Chapter Summary

Routers typically route traffic between different subnets. To forward traffic between subnets that aren't in the same location, the router uses connectivity that a telco provides. Although routers can use WAN links, the more popular option today is to use Frame Relay. Frame Relay requires less hardware at the telco's customer site, and the telco charges less money for the same amount of bandwidth between sites. Beyond that, the routers typically can send more traffic than was paid for, and the telco will send it, as long as it doesn't impact other customers.

The Frame Relay service from a telco acts like a big Ethernet switch in concept. Routers connect to the Frame Relay network using a leased line from the router to a Frame Relay switch in the local CO. By sending Frame Relay frames over these access links to the nearby switch, the switch looks at the frame headers and forwards the frames based on the DLCI value in the header. The DLCI is essentially an address field that Frame Relay defines.