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Washington School District WAN
Frame Relay Learning Objectives and Network Design
Learning Objectives
In Semester 4, chapter 6 we learned about that Frame Relay is a process for sending data over a public data network. It operates at the data-link layer, and uses HDLC encapsulation to handle multiple virtual circuits between connected devices across the Frame Relay cloud. The network can be either a carrier-provided public network or a network of privately-owned equipment, although this is relatively rare.
We learned about important terms that are used when setting up Frame Relay:
- Access Rate – The port speed of the connection to the Frame Relay cloud
- Data-link Connection Identifier (DLCI) – A number that identifies the end point in a Frame Relay network. It has local significance only. The Frame Relay switch uses DLCIs to create a virtual circuit.
- Local Management Interface (LMI) – Used for signaling between customer premises equipment and the Frame Relay switch. Can be used for keepalives, multicasts, to give DLCIs global significance, and a status mechanism.
- Committed Information Rate (CIR) – The minimum guaranteed rate that the service provider promises to provide.
- Forward Explicit Congestion Notification (FECN) and Backward Explicit Congestion Notification (BECN) – Used by Frame Relay switches to notify end devices of congestion, and adjust the rate at which data is being sent accordingly.
- Discard Eligbility (DE) Indicator – Bit in a frame indicating that the frame can be discarded in case of congestion. This bit is set on any traffic which is greater than the CIR.
In Frame Relay networks, the DLCI number is generally assigned by the service provider. It has only local significance, and end devices on the same permanent virtual circuit (PVC) can use different numbers to refer to the same PVC, and different devices in the Frame Relay cloud can all use the same DLCI number when accessing the cloud. Frame Relay switches use internal mechanisms to identify the PVCs and keep them distinct.
We also learned how LMI extensions to Frame Relay can be used to add functionality. There are four main extensions:
- Virtual Circuit Status Messages – Required to be implemented if LMIs are enabled, these messages are used to communicate the status of PVCs.
- Multicasting – Allows a single packet to be delivered to multiple recipients, which allows the efficient conveyance to routing protocol updates and address resolution protocols. This allows the router to dynamically build its Frame Relay map instead of the administrator needing to configure a static map. Optional.
- Global Addressing – Gives the DLCIs global significance, allowing the Frame Relay network to act like a LAN. Optional.
- Simple Flow Control – Allows flow control for devices that cannot use FECN/BECN bits. Optional.
Frame Relay also supports the use of subinterfaces, allowing a single physical interface to connect to multiple PVCs. This can cause problems if split-horizon is enabled, because the router will not send routing updates out the same physical interface from which it received an update, even if it logically connected to other routers.
Application of Concepts in the Network Design
On the District network, Frame Relay is being used for the connection between the Public Access router and the service provider. The provider has given us a CIR of 1.544Mbps, which means that the link will transfer data as fast as a T1 link. In theory, if there is low traffic on the Frame Relay cloud, we could transmit data even faster. The provider has given us a DLCI of 69. We have chosen not to implement LMI at the present time, because we have no need for the increased functionality. If in the future we choose to connect to other district networks through the Frame Relay cloud, we may choose to implement LMIs to make configuration and maintenance easier. In addition, since we only have a single link into the cloud, we have no need to use subinterfaces, although they could also be implemented in the future if we connect to other school district networks. The configuration of Frame Relay on the Public Access router can be found here.