Wan technologies (such as Frame Relay) typically define the Physical and Data Link layer connections. Frame Relay is both a Data Link layer encapsulation type implemented on the router and a Physical service provided by a telecommunications company. Frame Relay is a packet switching and encapsulation technology that functions at the Physical and Data Link layers of the OSI reference model. Frame Relay is a communications technique for sending data over high-speed digital connections operating anywhere form 56 kbps to 44.736 Mbps or higher.
Frame Relay is defined as a connection between the data terminal equipment (DTE) and the data communications equipment (DCE). DCE is switching equipment, supplied by a telecommunications provider, that services as a connection to the public data network (PDN). DTE is also known as customer premises equipment (CPE), because it is the equipment that belongs to, and is maintained by, the PDN customer. One advantage of a public Frame Relay Service is that the network equipment is managed by the service provider.
Virtual Circuits
You can use Frame Relay with nearly any serial interface. It operates by multiplexing, which means that it combines multiple data streams onto one physical link. Frame Relay separates each data stream into logical (software-maintained) connections called virtual circuits, which carry the data transferred on the connection. Two types of virtual circuits, switched virtual circuits (SVC) and permanent virtual circuits (PVC), connect Frame Relay ports.
DLCI
Frame Relay connections identify virtual circuits by Data Link Connection Identifier (DLCI) numbers. The DLCI (pronounced dell-see) numbers map virtual circuits to layer 3 protocol addresses. For example, a DLCI number associates an IP address with a specific virtual circuit. DLCI numbers are not unique identifiers on the network; instead, they have only local significance, which means they are important only to the local router and Frame Relay switch. Frame Relay uses DLCIs to identify logical connections.
Frame Relay Map
DLCI numbers are mapped, or assigned, to a specific interface. Each router that supports Frame Relay will have a Frame Relay Map, which is a table that defines the specific interface to which a specific DLCI number is mapped. The definition will contain a DLCI number and an interface identifier. The router uses the Frame Relay map to determine the DLCI of the next hop.
Incoming/outgoing ports and DLCIs are found in a Frame Relay switching table.
Basic (not extended) Frame Relay supports only PVCs with local significance.
Subinterfaces
With current technology, a single router serial interface can now service multiple PVCs through, a single physical serial interface. In order to allow a single serial interface to support multiple PVCs, the IOS divides the interface into logical subinterfaces. By dividing a single physical interface into several logical subinterfaces, the cost of implementing multiple Frame relay virtual circuits is reduced because only one port is required on the router. Also, the network administrator has to configure and maintain fewer physical connections.
LMI
Frame Relay engineers designed Local Management Interface (LMI) to exchange information about PVC status and to ensure that the link between two points was operating correctly. LMI is a standard signaling mechanism used to manage connections between the CPE (usually a router) and the Frame Relay connection.
One LMI purpose is to determine the operational status of PVCs.
Using Cisco IOS 11.2 or later the LMI type set is set by autosensing.
LMI uses keepalive packets to verify the Frame Relay link and to ensure the flow of data. The Frame Relay switch in turn provides to the frame Relay connectivity device the status of all virtual circuits that the device can utilize. Common LMI extensions are used to determine the status of virtual circuits. Each virtual circuit, represented by its DLCI number, can have one of three connection states:
· Active: The connection is working and routers can sue it to exchange data.
· Inactive: the connection from the local router to the switch is working, but he connection to the remote router is not available.
· Deleted: No LMI information is being received from the Frame Relay switch; this can indicate that the connection between the CPE and DCE is not functional.
The Frame Relay switch reports this status information to the Frame Relay map on the local router. The status information is used by the Frame Relay connectivity devices to determine whether data can be transmitted over the configured virtual circuits.
The LMI Frame must include a byte that identifies the protocol.
Inverse ARP
A Frame Relay map includes DLCIs and their corresponding remote IP address. Routers use the protocol inverse ARP to send a query using the DLCI number to find an IP address. As other routers respond to the Inverse ARP queries, the local router can build its Frame Relay map automatically. In order to maintain the Frame Relay map, routers exchange Inverse ARP messages every 60 seconds, by default.
Routers use Inverse ARP reequests to build Frame Relay maps. The Frame Relay map is used to identify the next hop router address.
Encapsulation Types
LMI has several different protocol encapsulation types that it can use for management communications. Different Frame Relay switches, CPE, and Frame Relay connectivity equipment employ or support different types of LMI encapsulation. Cisco routers support these types of LMI encapsulation:
· cisco: This type allows for 992 virtual circuit addresses and used DLCI 1023 as a management circuit, which transfers link and DLCI status messages. This is the default Frame Relay encapsulation type used to encapsulate data end to end.
· anso: This type provides for 976 virtual circuit addresses and uses DLCI 0 as the management circuit.
· Q933a: This type is similar to ansi above and uses DLCI 0 as a management circuit.
Non Cisco routers use this type of Frame Relay encapsulation - IETF.
Cisco routers (using IOS Release 11.2 or later) can autosense the LMI type used by the Frame Relay switch.
Split Horizon
Split horizon is a routing technique that reduces the chance of routing loops on a network. A split horizon implementation prevents routing update information received on one physical interface from being rebroadcast to other devices through that same physical interface. Routing updates sent to a particular neighbor router should not contain information about routes that were learned from that neighbor. Frame Relay uses split Horizon to prevent routing loops.
Point-to-Point or Multipoint Connections
The network administrator can set each sub-interface as a point-to-point connection or a multipoint connection. Point-to-point connections allow you to divide a single serial interface into multiple sub-interfaces, each supporting a separate virtual connection. The network administrator must configure each sub-interface with its own subnet identifier in a point-to-point configuration. On a point-to-point Frame Relay connection, you need a separate sub-interface for each connection.
In a multipoint configuration, the network administrator can configure a single subinterface to support multiple connections to physical or logical interfaces on other routers. On a multipoint connection you only need one subnet to connect to several interfaces.
Performance Parameters
When organizations contract for Frame Relay services from a telecommunications provider the contract specifies parameters by which the connection is expected to function. Terms that appear in the contract may include:
· Access rate: or local access rate is the clock speed (port speed) or rate
at which data travels into or out of the network.
· Committed Information Rate (CIR): This is the minimum data transfer rate
that the Frame Relay switch aggress to transfer data. The service provider agrees to
always allow the customer to transfer information at not less than the transfer rate
specified by the CIR. This is a guaranteed rate.
· Committed Burst Size (CBS): The maximum amount of data bits that the service provider agrees to transfer in a set time period under normal conditions.
· Excess Burst Size (EBS): The amount of excess traffic (over the CBS) that the network will attempt to transfer during a set time period. The network can discard EBS data, if necessary.
· Oversubscription: When the sum of the data arriving over all virtual circuits exceeds the access rate, the situation is called oversubscription. This can occur when the CIR is exceeded by burst traffic from the virtual circuits. Oversubscrilption results in dropped packets. In such a case, the dropped packets must be retransmitted.
Congestion
Frame Relay switches attempt to control congestion on the network. When the Frame Relay switch recognizes congestion, it sends a forward explicit congestion notification (FECN) message to the destination router. This message tells the router that congestion was experienced on the virtual circuit. In addition, the switch sends a backward explicit congestion notification (BECN) message to the transmitting, or source, router. The router's reaction to the BECN should be to reduce the amount of traffic it is sending.
A network administrator can configure certain types of traffic at the router as discard eligible (DE). Thus, during times of congestion, the router can discard DE frames in order to provide a higher, more reliable, service to those frames that are not discard eligible.
Frame Format
Frame Relay devices can utilize different Frame Relay frame formats. Since this course focuses on Cisco devices, this section will focus on the proprietary Frame Relay frame format. The Frame Relay format has specific parts:
· Flag: An eight-bit binary sequence is used to indicate the beginning or ending of the frame.
· Address: Two or four bytes that contain several pieces of Frame Relay information.
· Ethertype: Identifies the type of higher-layer protocol being encapsulated (IP, IPX or AppleTalk); this data field is specific to the Cisco proprietary frame format.
· Data: A variable length field that contain the information from the higher layers encapsulated in the Frame Relay frame.
· FCS: Frame Check Sequence (FCS) is a mathematical computation placed at the end of the frame and is used to ensure that the frame was not corrupted during transmission.
· Flag: An eight-bit binary sequence that indicates the beginning or ending of the frame.
Frame Relay Configuration
(will be added later)
A few important topics to be expanded upon later
Frame Relay provides a connection oriented service.
Frame Relay uses TCP for error correction.
Frame Relay is a layer 2 (data link) protocol.
When configuring an interface for Frame Relay, bandwidth should be set in order to allow protocols to calculate the metric.
The router command that will display the Frame Relay tables is show frame-relay map.
When using Frame Relay the router command, show interfaces serial, will display both the DLCI and LMI information.
The Frame Relay command, show interface, will display whether a Frame Relay connection is sending and receiving data.
To set Frame Relay encapsulation use this command:
Router(config-if)#encapsulation frame-relay
The command used to verify frame Relay configurations is: show frame-relay pvc
To configure a subinterface that will forward bradcasts and routing updates
use this command:
Router(config-if)#interface serial 1.1 multipoint