Summary

When choosing a MAN solution, you should consider cost, scalability, transparency, level of service, and type of connection(s) needed. There are many solutions to provide MAN services, including SONET, DWDM, CWDM, Ethernet, IP, and ATM. SONET is good for point-to-point connections. CWDM is used for last-mile connections and DWDM is used as the infrastructure for a MAN backbone.

There are different methods of attaching to a MAN: TLS (access link) and DVS (802.1Q trunk). Both have problems with scalability. The service provider can support only 4,096 VLANs for all customers. Q-in-Q (tag stacking) and EoMPLS address this issue. All of these solutions have issues when customer redundancy is implemented, especially when it comes to STP.

Q-in-Q has the provider insert an additional 4-byte VLAN tag before your trunking tag. This is a proprietary Cisco method that is currently in an RFC draft state. Q-in-Q allows the tunneling of BPDUs and CDP frames. One limitation of Q-in-Q is that the provider is still limited to 4,096 internal VLANs. EoMPLS overcomes this by using a larger tag value. EoMPLS utilizes a tunnel and VC tag. The tunnel tag is used to switch the frame through the provider's network and the VC tag is used to find the exit interface on the provider's egress device.