Chapter 6- Semester4 Carl Marandola CCRI

What is Frame Relay? Frame Relay is an industry-standard, switched data link-layer protocol that handles multiple virtual circuits using High-Level Data Link Control (HDLC) encapsulation between connected devices. Frame Relay uses virtual circuits to make connections through a connection-oriented service.

Frame Relay Features CCITT and ANSI standard that defines a process for sending data over a public data network (PDN). high performance, efficient data technology used in networks throughout the world. Sends information over a WAN by dividing data into packets. Operates at the physical and data link layers of the OSI reference model. Relies on upper-layer protocols such as TCP for error correction. Frame Relay interface can be either a carrier-provided public network or a network of privately owned equipment, serving a single enterprise.

Terms used in Frame Relay Access rate -- The clock speed (port speed) of the connection (local loop) to the Frame Relay cloud. It is the rate at which data travels into or out of the network. Data-link connection identifier (DLCI) – A DLCI is a number that identifies the end point in a Frame Relay network. This number has significance only to the local network. The Frame Relay switch maps the DLCIs between a pair of routers to create a permanent virtual circuit.

Terms used in Frame Relay- Continue Local management interface (LMI) -- A signaling standard between the customer premises equipment (CPE) device and the Frame Relay switch that is responsible for managing the connection and maintaining status between the devices. LMIs can include support for a keepalive mechanism, which verifies that data is flowing. a multicast mechanism, which can provide the network server with its local DLCI. multicast addressing, providing a few DLCIs to be used as multicast addresses and the ability to give DLCIs global (whole Frame Relay network) significance, rather than just local significance (DLCIs used only to the local switch); a status mechanism, which provides an ongoing status on the DLCIs known to the switch. There are several LMI types, and routers need to be told which LMI type is being used. Three types of LMIs are supported: cisco, ansi, and q933a.

Terms used in Frame Relay- Continue Committed information rate (CIR) -- The CIR is the guaranteed rate, in bits per second, that the service provider commits to providing. Committed burst -- The maximum number of bits that the switch agrees to transfer during a time interval. (It is noted Bc) Excess burst -- The maximum number of uncommitted bits that the Frame Relay switch attempts to transfer beyond the CIR. Excess burst is dependent on the service offerings available by the vendor, but is typically limited to the port speed of the local access loop. Forward explicit congestion notification (FECN) -- A bit set in a frame that notifies a DTE that congestion avoidance procedures should be initiated by the receiving device. When a Frame Relay switch recognizes congestion in the network, it sends a FECN packet to the destination device, indicating that congestion has occurred.

Terms used in Frame Relay- Continue Backward explicit congestion notification (BECN) -- A bit set in a frame that notifies a DTE that congestion avoidance procedures should be initiated by the receiving device. As shown in Figure when a Frame Relay switch recognizes congestion in the network, it sends a BECN packet to the source router, instructing the router to reduce the rate at which it is sending packets. If the router receives any BECNs during the current time interval, it decreases the transmit rate by 25%.

Terms used in Frame Relay- Continue Forward explicit congestion notification (FECN) -- A bit set in a frame that notifies a DTE that congestion avoidance procedures should be initiated by the receiving device. When a Frame Relay switch recognizes congestion in the network, it sends a FECN packet to the destination device, indicating that congestion has occurred. Backward explicit congestion notification (BECN) -- A bit set in a frame that notifies a DTE that congestion avoidance procedures should be initiated by the receiving device. when a Frame Relay switch recognizes congestion in the network, it sends a BECN packet to the source router, instructing the router to reduce the rate at which it is sending packets. If the router receives any BECNs during the current time interval, it decreases the transmit rate by 25%.

Terms used in Frame Relay- Continue Discard eligibility (DE) indicator -- A set bit that indicates the frame may be discarded in preference to other frames if congestion occurs. When the router detects network congestion, the Frame Relay switch will drop packets with the DE bit set first. The DE bit is set on the oversubscribed traffic (that is, the traffic that was received after the CIR was met).

Frame Relay operation You deploy a public Frame Relay service by putting Frame Relay switching equipment in the central office of a telecommunications carrier. In this case, users get economic benefits from traffic-sensitive charging rates, and don't have to spend the time and effort to administer and maintain the network equipment and service. The lines that connect user devices to the network equipment can operate at a speed selected from a broad range of data rates. Speeds between 56 kbps and 2 Mbps are typical, although Frame Relay can support lower and higher speeds

Frame Relay DLCI Frame Relay provides a means for multiplexing many logical data conversations, referred to as virtual circuits, through a shared physical medium by assigning DLCIs to each DTE/DCE pair of devices. Frame Relay's multiplexing provides more flexible and efficient use of available bandwidth. Therefore, Frame Relay allows users to share bandwidth at a reduced cost.

Frame Relay DLCI Frame Relay standards address permanent virtual circuits (PVCs) that are administratively configured and managed in a Frame Relay network. Frame Relay PVCs are identified by DLCIs The service provider's switching equipment constructs a table mapping DLCI values to outbound ports. When a frame is received, the switching device analyzes the connection identifier and delivers the frame to the associated outbound port. The complete path to the destination is established before the first frame is sent.

Frame Relay Frame Format The Frame Relay frame format is shown in the Figure. The flag fields indicate the beginning and end of the frame. Following the leading flag field are 2 bytes of address information. 10 bits of these 2 bytes make up the actual circuit ID (that is, the DLCI). Congestion Control -- The last 3 bits in the address field, which control the Frame Relay congestion notification mechanisms. These are the FECN, BECN, and discard eligible (DE) bits.

Frame Relay addressing DLCI address space is limited to 10 bits. This creates a possible 1024 DLCI addresses. The usable portion of these addresses are determined by the LMI type used. The remaining DLCI addresses are reserved for vendor implementation. This includes LMI messages and multicast addresses. Examples: The Cisco LMI type supports a range of DLCI addresses from DLCI 16-1007 for carrying user-data. The ANSI/ITU LMI type supports the range of addresses from DLCI 16-992 for carrying user-data.

LMI operation & extensions In addition to the basic Frame Relay protocol functions for transferring data, the Frame Relay specification includes LMI extensions that make supporting large, complex internetworks easier. A summary of the LMI extensions follows: Virtual circuit status messages (common) -- Provide communication and synchronization between the network and the user device, periodically reporting the existence of new PVCs and the deletion of already existing PVCs, and providing general information about PVC integrity.

LMI extensions -Continue Multicasting (optional) -- Allows a sender to transmit a single frame but have it delivered by the network to multiple recipients. Global addressing (optional) -- Gives connection identifiers global rather than local significance, allowing them to be used to identify a specific interface to the Frame Relay network. Global addressing makes the Frame Relay network resemble a local-area network (LAN) in terms of addressing; Simple flow control (optional) -- Provides for an XON/XOFF flow control mechanism that applies to the entire Frame Relay interface. It is intended for devices whose higher layers cannot use the congestion notification bits and that need some level of flow control

LMI frame Format The Frame Relay specification includes the LMI procedures. LMI messages are sent in frames distinguished by an LMI-specific DLCI (defined in the consortium specification as DLCI = 1023).

LMI Feature- Global addressing In normal Frame Relay there are no addresses that identify network interfaces, or nodes attached to these interfaces. Because these addresses do not exist, they cannot be discovered by traditional address resolution and discovery techniques. So, static maps must be created to tell routers which DLCIs to use to find a remote device and its associated internetwork address. Global addressing provides significant benefits in a large, complex network. The Frame Relay network now appears to the routers on its periphery like any LAN.

LMI Features- Multicasting Multicast groups are designated by a series of four reserved DLCI values (1019 to 1022). Frames sent by a device using one of these reserved DLCIs are replicated by the network and sent to all exit points in the designated set. The multicasting extension also defines LMI messages that notify user devices of the addition, deletion, and presence of multicast groups.

LMI Features- Inverse ARP The Inverse ARP mechanism allows the router to automatically build the Frame Relay map, as shown in the Figure. The router learns the DLCIs that are in use from the switch during the initial LMI exchange. The router then sends an Inverse ARP request to each DLCI for each protocol configured on the interface if the protocol is supported. The return information from the Inverse ARP is then used to build the Frame Relay map.

LMI Features- Frame Relay mapping The router next-hop address determined from the routing table must be resolved to a Frame Relay DLCI, as shown in the Figure. The resolution is done through a data structure called a Frame Relay map. The routing table is then used to supply the next-hop protocol address or the DLCI for outgoing traffic. This data structure can be statically configured in the router, or the Inverse ARP feature can be used for automatic setup of the map.

LMI Features- Frame Relay switching table The router next-hop address determined from the routing table must be resolved to a Frame Relay DLCI, as shown in the Figure. The resolution is done through a data structure called a Frame Relay map. The routing table is then used to supply the next-hop protocol address or the DLCI for outgoing traffic. This data structure can be statically configured in the router, or the Inverse ARP feature can be used for automatic setup of the map.

LMI Features- Frame Relay Switching The Frame Relay switching table consists of four entries: two for incoming port and DLCI, and two for outgoing port and DLCI, as shown in the Figure. The DLCI could, therefore, be remapped as it passes through each switch; the fact that the port reference can be changed is why the DLCI does not change even though the port reference might change.

Frame Relay Subinterfaces Subinterfaces are logical subdivisions of a physical interface. By logically dividing a single physical WAN serial interface into multiple virtual subinterfaces, the overall cost of implementing a Frame Relay network can be reduced.

Spilt Horizon Use in Frame Relay if a remote router sends an update to the headquarters router that is connecting multiple PVCs over a single physical interface, the headquarters router cannot advertise that route through the same physical interface to other remote routers

Subinterfaces You can configure subinterfaces to support the following connection types: Point-to-point -- A single subinterface is used to establish one PVC connection to another physical interface or subinterface on a remote router. Multipoint -- A single subinterface is used to establish multiple PVC connections to multiple physical interfaces or subinterfaces on remote routers.

Writing the IOS command sequence to completely configure Frame Relay Select the interface and go into interface configuration mode Router(config) # interface serial 0 Configure a network-layer address, for examlple, an IP address Router(config-if) # ip address 212.14.249.197 255.255.255.0 Select the encapsulation type used for data traffic end-to-end Router(config-if) # encapsulation frame-relay [cisco|ietf] For Cisco IOS 11.1 or earlier specify the LMI type used in the Frame Relay Switch, Router(config-if) # frame-relay lmi-type {ansi|cisco|q933i} for later versions of the Cisco IOS LMI type is autosensed Configure the bandwidth of the link Router(config-if) # bandwidth kilobits If the inverse ARP was disabled on the router, re-enable it Router(config-if) # frame-relay inverse-arp [protocol] [dlci]

Configuring subinterfaces Select the interface Remove any network layer address assigned to the physical interface Configure Frame Relay encapsulation as stated perviusly Select the subinterface you want to configure Router(config-if)# interface serial number.subinterface-number {multipoint|point-to-point} Interface number 1-4294967293 Multipoint is used when you want the router to forward broadcasts and routing updates it receives Point-to-point is used when you do not want the router to forward broadcasts and routing updates it receives. Configure the network layer address on the subinterface If you configured the subinterface as multipoint or point-to-point you must configure a local DLCI Router(config-if)# frame-relay interface-dlci dlci-number

Router(config-if)# frame-relay map protocol protocol address dlci [broadcast] [ietf|cisco|payload-compress packet-by-packet]

GOOD LUCK Carl M