Routing

The SRX comes by default with two interfaces (fxp0 and Irb0) as shown below:

--------------------------------

fxp0 {
unit 0 {
family inet {
address 192.168.1.1/24;
}
}
}
irb {
unit 0 {
family inet {
address 192.168.2.1/24;
}
}
}

----------------------------------------------

The fxp0 is the OOB management port but what is the use of the irb0. I really don't get it.

Most of the SRX have a series of interfaces linked together as a layer 2 switch for local devices. 

The irb interface is the layer 3 gateway for this grouping of local switch ports.

Hi,

SRX can also work in Ethernet Switcing and Transparent mode as well.

When configured in Transparent mode, IRB interface is specifically used for local management purpose as that will be the only Layer3 interface in this mode. High end devices can also use fxp0 interface for management purpose along with irb.

When in Ethernet switching mode, IRB interface is associated to layer-2 VLANs to allow L3-based routing. For more information on usage on IRB, please refer to https://www.juniper.net/documentation/en_US/junos/topics/topic-map/irb-and-bridging.html

Also refer to https://kb.juniper.net/InfoCenter/index?page=content&id=KB23823 on how to configure management access for devices in transparent mode.

Hope this helps.

Thanks,
Pradeep
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