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How Does Frame Relay Work?

by Team Nuggets
Networking Basics: How Does Frame Relay Work? picture: A
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Updated on March 11, 2024

Quick definition: Frame relay is a cost-effective way to connect Local Area Networks (LANs) or transport data between endpoints in Wide Area Networks (WANs).

In networking, there is one constant — things change. Like frame relays, older techs are often replaced as newer technologies emerge. Frame relay is a packet-switching protocol used to connect local area networks (LANs) and send data over wide area networks (WANs.)

So, why do you need to understand a technology that isn't commonly used? At some point in your career, you may work on legacy systems. Older systems that haven't been updated may still use frame relays. They can also be used as a backup in networks where continuity of service is crucial.

Understanding frame relays can also serve as a basis for learning more complex networking topics, like those found on the CCNA exam.

After reading this post, you'll walk away with a deep understanding of frames, how frame relays work, and how to configure them.

What is a Frame?

A frame refers to the data unit used in Layer 2 of the OSI model. It is a specific data structure that includes the information being sent and the control info needed to transmit the data. 

The transport, network, data link, and physical layers of the OSI Model each have a different protocol data unit (PDU):

  • Layer 4: Transport layer = Segment

  • Layer 3: Network layer = Packet

  • Layer 2: Data link layer = Frame

  • Layer 1: Physical layer = Bits

While network layer devices like routers transport packets, devices in the data link layer use frames. It's mostly a semantic difference. A frame, however, has both a header and trailer. A packet only has a header.

What is Frame Relay?

Frame relay is a protocol used to transmit data over a network, typically a Wide Area Network (WAN). It operates on Layer 2 of the OSI model and is based on packet-switching technology.

Frame relay was widely used in the 1990s and early 2000s as a cost-effective solution for connecting remote locations in a WAN. However, its popularity has waned with the advent of more modern technologies such as MPLS (multiprotocol label Switching) and Ethernet-based solutions.

How Does Frame Relay Work?

Frame relay is typically used to transfer data between geographically separated LANs or across WANs. Frame relays can also be useful in home lab environments. CBT Nuggets trainer Jeremy Cioara recounts that while studying for his CCIE R&S, he was spending too much time recabling his lab.

"I had to re-cable every time I wanted a new topology. So, instead, I took all of my routers and connected them to a single-frame relay switch in a rack of equipment. And then, just by re-configuring that frame relay switch, I could say which routers were connected to what without actually physically moving cables around."

Here's how frame relay works:

  1. Frame relay breaks data into small units called frames, which are then transmitted over the network. These frames can contain data from multiple sources and are switched dynamically across network paths to reach their destination.

  2. Frame relay establishes virtual circuits between endpoints in the network. These virtual circuits provide a logical connection between devices, allowing them to communicate as if they were directly connected, even though long distances may physically separate them.

  3. Each virtual circuit in the frame relay is identified by a data link connection Identifier (DLCI), which is a unique identifier assigned to each connection. DLCIs are used by the network devices to route frames to the correct destination.

Frame relays are more efficient than traditional circuit-switched networks, allowing multiple virtual circuits to share the same physical network infrastructure. This allows for more efficient use of network resources and higher data throughput.

Switched Virtual Circuits & Permanent Virtual Circuits: What's the Difference?

Rather than using a full-time leased line between remote sites, frame-relay devices create one of two types of connections: Switched virtual circuits (SVC) or permanent virtual connections (PVC).

SVCs are temporary connections set up per session, meaning they are created when data needs to be transferred and close when they aren't in use. Permanent virtual connections (PVC) keep the connection between two locations always open. PVCs are much more common.

What are Data Link Connection Identifiers?

These dedicated connections link endpoints, usually routers configured as frame relay switches or provided by a service provider. In either case, the endpoints are labeled with unique Data Link Connection Identifiers (DCLI) identifiers. Frames are then transferred between two static points.

Frame relay is bizarre because you don't go directly from a source router to a destination router. You use a Permanent Virtual Connection (PVC) to send data. On either side, a Data Link Connection Identifier (DLCI) will be the source and destination. In this example, the source will be DLCI 102, and the destination will be DLCI 201.

In conclusion, when Router One wants to send data to Router Two, it will send data from DLCI 102, fly through the air, and then come out on DLCI 201. It's kind of like leaving out the gates of an airport. When Router Two sends data back, the data goes into 201, flying through the air, and comes out on 102.

How to Configure a Frame Relay Switch on Cisco IOS

You must first figure out your topology to configure a frame relay switch on Cisco IOS.

In this case, we'll use a topology where:

  • PVC A will connect DLCI 102 and DLCI 201. When R2 wants to send data to R1, it will send it to the destination of DLCI 201.

  • PVC B will connect DLCI 103 and DLCI 301. When R1 wants to send data to R2, it will send it to the destination of DLCI 102.

Use these Cisco IOS commands to configure a PVC from R2 to R1:

frame-relay route ? <input dlci to be switched> frame-relay route 102 ? <interface outgoing interface for pvc switching> frame-relay router 102 interface ? frame-relay router 102 interface s0/1 ? <output dlci to use when switching> frame-relay router 102 interface s0/1 201

You'll need to create connections between every endpoint in both directions. To configure a PVC from R2 to R1, update the final command with the DLCI and endpoint:

frame-relay router 201 interface s0/0 102

Finally, create connections for the remaining connections in both directions:

frame-relay router 301 interface s0/0 103
frame-relay router 103 interface s0/2 301

To check your connections, #frame route will show all the mappings.

Learn Frame Relay with CBT Nuggets Courses

In 2016, Cisco significantly reduced frame relay content on the CCNA 200-105 ICND2 exam. In place of frame relay, Cisco introduced BGP, modern WAN technologies, and SDN topics into the exam. 

As CBT Nuggets trainer Jeremy Cioara points out, frame relay is still good to learn because it covers key networking basics.

Preparing for the CCNA exam? Sign up for CBT Nuggets to get unlimited access to CCNA training. 


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