Juniper MX Series

Integrated Routing and Bridging

How does one take the quantum leap from Layer 2 into Layer 3? The secret is that you need a gateway that has access to a Routing Information Base (RIB) and sits in the same bridge domain as you. The Junos way of making this happen is through a logical interface called irb. The astute reader knows that irb stands for Integrated Routing and Bridging.

Although IRB is a bit more than a simple gateway, it has other features such as handling control packets for routing protocols such as OSPF, IS-IS, and BGP. If you’re running a multicast network, it will also handle the copying of frames for the bridge domain.

Figure 2-38. Illustration of IRB and Bridge-Domain Integration.

The hierarchy of Figure 2-38 should look familiar, as it follows the same interface hierarchy discussed previously in the chapter. At the top of the hierarchy sits the interface irb—this is a pseudo interface inside of Junos that acts as the gateway between bridge domains at the RIB. The irb is able to do this because it has both Layer 2 and Layer 3 that are associated to bridge domains and route tables. Let’s take a look at a basic example:

interfaces { irb { unit 100 { family inet6 { address 2001:db8:1:10::1/64; } } unit 200 { family inet6 { address 2001:db8:1:20::1/64; } } } } bridge-domains { BD100 { vlan-id 100; routing-interface irb.100; } BD200 { vlan-id 200; routing-interface irb.200; ...