Explain Frame Relay

Exam: Cisco 300-101 - Implementing Cisco IP Routing (ROUTE v2.0)

Under the exam number 300-101 Route there is a sub topic of “explain frame relay”. In this chapter we will be discuss the topic in details so that the readers get a better idea of the same.

Frame relay is a wide area network technology that will specify the logical and the physical link layers in a digital telecommunication channel that uses a packet switching technology. It is a cost efficient data transmission for traffic between local area networks. It can also connect the local area network to wide area networks. In frame relay the data is put in a variable size unit that is called the frame.

It leaves possibility of error correction to the end points. This ensures that the speed of the data transmission is increased considerably. You must know that an enterprise has the right to prioritize some frames and make the others less important. The frame relay is offered by many service providers.

The Frame Relay can also be provided on a full carrier system or a fractional T1. The Frame Relay is based on the X.25 technology but it is much faster. If an error is found in the frame no effort is made to correct it instead it is just dropped. This is exactly how Frame Relay becomes very fast and efficient.

The Frame Relay was initially designed only for ISDN (integrated services digital network) infrastructure. Today it is used in many other network interfaces as we have already explained. The Frame Relay switches create a virtual circuit that can help to connect the LAN with the WAN.

The Frame Relay concept may seem simple but you must have a deep knowledge of the matter in order to trouble shoot the Frame Relay. Sometimes the Frame Relay will exactly reflect the LAP-D structure. With the help of traffic analysis you can differentiate between the two. The LAP-D will mostly lack a control field.


Each Frame Relay consists of PDU (protocol data unit) that will contain the following fields.

  1. Flag field – this is a field that is used to perform a high level data synchronization that indicates the start and the end of the frame. It is done with the help of a unique pattern and ensuring that this pattern do not appear anywhere within the frame.
  2. Address field – this is the second field. It may occupy any of the octets depending on the range of the address that is in use.
  3. Information field – it is the third field. A system parameter will define the maximum number of data bytes that a host can put in a frame. In order to work effectively the frame relay ensures that a maximum value of 1600 Octets is used.
  4. Frame check sequence field – Each switching node must implement an error detection in order to avoid the wastage of bandwidth that may happen due to the transmission of erroneous frames. The cyclic redundancy check is basically used for error detection in a frame.

To control the congestion the frame relay uses a simple protocol at each switching node. This simplicity can only be obtained by removing the link by link flow control. This ensures that the offered load determines the performance of the frame relay. The elements that form part of the congestion control are:

  1. Admission control – this provides the basic mechanism that is needed in the frame relay to ensure that there is a guarantee of resource requirement once they are accepted. This also helps in achieving high level of performance. It is the network that will decide if the new connection request must be accepted.
  2. Committed information rate – it is the average rate in which the network will guarantee to transfer information units over a measurement interval.
  3. Committed Burst size – it is nothing but the maximum number of information units that can be transmitted in an interval.
  4. Excess Burst size – it is the maximum number of uncommitted information units that a network will try to carry during the interval.

When the connection is established in the network the edge node of the frame relay network monitors the connection traffic flow and ensures that the actual usage of the network do not exceed that of the specification. The user’s information is discarded if the subscribed access rate is exceeded.     


In a frame relay network a single VC will be designed for a point to point connection. The VC will generally originate at the local end and will terminate at the remote end. A subnet address will be assigned to each of the point to point connection.

In a point to point subinterface the destination will be defined and it will be configured with the command “frame relay interface –dlci”. DLCI is data link connection identifier it is used to assign frames to a SVC or a PVC.

This command must be given in interface configuration mode. You must remember here that for point to point subinterface there is no need to use the frame-relay map command for performing the static address mapping.

This is because it is always assumed that the termination point of the point to point connection will be found on the same subnet as that of the beginning point. In this there is also no need to disable or enable the inverse ARP. This is because of the fact that there is a unique one and only remote destination on a point to point interface.

Multipoint Subinterfaces

In a Cisco router by default the physical interfaces are multipoint interfaces. You must note that when a multipoint subinterface is created then a DLCI must be assigned to the multipoint subinterface. The Cisco IOS allocates all the unassigned DLCI to a physical interface in the router.

The frame relay interface dlci command can be used many a times for associating more than one DLCI to a multipoint subinterface. In the multi-point subinterface you can use both static as well as dynamic mapping.

This depends on the network configurations completely. The frame relay map protocol - protocol address dlci is a command that is used specially assigns the DLCI multipoint subinterfaces. Using the optional broadcast command the multicast traffic can be sent over a specific dlci.

We hope that this chapter on explain frame relay will help you to understand the topic better and get a better score in the exam.

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