JUNOS Enterprise Switching

Inter-VLAN Routing

EX switches support the notion of a Routed VLAN Interface used to route IP traffic between VLANs. The RVI is a purely logical interface construct that does not require any additional interface hardware or cabling.

Figure 7-1 depicts the VLAN topology example, which is pretty much the exit state from Chapter 5.

Thinking back to Chapter 5, recall that things began with a single flat Layer 2 network that encompassed all four hosts. This result was a single logical IP subnet (LIS) shared among all hosts, which then had full IP layer connectivity.

Then, two VLANs were deployed, and continued connectivity within each VLAN was confirmed, along with a lack of communications for inter-VLAN traffic. The original LIS was intentionally not renumbered along with the VLAN deployment to help demonstrate the inherent isolation of VLANs. After all, stations on separate subnets require a router to communicate, and there was no router in Chapter 5. Therefore, leaving the stations on the same LIS gave them the maximum chance for continued connectivity, which, as noted, failed after VLANs were deployed.

Figure 7-1. VLAN switching topology

The failure to communicate is because VLAN-aware switches segregate traffic on access links, permitting only traffic that is associated with the access link’s VLAN to be sent out that port. With VLAN segregation successfully demonstrated, and in preparation ...