Frame Relay Configuration

Having looked at both Frame Relay concepts and an example of the communication process, let’s take a look at configuring a Cisco router to use Frame Relay. Our network in this example is configured as illustrated in the figure below. Notice that the PVC between Chicago and Toronto is a point-to-point interface, while the PVCs between Chicago and San Jose, as well as Chicago and Miami are part of a multipoint configuration. Your goal is to properly configure Frame Relay on the Chicago router.

Figure: Frame Relay network used in configuration example.

You’ll begin by configuring settings on the Chicago router’s S0 interface. This includes the encapsulation method to be used, as well as the LMI type. After these steps are complete, you will configure two subinterfaces, one for DLCI 88 (the PVC to Toronto), and one other for DLCIs 92 and 107 (the PVCs to Miami and San Jose respectively).

Chicago#config t
Enter configuration commands, one per line. End with CNTL/Z.
Chicago(config)#int s0
Chicago(config-if)#encapsulation ?
atm-dxi ATM-DXI encapsulation
frame-relay Frame Relay networks
hdlc Serial HDLC synchronous
lapb LAPB (X.25 Level 2)
ppp Point-to-Point protocol
smds Switched Megabit Data Service (SMDS)
x25 X.25
Chicago(config-if)#encapsulation frame-relay ?
ietf Use RFC1490/RFC2427 encapsulation
Chicago(config-if)#encapsulation frame-relay
Chicago(config-if)#frame-relay lmi-type ?
Chicago(config-if)#frame-relay lmi-type cisco

By default, the encapsulation method used on a Frame Relay interface will be Cisco, unless IETF is specified. Recall that it isn’t usually necessary to specify an LMI type, since the interfaces will auto-sense the LMI type in use by the local Frame Relay switch as of IOS version 11.2.

With the basic interface settings completed, you can move on to the configuration of the subinterfaces. You’ll begin with the subinterface that will be used to connect to the Toronto office, S0.88. This link represents a point-to-point connection, as shown below.

Chicago(config)#int s0.88 ?
multipoint Treat as a multipoint link
point-to-point Treat as a point-to-point link
Chicago(config)#int s0.88 point-to-point

Your next step involves configuring an IP address on subinterface S0.88, as well as its DLCI number.

Chicago(config-subif)#ip address
Chicago(config-subif)#frame-relay interface-dlci 88

At this point, configuration of a point-to-point Frame Relay interface on the Chicago router is complete. Of course, communication will not be possible until the Toronto router is also configured, and the no shutdown command is issued on interface S0. DLCI to IP address mappings are not configured on point-to-point Frame Relay interfaces, since only one other possible communication endpoint exists on the virtual circuit.

The configuration of multipoint interfaces is slightly different, and is the reason why I included both types in this example. The encapsulation method and LMI type have already been specified on interface S0, so there is no need to issue them again (remember that subinterfaces inherit settings from their “parent” interface). Because the connection between Chicago, Miami, and San Jose is multipoint (a single subnet), you can configure both virtual circuits on a single subinterface. You’ll use the DLCI number for the PVC to Miami as our subinterface number, if for no other reason than the fact that it’s the lower of the two.

Chicago(config)#int s0.92 ?
multipoint Treat as a multipoint link
point-to-point Treat as a point-to-point link
Chicago(config)#int s0.92 multipoint
Chicago(config-subif)#ip address
Chicago(config-subif)#frame-relay interface-dlci 92
Chicago(config-subif)#frame-relay interface-dlci 107

Communication over a multipoint interface requires that you somehow map remote IP addresses to the local DLCI number of the corresponding virtual circuit. By default, a multipoint interface will automatically map this pairing of information using inverse ARP. However, it is also possible (and generally recommended) to create static mappings. This is accomplished using the frame-relay map command, as outlined below.

Chicago(config-subif)#frame-relay map ip 92 broadcast
Chicago(config-subif)#frame-relay map ip 107 broadcast

If you take a closer look at the commands above, you’ll notice that they create a mapping between the IP address of the remote router and the local DLCI number of the appropriate virtual circuit. When the Chicago router needs to reach address (the Miami office), it now knows that frames must be assigned to the virtual circuit identified by DLCI 92. When traffic needs to reach address (the San Jose office), the virtual circuit identified by DLCI 107 should be used. You may have also noticed the use of the broadcast keyword at the end of the frame-relay map commands. The broadcast keyword tells the router to forward broadcasts (like routing protocol updates) across the PVC as required. This is necessary because by default, Frame Relay networks are non-broadcast multi-access (NBMA). As such, they would not forward routing table updates from broadcast-based protocols like RIP or IGRP without the broadcast keyword configured. For practical purposes, it’s worth noting that broadcasts will be forwarded across point-to-point Frame Relay interfaces by default as of IOS 11.1

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.