What is FRAME RELAY Frame Relay is a

What is FRAME RELAY ? • Frame Relay is a way of sending information over a WAN by dividing data into packets • It operates at the Physical and Data Link layers of the OSI reference model • It relies on upper-layer protocols such as TCP for error correction • Frame Relay is a switched data link-layer protocol that handles multiple virtual circuits using (HDLC) encapsulation • Frame Relay interface can be either a carrier-provided public network or a network of privately owned equipment, serving a single enterprise

Benefits of FRAME RELAY Ø Reduced internetworking costs • Statistically multiplexed traffic from multiple sources over private backbone networks can reduce the number of circuits and corresponding cost of bandwidth • Lower Equipment Costs • Lower cost than dedicated leased lines Ø Increased performance & reduced network complexity • Reduces the amount of processing (as compared to X. 25) • Efficiently utilizing high speed digital transmission lines, frame relay can improve performance and response times of applications. Ø Increased interoperability via international standards • Frame relay can be implemented over existing technology • Access devices often require only software changes or simple hardware modifications to support the interface standard • Existing packet switching equipment and T 1/E 1 multiplexers often can be upgraded to support frame relay over existing backbone networks.

FRAME RELAY Overview Ø Packet Switched Ø Uses Virtual Circuits (Connection Oriented Service) Logical connection created between two (DTE) devices across a Frame Relay packet-switched network (PSN)

FRAME RELAY Technology Ø Access rate • The clock speed (port speed) of the connection (local loop) to the Frame Relay cloud Ø Data-link connection identifier (DLCI) • DLCI number identifies the end point in a Frame Relay network • Each Virtual Circuit is uniquely identified by a DLCI number • The Frame Relay switch maps the DLCIs between a pair of routers to create a permanent virtual circuit Ø Local management interface (LMI) • Signaling standard between the customer premises equipment (CPE) device and the Frame Relay switch. It includes: 1. A keepalive mechanism, which verifies that data is flowing 2. Multicast mechanism, which conserves bandwidth

FRAME RELAY Technology Ø DLCI Ø LMI

Frame Relay Addressing Frame Relay DLCIs have local significance The values themselves are not unique in the Frame Relay WAN Two DTE devices connected by a virtual circuit might use a different DLCI value to refer to the same connection

Functions of LMI (Local Management Interface) • Determines the operational status of the various PVCs that the router knows about • To transmit keepalive packets to ensure that the PVC stays up and does not shut down due to inactivity • Three LMI types can be invoked by the router: ansi, cisco, and q 933 a

LMI EXTENSIONS § Are set of enhancements to the basic Frame Relay specifications § It offers a number of features (called extensions) for managing complex internetworks § Key Frame Relay LMI extensions include: - Global addressing - Virtual-circuit status messages - Multicasting

LMI EXTENSIONS • Virtual circuit status messages (common) PVC Integrity and information about new and existing PVC. • 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; address resolution protocols therefore perform over Frame Relay exactly as they do over a LAN. • 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

FRAME RELAY Technology • 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. • Excess burst The maximum number of uncommitted bits that the Frame Relay switch attempts to transfer beyond the CIR. Excess burst is typically limited to the port speed of the local access loop (Your router’s connection to the Frame Relay Switch). • 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 (Anything over the CIR).

FRAME RELAY Congestion • 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%.

FRAME RELAY Congestion

FRAME RELAY Multiplexing Statistical Time Division Multiplexing (STDM) Multiplexes multiple virtual circuits, through a shared physical medium by assigning DLCIs to each DTE/DCE pair of devices

INVERSE ARP Allows the router to automatically build the Frame Relay map 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.

FRAME RELAY Mapping Frame relay maps (which bind next router hop IP addresses to DLCIs and work together with standard routing tables) can be statically configured, or can be dynamically created by the invocation of inverse ARP

FRAME RELAY Switching tables The Frame Relay switching table consists of four entries: • 2 for incoming port and DLCI • 2 for outgoing port and DLCI

Subinterfaces • A single physical interface can be split into multiple logical interfaces • Subinterfaces can resolve split horizon issues • Routing updates can be sent out subinterfaces as if they were separate physical interfaces • Overall cost of implementing a Frame Relay network can be reduced.

FRAME RELAY Implementation without subinterfaces Router (DTE device) have a WAN serial interface for every PVC

Basic FRAME RELAY Configuration BASIC FRAME RELAY CONFIGURATION ASSUMES THAT: • Configure Frame Relay on one or more physical interfaces • LMI and Inverse ARP are supported by the remote router(s)

Basic FRAME RELAY Configuration 1. Select the interface (S 0, S 1) & get into the interface configuration mode 2. Configure network layer address (IP address) 3. Configure the encapsulation type (cisco is default, ietf is used if connecting to non-cisco routers) 4. Cisco IOS release 11. 1 or earlier, specify the LMI type used by the Frame relay switch {ansi | cisco | q 933 a} 5. Cisco IOS 11. 2 or later, the LMI type is autosensed 6. Configure bandwidth for the link (Affects many routing protocols which uses it for a metric) 7. Inverse ARP is on by default

Verifying FRAME RELAY Operation After configuring Frame Relay, you can verify that the connections are active by using the show commands • Show interface serial (Displays DLCI used on the configured interface, LMI DLCI used for the LMI) • Show frame-relay pvc (Displays status of each configured connection & view the number of BECN & FECN packets received by the router) • Show frame-relay map (Displays the IP address & associated DLCI for each remote destination to which the router is connected to) • Show frame-relay lmi (Displays LMI traffic statistics- it will show the number of status messages between the router & the FR Switch)

Configuring Subinterfaces 1. Select interface & get into interface configuration mode 2. Remove any existing network-layer address assigned to the physical interface 3. Configure Frame Relay encapsulation router(config-if)#encapsulation frame-relay 4. Select the subinterface you want to configure router(config-if)#interface serial 0. 1 {multipoint | point to point} 5. Configure the network-layer address on the subinterface 6. Configure the DLCI for the subinterface to distinguish it from the physical interface router(config-if)#frame-relay interface-dlci-number

Multipoint Subinterfaces

Point to point Subinterfaces