Tuesday, April 6, 2010


Frame Relay was invented as the result of ISPs monitoring leased lines. They found out that leased lines are rarely fully used. Frame Relay is a class of packet switched technology. Frame Relay came from X.25, and later evolved into ATM. The concept behind Frame Relay is later used to develop MPLS.
Frame Relay terminologies are:
CIR - Committed Information Rate
The CIR is the minimum bandwidth that the provider guarantees. You can't go lower than it. At times you can actually burst above the CIR.

LAR - Local Access Rate
This is literally how fast the circuit can go. This is the real clocking rate of the circuit. If you're paying for a CIR that's lower than the LAR, you can only output short bursts of data that averages to your CIR.

LMI - Local Management Interface
The language that the router speaks between the router and the service provider. It is a signaling protocol that the provider can use to send statistics about the line. LMI also gives DLCI information.

DLCI - Data Link Connection Identifier
DLCI is the Frame Relay's equivalent of MAC addresses. DLCIs are locally significant and is used to identify the local "gate" to get to the other end. DLCI numbers go up to 1024.

Suppose that a RA has DLCI 102, 103, RB has DLCI 201, and RC has DLCI 301. They are connected by a Frame Relay PVC. For RA to reach RB, he would have to send his packet to DLCI 102. From the perspective of RB, the packet would be coming from DLCI 201. Similarly, when RC sends a packet to RA, he would send it to a destination of 301. However, from RA, the packet would be seen coming from DLCI 103.

PVC - Permanent Virtual Circuit
Frame Relay operates through Frame Relay switches. When you sign up for Frame Relay, a path is programmed into the Frame Relay switches which your packets can flow through. Since PVCs have recurring fees, companies use a hub and spoke topology to connect their sites.

There are three types of topology:

The hub-and-spoke topology is the cheapest out of all the topologies because it requires the least PVCs. However, you now have a single point of failure and VoIP calls may get higher latency.


A partial mesh usually starts out as a hub and spoke. As it grows, the important sites get more links and the less important sites get less links.


Full mesh is when all sites have direct connectivity to each other. This may become expensive and unmanageable as the number of sites increase. The formula for finding the number of links in a full mesh topology is: n(n-1)/2.

For the interfaces in a Frame Relay topology, you have two choices. You can either use a multi-point design, or a point-to-point design.

In a multi-point design, all routers are on the same subnet. Multiple DLCIs are mapped to an interface. The routers will think that they can reach all routers directly like in an Ethernet network, but only the hub of the topology can do that. This can cause problems with split horizon with DV routing protocols.

In a typical split horizon problem scenario, the hub would have all routes, while the spokes would only have their own routes. This is because the hub will not send updates back out of the interface it received it from. The solution in a multi-point design is to turn off split horizon.

The other way is the point-to-point design. In a point-to-point design, all routers are on different subnets. For each DLCI, a new sub-interface is created. There are no problems with split-horizon. The choice of point-to-point and multi-point is made by the network administrator. There is no need to coordinate with the ISP.

1 comment :

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