Frame Relay Overview & Configuration

Frame-Relay Basics

Frame-Relay networks are multi-access, meaning that multiple devices can be attached to the network similar to Ethernet LANs. But unlike LANs Frame-Relay networks do not support native broadcast but instead broadcast is replicated as unicast, thus this type of network is known as NBMA (Non-Broadcast Multi-access). Figure 1 shows an example of Frame-Relay Wide Area Network  

We can consider Routers A, B and C as there sites of a company who have purchased Frame-Relay service from a provider.  Each site connects with the Service Provider using a single link but using Frame-Relay technology virtual connection can be provisioned either on-demand or permanent to create various topologies like Hub-and-Spoke, Partial Mesh or Full Mesh.  Figure 2 shows the concept of Virtual Circuits.

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Table 1 specifies the various Frame-Relay Terminologies used in Figure 1 and 2.

Frame-Relay Signaling and Encapsulation

LMI is signaling mechanism used between a Router (DTE) and Frame-Relay Switch (DCE) for managing and maintaining the connection between devices. A Cisco router supports three types of LMI mentioned below

• Cisco: LMI type defined by Cisco, StrataCom and DEC 
• ANSI
• ITU-T Q.933A

 A Frame-Relay router encapsulates a Layer 3 Packet in a Frame-Relay header and trailer. There are two options available on Cisco routers as mentioned below

• Cisco
• IETF

 Frame-Relay Virtual Circuits

DTE devices exchange data over a frame-relay network by using virtual circuits. Multiple virtual circuits can be provisioned over a single physical link.  Since SVCs are not in the scope of CCNA Certification, we will focus on the explanation and configuration of frame-relay PVCs. Each PVC at the DTE end is identified using a 10-bit address called Data-Link Connection Identifier (DLCI). As shown in Figure 2, Router A has two virtual circuits. A DLCI of 102 identifies the VC to Router B and 103 identifies a VC to Router C. At the other end on Router B a VC to Router A is identified by DLCI 201.

The DLCI is mapped to a Network layer destination address and when date needs to be transported over the frame-relay network to the remote destination it is encapsulated inside a frame-relay header. Each DLCI is mapped to a corresponding remote destination network address. As the frames are transported across the Provider, the frame-relay switch will change the DLCI values and when the frames are received the DLCI value will not be the same as it was when the frame entered the frame-relay network, hence the value of a DLCI is of a local significance only. Thus each Virtual Circuit between two DTE devices will have at least two DLCI. This type of addressing is known as local addressing

Frame-Relay Address Mapping

As mentioned previously frame-relay is non-broadcast multi-access media, which means that there must be a mechanism for Layer 3 to Layer 2 resolution. Consider figure 2, a company lets say ABC has purchased frame-relay WAN service from a provider to connect three of its offices. It will be using IP network 192.168.1.0/24 on the WAN.
Table 2 shows the WAN IP Address and Interface for each site

If Router A needs to communicate to Router B, it needs to map the IP Address of Router B to the local DLCI assigned by the Provider for the VC between A and B. Layer 3 to Layer 2 mapping can be done either manually or dynamically through the use of Inverse ARP.

Frame-Relay Interface

Apart from using the physical interface for Frame-Relay, Cisco IOS includes a feature called logical sub-interfaces.
An interface can be point-to-point which terminates a single VC or multipoint which can terminates more than one VC.
The Physical interface is a multipoint interface which can terminate multiple VCs. By using these features we can create various topologies such as Hub-and-Spoke, Full-Mesh or Partial Mesh.

By default LMI will associate all DLCIs to the physical interface. To associate an interface or a sub-interface with different DLCIs, frame-relay map or frame-relay interface-dlci commands are used.

Frame-Relay Configuration

We will use Figure 2 and Table 2 in this section for reference.

We will utilize the physical interface for Frame-Relay configuration and as previously mentioned the physical interface is a multipoint interface which can terminate more than one VC.

RouterA(config)# Interface Serial 0/0
RouterA(config-if)# ip address 192.168.1.1 255.255.255.0
RouterA(config-if)# encapsulation frame-relay