What is a function of an integrated routing and bridging IRB interface?

In Junos-based data center switching, an IRB interface is the Layer 3 gateway that is logically associated with a Layer 2 VLAN or bridge domain. The VLAN provides Layer 2 bridging inside the broadcast domain, while the IRB interface provides the routed interface that enables hosts in that VLAN to reach destinations outside their local subnet. This is the standard mechanism used for inter-VLAN routing on Juniper switches and for providing default gateway services to servers connected to access ports or VLAN-tagged trunks.

Operationally, endpoints in a VLAN use the IRB interface IP address as their default gateway. Frames destined to a remote subnet are bridged at Layer 2 to the IRB gateway MAC address, and then the packet is routed at Layer 3 based on the routing table. This allows a single device to perform both bridging within the VLAN and routing between VLANs or to other routed interfaces, which is why the concept is called integrated routing and bridging.

IRB does not encrypt traffic and does not provide NAT by itself; those functions are typically associated with security services features and firewall platforms. IRB is also not the mechanism that performs pure bridging within the same VLAN, because bridging is handled by the VLAN or bridge domain and the Ethernet switching table.