The process of provisioning Frame Relay services for a corporate network involves a certain amount of pre-planning. Issues to be considered include bandwidth requirements for each PVC, the core bandwidth required at central locations, and the type of hardware that will support your needs. Each of these can vary greatly depending upon the number of sites and business requirements of the implementation. The sections below outlines important considerations in each area.
Choosing Frame Relay CIRs
Choosing CIRs can be difficult, especially if you are not sure of exactly how much data will need to be transferred over a given virtual circuit. In most cases this is accomplished by attempting to define and characterize the types of data traffic that will travel over the PVC between locations. For example, a PVC that will be handling sustained file transfer traffic will likely need a higher CIR than one used primary for the purpose of connecting to corporate email servers. This analysis should be done on a per-virtual circuit basis, as different locations may have different requirements.
Determining Frame Relay Core Bandwidth Requirements
Most Frame Relay networks are designed using a hub-and-spoke model where branch locations connect to a central location. Determining the total bandwidth required at the central location is easy – simply calculate the sum of all CIRs used by all of the virtual circuits that will connect to the central location. For example, a company with 6 branch offices with PVCs that connect to the central location with CIRs of 64Kbps each will require at least a 384Kbps link at the central location.
Choosing Router Models for a Frame Relay Network
Choosing an appropriate central office router model is an important consideration when planning a Frame Relay network. Firstly, you must be sure that the interface used to connect to the provider network is at least as fast as the sum of the CIRs on circuits that will be connecting to it. For example, one standard serial interface (which supports up to 1.544Mbps) could easily handle the 384Kbps bandwidth requirement just discussed, but this doesn’t take router load or utilization into consideration. Conversely, a router with 20 64Kbps PVCs connecting to it would necessitate the use of an additional serial interface (or a higher speed interface) based on bandwidth needs alone. As a general rule, Cisco recommends that no more than 30-50 DLCIs be configured per router interface. By the same token, the router model selected will have a measurable impact on performance – while a 1600 series router might be a great choice for a branch office, a 3600 series would probably be a much better choice for a central or “hub” location.
