Frame Relay Basics

Frame Relay is an OSI Layer 2 Wide Area Network Protocol. It is commonly implemented as a WAN access protocol on the link between the customer and the provider edge. It also can be implemented as core protocol.
Frame Relay permits multiple Permanent Virtual Circuits (PVC) to use the same physical link, providing a kind of Layer 2 VPN between customer locations. These PVCs can be flexibly designed as Full Mesh or Hub and Spoke depending on traffic patterns and budgets.

Frames are relayed using the Data Link Connection Identifier (DLCI). The DLCI uniquely identifies traffic for a given PVC on a given physical channel, it is a 10 bit number that can vary between 0 and 1023. I will use the diagram below to explain the usage of DLCI.

If HQ want to send packets destinate to Branch A, HQ's router need to know which DLCI to use for packet with next hop of 123.3. This mapping of far side Layer 3 address to near side Layer 2 address can be created statically or dynamically using inverse arp.
In mapping table the router determine the traffic with the next hop of 123.2 should go out on the PVC designated by DLCI 102. It encapsulates the packet in the frame and search the DLCI of 102 address in the frame header, and send it out the interface to Switch1. The Switch1 replaces the DLCI of 102 in the frame header with DLCI 112 and sends it on Switch2. Switch2 rewrites the header with DLCI 201 and sends the frame on to the Branch A router. The same process takes places from packets from HQ destinated to Branch B.

In the diagram, you see the Headquarters and Branches connected over Frame Relay. The Frame Relay Service Provider cloud consists of 2 SW1 and SW2 which are Cisco Routers that configured for Frame Relay Switching. Notice that the customer router connected to Frame Relay cloud and shared the same subnet 172.16.123.x.

Frame Relay interfaces in Cisco can be configured as:

• DTE (Data Terminal Equipment), Serial interfaces at R1, R2, and R3 as routed interfaces
• DCE (Data Communications Equipment), Serial 1/0 at Switch 1, Serial 0 and Serial 2 at Switch S2
• NNI (Network-to-Network Interface), Serial 0/0 at Switch 1, Serial 1 at Switch S2

Configuration of DTE interfaces in R1:

interface serial 1/0
ip address 172.16.123.1 255.255.255.0
clock rate 128000
no dce-terminal-timing-enable

Configuration of DCE interfaces in SW1:

interface serial 1/0
no ip address
encapsulation frame-relay
frame-relay intf-type dce
frame-relay route 102 interface serial 0/0 112
frame-relay route 103 interface serial 0/0 113

Configuration of NNI interfaces in SW1:

interface serial 0/0
no ip address
encapsulation frame-relay
no fair-queue
frame-relay intf-type nni
frame-relay route 112 interface serial 0/0 102
frame-relay route 113 interface serial 0/0 103