Chapter 11. Metro Ethernet

Terms you'll need to understand:

• MAN, SONET, DWDM, and CWDM

• Transparent LAN (TLS) and directed VLAN (DVS) services

• Q-in-Q tunneling and tag stacking

• Ethernet over MPLS

Techniques you'll need to master:

• Choosing a MAN service

• Understanding the similarities and differences in access methods to MANs: access links, 802.1Q trunks, Q-in-Q, and EoMPLS

• Understanding how Q-in-Q and EoMPLS tags frames

• Understanding STP issues with redundant MAN connections

Until recently, most WAN and MAN (metropolitan area network) connections required you to use WAN connection services, such as Frame Relay, ATM, and dedicated leased lines. There are problems with deploying these solutions, though. First, you must use some type of Layer 3 device that supports these WAN connections. This means that for a MAN, you cannot connect one Ethernet switch at one site directly to an Ethernet switch at a remote site you're forced to purchase a router. Second, you're introducing a delay in your traffic streams because your Layer 3 device must encapsulate your user's data in a different Layer 2 frame format. This can create problems for delay-sensitive traffic such as voice and video.

To deal with these and other problems, carriers have developed MAN solutions that more easily fit into a customer's network. Basically, carriers allow their customers to send Ethernet frames into the carrier's networks. However, this presents problems to carriers in how Ethernet frames should be transferred over a Layer-2 transport that is not typically used for Ethernet and how the carrier should establish and maintain these connections for hundreds of customers at a time. This chapter focuses on the problems and solutions that carriers use to transport Ethernet across their MAN backbones.