Configure Frame Relay on a Cisco Router

Contributed by abanks February 10, 2004  
Tagged: Cisco router

Frame relay is an attractive choice for wide-area network (WAN) connections. It offers simple configuration, and allows one telco circuit to connect many locations.

First, some terminology:

DLCI - Data Link Connection Identifier - A number which directs traffic to different destinations

CIR - Committed Information Rate - Packets transmitted in excess of this rate will be marked as DE (Discard Eligible)

BECN - Backwards Explicit Congestion Notification - Notifies devices of congestion in the network

FECN - Forward Explicti Congestion Notification - Notifies devices of congestion in the network

DE - Discard Eligible - These frames may be dropped when congestion occurs.

NOTE: An unconfigured router is assumed, with serial 0/0 and FastEthernet 0/0 interfaces available. This end of the frame relay connection will use DLCI 100. The 192.168.0.4/30 network will be used on the WAN link, 10.1.1.0/24 on the Ethernet, and 10.2.2.0/24 on the remote side.

Connect to the router’s console port, enter the appropriate passwords, then:

config t
Short for configure terminal, allows entry of configuration commands.

interface FastEthernet0/0
Enters into interface configuration mode. The prompt should change to “router#(config-if)”

ip address 10.1.1.1 255.255.255.0
Configures the FastEthernet port to use address 10.1.1.1. Machines on this subnet would use this address as the gateway to reach 10.2.2.0.

no shutdown
Enables the port. By default, ports are administratively down.

interface serial 0/0
Change to interface configuration mode for the serial interface.

no shutdown
Enables the port. By default, ports are administratively down.

encapsulation frame-relay
Tells the router to transmit frame relay on this interface, instead of serial (HDLC)

Now, there is a choice. The most common method is subinterfaces:

interface serial 0/0.100 point-to-point
Creates the subinterface. Note the 100 can be any number, but using the DLCI number makes troubleshooting easier.

ip address 192.168.0.5 255.255.255.252
Assigns the IP address to the subinterface.

frame-relay interface-dlci 100
Instructs the router to use DLCI 100 for traffic going out this interface and route inbound traffic from DLCI 100 through this interface.

All done! Now, the other method, frame mapping:

frame-relay map ip 192.168.0.6 100 broadcast
Associates the destination IP address with a DLCI.

Although the configuration is longer, subinterfaces are easier to troubleshoot. If a DLCI fails, the subinterface will appear down in the show ip interfaces brief output. Other useful commands include show frame-relay pvc and show frame lmi