JUNOS Enterprise Switching

Chapter 5. Virtual LANs and Trunking

LAN switches filter and forward traffic based on learning Source MAC (SMAC) addresses, and then later filter traffic based on Destination MAC (DMAC) addresses. Although a regular switch does terminate a collision domain, it does not isolate broadcast and multicast traffic; such traffic must be flooded to all LAN members because these addresses are never used as an SMAC, and thus can never be learned. By definition, this Broadcast, Unknown, or Multicast (BUM) traffic by its nature has to be flooded to all stations anyway.

LAN virtualization, also known as a virtual LAN or VLAN, allows switches to logically group end stations to provide isolation of BUM traffic, enhancing both performance and security. This chapter covers VLAN tagging and the configuration and monitoring of VLAN switching on the EX platform in the context of integrating a JUNOS switch into a Cisco environment.

The topics covered in this chapter include:

• Virtual LANs and trunking

• EX to catalyst VLAN integration (and troubleshooting)

Virtual LANs and Trunking

Ethernet LAN and LAN switching concepts, also known as bridging, are described in Chapter 1. This chapter focuses on using VLAN tags to logically partition the LAN into multiple broadcast domains.

In a conventional Ethernet LAN, all nodes must be physically connected to the same MAC broadcast domain to communicate at Layer 2. With VLANs, the physical location of the nodes is not important, as you can group network devices in ways ...