Frame Relay 101

It does seem to be a little silly to be reviewing Frame Relay in such depth for the 640-802 CCNA exam, but I suppose that's one of the reasons Cisco is retiring the exam and old guys like me are cramming to get this taken.  Anyhow...

Frame Relay is a technology that is packet-switched instead of cell-switched like ATM.  The fundamentals are that each router on either end of the Frame Relay circuit connects to the provider network, and there is a virtual circuit inside the provider network that makes those routers think they are directly connected.  Doing a traceroute from one end of the virtual circuit to another shows only one hop, even though there are undoubtedly several devices within the provider network the packet traverses.

The 3 Major Designs are:

1. Hub and Spoke - all branch offices connect to a central site.  The main benefit is cost savings, while the main sacrifices are redundancy and tandem switching (routing all traffic through the main hub)
2. Partial Mesh - key sites have multiple virtual circuit connections
3. Full Mesh - all sites connected to every other site, which is expensive.

 PVC (permanent virtual circuit) is an always-on virtual circuit, no set up time

SVC (switched virtual circuit) is an on-demand virtual circuit

LMI (Local Management Interface) is the language used for the provider to send status info about the state of the virtual circuit to the router

DLCI - because serial interfaces have no MAC address, they use a Data Link Connection Identifier to determine which interface with which to communicate.  DLCI numbers are locally significant in that, when traffic is destined for a remote IP address, it goes out a local DLCI instead of heading to a destination DLCI like in the case of using a MAC address. 

CIR (Committed Information Rate) is the minimum speed the provider promises to you at all times.  Each PVC has its own CIR, and anything over that minimum rate is marked "De" or discard eligible - which means it can be dropped if there is not sufficient bandwidth.

BECN (Backward Explicit Congestion Notice) alerts the sending device that it should slow down.  BECN goes away from the congested resource to the sending resources

FECN (Forward Explicit Congestion Notice) is data used to generate traffic back to the sender to tell it to slow down, and is sent to the receiving device by the FR provider.