Frame Relay Tutorial

In the last part we will mainly learn about LMI, which is the signaling protocol of Frame Relay

LMI

Local Management Interface (LMI) is a signaling standard protocol used between your router (DTE) and the first Frame Relay switch. The LMI is responsible for managing the connection and maintaining the status of your PVC.

LMI includes:
+ A keepalive mechanism, which verifies that data is flowing
+ A multicast mechanism, which provides the network server (router) with its local DLCI.
+ A status mechanism, which provides PVC statuses on the DLCIs known to the switch

In our example, when HeadQuarter is configured with Frame Relay, it sends an LMI Status Inquiry message to the DCE. The response from the DCE might be a small Hello message or a full status report about the PVCs in use containing details of all the VCs configured (DLCI 23 & 51). By default, LMI messages are sent out every 10 seconds.

The four possible PVC states are as follows:
+ Active state: Indicates that the connection is active and that routers can exchange data.
+ Inactive state: Indicates that the local connection to the Frame Relay switch is working, but the remote router connection to the Frame Relay switch is not working.
+ Deleted state: Indicates that no LMI is being received from the Frame Relay switch, or that there is no service between the customer router and Frame Relay switch.
+ Static state: the Local Management Interface (LMI) mechanism on the interface is disabled (by using the “no keepalive” command). This status is rarely seen so it is ignored in some books.

We can use the “show frame-relay lmi” to display LMI statistics of Frame Relay on enabled interfaces of the router. The output shows the LMI type used by the Frame Relay interface and the counters for the LMI status exchange sequence, including errors such as LMI timeouts.

Cisco routers support the following three LMI types:
* Cisco: LMI type de?ned jointly by Cisco, StrataCom, Northern Telecom (Nortel), and Digital Equipment Corporation
* ANSI: ANSI T1.617 Annex D
* Q.933A: ITU-T Q.933 Annex A

Notice that three types of LMI are not compatible with each others so the LMI type must match between the provider Frame Relay switch and the customer DTE device.

From Cisco IOS Release 11.2, the router attempts to automatically detect the type of LMI used by the provider switch.

Note: LMI is required for Inverse ARP to function because it needs to know that the PVC is up before sending out Inverse ARP Request.

Now you learn most of Frame Relay mentioned in CCNA, some other Frame Relay’s characteristics you should know are mentioned below.

Other Frame Relay characteristics

+ Frame Relay provides no error recovery mechanism. It only provides CRC error detection.
+ Unlike with LANs, you cannot send a data link layer broadcast over Frame Relay. Therefore, Frame Relay networks are called nonbroadcast multiaccess (NBMA) networks.
+ Depending on the bandwidth needed for each virtual connection, the customer can order a circuit with a guaranteed amount of bandwidth. This amount is the Committed Information Rate (CIR). CIR defines how much bandwidth the customer is “guaranteed” during normal network operation. Any data transmitted above this purchased rate (CIR) is available for discard by the network if the network doesn’t have available bandwidth.
+ If the Frame relay switch begins to experience congestion, it sends the upstream site (to the source) a Backward explicit congestion notification (BECN) and the downstream site (to the destination) a Forward explicit congestion notification (FECN).

+ There are two Frame Relay encapsulation types: the Cisco encapsulation and the IETF Frame Relay encapsulation, which is in conformance with RFC 1490 and RFC 2427. The former is often used to connect two Cisco routers while the latter is used to connect a Cisco router to a non-Cisco router.

+ Frame Relay does not define the way the data is transmitted within the service provider’s network once the traffic reaches the provider’s switch. So the providers can use Frame Relay, ATM or PPP… inside their networks.

Layer 2 Encapsulation Protocols

Besides Frame Relay there are other Layer 2 Encapsulation Protocols that you can implement instead:

High-Level Data Link Control (HDLC): The default encapsulation type for Cisco routers on point-to-point dedicated links and circuit-switched connections. HDLC is a Cisco proprietary protocol.
Point-to-Point Protocol (PPP): Provides connections between devices over several types of physical interfaces, such as asynchronous serial, High-Speed Serial Interface (HSS1), ISDN, and synchronous. PPP works with many network layer protocols, including IP and IPX. PPP can use either Password Authentication Protocol (PAP) or Challenge Handshake Authentication Protocol (CHAP) for authentication.
X.25/Link Access Procedure, Balanced (LAPB): Defines connections between DTE and DCE for remote terminal access. LAPB is a data link layer protocol specified by X.25.
Asynchronous Transfer Mode (ATM): International standard for cell relay using fixed-length (53-byte) cells for multiple service types. Fixed-length cells allow hardware processing, which greatly reduces transit delays. ATM takes advantage of high-speed transmission media such as E3, T3, and Synchronous Optical Network (SONET).

If you want to learn how to configure Frame Relay in GNS3, please read my Frame Relay Lab in GNS3 tutorial.