functions, and the MPC delivers the ingress packets to the SPU and forwards the egress packets that are encapsulated by the SPU to the outgoing interfaces.

When the SRX Series device is configured in Layer 2 transparent mode, you can enable the interfaces on the MPC to work in Layer 2 mode by defining one or more logical units on a physical interface with the family address type as Ethernet switching. Later you can proceed with configuring Layer 2 security zones and configuring security policies in transparent mode. Once this is done, next-hop topologies are set up to process ingress and egress packets.

Configuring Out-of-Band Management on SRX Devices

You can configure the fxp0 out-of-band management interface on the SRX Series device as a Layer 3 interface, even if Layer 2 interfaces are defined on the device. With the exception of the fxp0 interface, you can define Layer 2 and Layer 3 interfaces on the device’s network ports.


There is no fxp0 out-of-band management interface on the SRX300, SRX320, and SRX550M devices. (Platform support depends on the Junos OS release in your installation.)