2.3 IRB Architecture and Interface Families

The Architectural Need for Layer 3 Inter-VLAN Routing

Virtual Local Area Networks enforce hard Layer 2 boundaries. Inside a switch, the Packet Forwarding Engine maintains an isolated Filtering Database (FDB)—also called a MAC address table—for each configured VLAN. When a host in VLAN 10 transmits a frame toward an IP address residing in VLAN 20, the switch cannot bridge that frame across broadcast domains. Bridging between VLANs would collapse their broadcast isolation, undermine security boundaries, and cause IP subnetting conflicts. Therefore, any communication between distinct VLANs mandates a Layer 3 routing function.

The Evolution of Inter-VLAN Routing Architecture

1. Router-on-a-Stick (Legacy Approach): In early campus networks, Layer 2 switches lacked routing capabilities. To route between VLANs, the switch connected via an 802.1Q trunk link to an external router. The router created logical subinterfaces (one per VLAN), each acting as the default gateway for its respective subnet. While functional, this design created a severe architectural bottleneck: every inter-VLAN packet had to travel up the physical trunk link to the router and immediately traverse back down the same trunk link to the switch. This "hairpinning" consumed double the trunk bandwidth and introduced significant forwarding latency.

2. Multilayer Switching with Internal Gateways (Modern Standard): Modern enterprise switches—such as Juniper Networks EX and QFX series—integrate full Layer 3 routing logic directly into their switching silicon. The switch hardware contains specialized Application-Specific Integrated Circuits (ASICs) capable of performing Layer 2 frame switching and Layer 3 IP routing concurrently at line rate. Instead of exiting the chassis to an external router, inter-VLAN traffic is routed internally across the switch backplane through internal logical routing interfaces, eliminating external bottlenecks and delivering hardware forwarding without the external router-on-a-stick path.

---

Junos Integrated Routing & Bridging (IRB) Architecture

Junos OS executes routing and switching through a rigorous separation of the Control Plane (managed by the Routing Engine, or RE) and the Forwarding Plane (managed by the Packet Forwarding Engine, or PFE). In multilayer switches, the RE runs routing protocols (such as OSPF, IS-IS, and BGP), resolves ARP requests, and populates the master routing table (inet.0). The RE then compiles this routing state into an optimized Forwarding Information Base (FIB) and downloads it directly into the PFE's Layer 3 forwarding ASICs.

PFE Packet Processing Mechanics

When an Ethernet frame ingresses a physical switch port, the PFE performs an instantaneous Layer 2 hardware lookup on the frame's Destination MAC (DMAC) address:

• Intra-VLAN Bridging: If the DMAC matches another host's MAC address learned within that same VLAN, the switch bridges the frame directly out the associated egress port. The Layer 3 IP header remains untouched, and the Time-to-Live (TTL) is not decremented.
• Inter-VLAN Routing: If the DMAC matches the switch's own internal Layer 3 interface MAC address (the default gateway MAC), the PFE hardware recognizes that the frame is addressed to the switch itself for routing. The PFE strips the incoming Layer 2 Ethernet header and passes the decapsulated IP packet to the Layer 3 FIB lookup engine. The FIB determines the egress route, identifies the outbound VLAN, decrements the TTL, recomputes the checksum, encapsulates the packet in a new Layer 2 header, and transmits it toward the destination host.

---

Platform Evolution: Legacy RVI (vlan.unit) vs. Modern ELS IRB (irb.unit)

Candidates studying for the JNCIS-ENT examination must understand the platform evolution between legacy Junos switching platforms and modern Enhanced Layer 2 Software (ELS) architectures:

1. Legacy Non-ELS Platforms (EX2200, EX3200, EX3300, EX4200, EX8200)

On original EX series switches running non-ELS Junos code, Layer 3 VLAN interfaces were designated as Routed VLAN Interfaces (RVIs) and configured under the vlan interface hierarchy:

[edit interfaces]
user@legacy-switch# set vlan unit 10 family inet address 192.168.10.1/24

[edit vlans]
user@legacy-switch# set SALES vlan-id 10 l3-interface vlan.10

2. Modern ELS Platforms (EX2300, EX3400, EX4300, EX4400, EX4600, QFX, MX)

Modern enterprise and data center switches running ELS, as well as Juniper MX Series 5G Universal Routing Platforms, standardized on Integrated Routing and Bridging (IRB) interfaces. The interface name is explicitly irb followed by a logical unit number:

[edit interfaces]
user@EX4400# set irb unit 10 family inet address 192.168.10.1/24

[edit vlans]
user@EX4400# set SALES vlan-id 10 l3-interface irb.10

Why Juniper Transitioned to ELS IRB

The shift from vlan.unit to irb.unit was driven by architectural standardization. In large-scale enterprise and service provider networks, engineers frequently deploy both EX enterprise switches, QFX data center fabrics, and MX edge routers. By unifying the switching and routing syntax under irb, Junos OS enforces identical configuration hierarchies, operational verification commands, and automation scripts across all platforms. Furthermore, irb interfaces natively integrate with advanced architectures such as Virtual Routing and Forwarding (VRF) instances, EVPN-VXLAN overlays, and sophisticated firewall filter policies.

---