To prepare you for this portion of the CCNA exam, this section demonstrates how to configure a Cisco router to be used in a Frame Relay network. It explains the steps for configuring a router to use the Frame Relay encapsulation, assigning a DLCI number, and choosing a Local Management Interface
(LMI) type. Additionally, you will learn how to create a subinterface and why you should use one. You’ll finish the configuration by creating a Frame Relay mapping.

Critical Information

The next few sections jump right into configuring Frame Relay’s LMIs, maps, and the router’s sub interfaces.

Necessary Procedures

It’s important to learn the commands and procedures for configuring a router for use on a Frame Relay network. Frame Relay is used in many internetworks, and Cisco requires you to understand how to configure it if you are going to be a CCNA.

When configuring Frame Relay on Cisco routers, the first thing you do is specify an encapsulation type on serial interfaces. The encapsulation type is used ensure that both routers communicating are speaking the same language; a mismatched encapsulation type would make it difficult for the routers to communicate. There are only two encapsulation types: Cisco and IETF (Internet Engineering Task Force). Use these commands:

RouterA(config)#int s0
RouterA(config-if)#encapsulation frame-relay ?
ietf Use RFC1490 encapsulation

Cisco is the default encapsulation unless you type ietf. Use Cisco encapsulation when you are connecting two Cisco devices. Use IETF encapsulation when you are connecting a Cisco device to a non-Cisco device using Frame Relay.

Data Link Connection Identifiers (DLCIs)

As mentioned earlier, Frame Relay virtual circuits are identified by Data Link Connection Identifiers (DLCIs). Because many virtual circuits can be terminated on a multipoint Frame Relay interface, many DLCIs are affiliated with one interface. For the IP devices at each end of a virtual circuit to communicate, their IP addresses are mapped to DLCIs. This is so that a multipoint device can point out the appropriate destination virtual circuit on the Frame Relay network to each packet sent over the single physical interface.

Each DLCI can have a local meaning. In other words, two DTE devices connected via a virtual circuit use different DLCI values when referring to the same connection. Here is an example that shows how to configure a DLCI number to an interface:

RouterA(config-if)#frame-relay interface-dlci ?
<16-1007> Define a DLCI as part of the current Æ’subinterface
RouterA(config-if)#frame-relay interface-dlci 16

Local Management Interface (LMI)

The protocol produced by this group became known as the Gang-of-Four LMI, or Cisco LMI. This ¡°gang took the basic Frame Relay protocol and added extensions to its features that allowed internetworking devices to communicate easily with a Frame Relay network. LMI is used only between the router and the service provider’s switch.

LMI messages provide information about the current DLCI values, the global or local significance of the DLCI values, and the status of virtual circuits. You will need to check with your Frame Relay provider to find out which LMI type to use. The default type is Cisco, but you may need to change to ANSI or Q.933A. You can display the three LMI types on your screen using the command shown here:

RouterA(config-if)#frame-relay lmi-type ?
cisco
ansi
q933a
All standard LMI signaling formats are supported by the following:
ANSI Annex D, defined by ANSI standard T1.617.
ITU-T (Q.933A) Annex A, defined by ITU-T Recommendation Q.933A.
Cisco LMI, defined by the ¡°Gang of Four (default).

Note With Cisco IOS version 11.2 and above, the LMI type is set to auto-detect by default.

Subinterfaces

You can have multiple virtual circuits on a single serial interface and treat each virtual circuit as a separate interface, called a subinterface.

Define subinterfaces with the command interface s0.subinterfacenumber, as shown in this example:

RouterA(config)#int s0.?
<0-4294967295> Serial interface number
RouterA(config)#int s0.16 ?

multipoint Treat as a multipoint link
point-to-point Treat as a point-to-point link

In the above example, we chose to use subinterface 16 because that is the DLCI number of that interface. You can choose any number between 0 and 4294967295

Multipoint subinterfaces are used when the router is at the center of a star of virtual circuits.

Mapping Frame Relay

As explained earlier, in order for IP devices at the ends of virtual circuits to communicate, their addresses must be mapped to the DLCIs. There are two ways to make this mapping happen:
Use the frame-relay map command.
Use the inverse-arp function.
For each packet sent out of a physical interface, mappings allow a multipoint device to identify a virtual circuit on the Frame Relay network.

This is an example program that uses the frame-relay map command:

RouterA(config)#int s0.16 point-to-point
RouterA(config-if)#encap frame-relay ietf
RouterA(config-if)#no inverse-arp
RouterA(config-if)#ip address 172.16.30.1 255.255.255.0
RouterA(config-if)#frame-relay map ip 172.16.30.17 20 Æ’cisco broadcast
RouterA(config-if)#frame-relay map ip 172.16.30.18 30 Æ’broadcast
RouterA(config-if)#frame-relay map ip 172.16.30.19 40

Here’s what we did: First, we chose our subinterface and set the encapsulation to IETF. Then we turned off inverse-arp (IARP) and mapped three virtual circuits and their corresponding DLCI numbers. (IARP would map our DLCIs to IP addresses dynamically, as demonstrated below.) Notice that we specified Cisco encapsulation on the first virtual circuit. The other two virtual circuits will use the encapsulation type specified in the interface command (IETF). The frame-relay map command is the only way to mix both Cisco and IETF encapsulation types. The broadcast keyword at the end of the map command tells the router to forward broadcasts for this interface to this specific virtual circuit. Instead of putting in map commands for each virtual circuit, you can use the inverse-arp (IARP) function to perform dynamic mapping of the IP address to the DLCI number. In that case, your configuration program would look like this:

RouterA(config)#int s0.16 multipoint
RouterA(config-if)#encap frame-relay ietf
RouterA(config-if)#ip address 172.16.30.1 255.255.255.0

Yes, this is a whole lot easier, but it’s not as stable as using the map command.

Note Frame Relay mapping isn’t something an administrator would typically do. This process is usually performed at the switching office. Check with your provider before doing any Frame Relay configurations.

Related posts:

1. Frame Relay LMIs, Maps, and Subinterfaces
2. Commands to Monitor Frame Relay Operation in the Router
3. key Frame Relay Terms and Features
4. Monitor Frame Relay Operation in the Router
5. key Frame Relay Terms and Features Exam Points
6. Configure IP Addresses
7. Configure Standard Access Lists to Figure IP Traffic Lab
8. Control Router Passwords, Identification and Banner
9. Configure IP addresses Exams Points
10. Initial Configuration in Router Using the Setup Command