Frame Relay

Why do we need Frame Relay? Frame Relay is more complex a technology than point-to-point WAN links but also provides more features and benefits

Frame Relay Frame Relay is a high-performance WAN protocol that operates at the physical and data link layers of the OSI reference model. It offers lower-cost data transfer when compared to typical point-to-point applications, by using virtual connections within the frame relay network and by combining those connections into a single physical connection at each location.

Frame Relay Network access switches

Frame Relay Network Frame Relay networks are multiaccess networks, which means that more than two devices can connect to the network. A Frame Relay network is made up of a large number of Frame Relay switches dispersed all over the coverage area of a Frame Relay service provider (e.g. region or country)

Frame Relay Network The switches are interconnected in a complex mesh topology. Frame Relay switches: – terminate user circuits, in addition to connecting to other switches, and are called access switches. – Other Frame Relay switches do not terminate user circuits, connecting to other Frame Relay switches only, and make the backbone of the Frame Relay network.

Frame Relay Network Unlike with LANs, Frame Relay cannot send a data link layer broadcast over. Therefore, Frame Relay networks are called nonbroadcast multiaccess (NBMA) networks.

Frame Relay Terminologies DCE & DTE In the context of Frame Relay, the router is the data terminal equipment (DTE) while the Frame Relay switch is the data circuit-terminating equipment (DCE). The purpose of DCE equipment is to provide clocking and switching services in a network.

Frame Relay Terminologies Virtual Circuits The logical connection through the Frame Relay network between two DTEs. The term “virtual” here means that the two DTEs are not connected directly but through a network.

Frame Relay Terminologies There are two types of VCs Permanent virtual circuits (PVCs): A predefined VC. A PVC can be equated to a leased line in concept.

Frame Relay Terminologies Switched virtual circuits (SVCs): – Are temporary connections used in situations requiring only sporadic data transfer between DTE devices across the Frame Relay network. – SVC is set up dynamically when needed. – SVC connections require call setup and termination for each connection.

A PVC between the same two DTEs will always be the same. An SVC between the same two DTEs may change. Path may change. Always same Path.

Frame Relay Frame Formats Frame Relay is a data link protocol and the customer router encapsulates each Layer 3 packet inside a Frame Relay frame comprising a header and trailer before it is sent out the access link. The header and trailer used is actually defined by the Link Access Procedure Frame Bearer Services (LAPF) specification

Frame Relay Frame Formats The simple LAPF header was extended to compensate for the absence of a Protocol Type field:

Frame Relay Frame Formats Header – DLCI, fields related to congestion management Trailer – Frame Relay provides no error recovery mechanism. It only provides CRC error detection. You should keep in mind that Frame Relay encapsulation should match on the routers at the two ends of a VC.

Rick Graziani 18 cisco - Default. – Use this if connecting to another Cisco router. Ietf - – Select this if connecting to a non-Cisco router. Router(config-if)#encapsulation frame-relay {cisco | ietf} Frame Relay Encapsulation

Rick Graziani HubCity(config)# interface serial 0 HubCity(config-if)# ip address HubCity(config-if)# encapsulation frame-relay Spokane(config)# interface serial 0 Spokane(config-if)# ip address Spokane(config-if)# encapsulation frame-relay

DLCI Frame-relay uses data-link connection identifiers (DLCIs) to build up logical circuits. The identifiers have local meaning only, that means that their values are unique per router, but not necessarily in the other routers.

DLCI Before DLCI can be used to route traffic, it must be associated with the IP address of its remote router

DLCI The HeadQuarter will need to map Branch 1 IP address to DLCI 23 & map Branch 2 IP address to DLCI 51. After that it can encapsulate data inside a Frame Relay frame with an appropriate DLCI number and send to the destination.

Mapping of DLCI The mapping of DLCIs to Layer 3 addresses can be handled manually or dynamically. Manually (static): the administrators can statically assign a DLCI to the remote IP address. Dynamic: the router can send an Inverse ARP Request to the other end of the PVC for its Layer 3 address.

Mapping of DLCI In short, Inverse ARP will attempt to learn its neighboring devices IP addresses and automatically create a dynamic map table. By default, physical interfaces have Inverse ARP enabled.

Now all the routers have a pair of DLCI & IP address of the router at the other end so data can be forwarded to the right destination.

Rick Graziani HubCity# show frame-relay map Serial0 (up): ip dlci 101, dynamic, broadcast, status defined, active dynamic refers to the router learning the IP address via Inverse ARP The DLCI 101 is configured on the Frame Relay Switch by the provider.

Rick Graziani 28 Static Frame Relay Address Mapping Use the frame-relay map command to configure static address mapping. Once a static map for a given DLCI is configured, Inverse ARP is disabled on that DLCI. The broadcast keyword is commonly used with the frame-relay map command. The broadcast keyword provides two functions. – Simplifies the configuration of OSPF for nonbroadcast networks that use Frame Relay. Router(config-if)#frame-relay map protocol protocol-address dlci [broadcast] [ietf | cisco]

Rick Graziani Remote IP Address Local DLCI Uses cisco encapsulation for this DLCI (not needed, default) By default, cisco is the default encapsulation

Rick Graziani If the Cisco encapsulation is configured on a serial interface, then by default, that encapsulation applies to all VCs on that serial interface. If the equipment at the destination is Cisco and non-Cisco, configure the Cisco encapsulation on the interface and selectively configure IETF encapsulation per DLCI, or vice versa. These commands configure the Cisco Frame Relay encapsulation for all PVCs on the serial interface. Except for the PVC corresponding to DLCI 49, which is explicitly configured to use the IETF encapsulation. Applies to all DLCIs unless configured otherwise

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

LMI LMI includes: – A keepalive mechanism, which verifies that data is flowing – A multicast mechanism, which provides the router with its local DLCI. – A status mechanism, which provides PVC statuses on the DLCIs known to the switch LMI is to allow DTE and DCE to exchange status information about the VCs and themselves

LMI The three possible states that PVC can be in are: Active state: Indicates that the connection is active and that routers can exchange data. Inactive state: Indicates that the access link to the Frame Relay switch is working, but the remote router connection to the Frame Relay switch is not working. Deleted state: means that there is a problem between the router and the Frame Relay switch.

LMI Cisco routers support the following three LMI types: 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. Router(config-if)#frame-relay lmi-type {ansi | cisco | q933a}

CIR 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.

Congestion Control There are three flag bits inside the Frame Relay header that can be used to control how the switches control the network when the network gets congested. – Forward Explicit Congestion Notification (FECN) – Backward Explicit Congestion Notification (BECN) – Dicard Eligibility (DE)

Congestion Control Frame Relay implements two congestion- notification mechanisms: – FECN – BECN FECN and BECN each is controlled by a single bit contained in the Frame Relay frame header. The Frame Relay frame header also contains a DE bit, which is used to identify less important traffic that can be dropped during periods of congestion.

Default Sitting Cisco IOS Software uses the following defaults for Frame Relay: LMI Cisco IOS automatically senses the LMI type by default and this feature is referred to as LMI autosense. If you manually configure the LMI using the frame- relay lmi-type command, LMI autosense is silently disabled.

Default Sitting IARP Cisco IOS automatically discovers the next-hop IP address associated with a DLCI or VC using Inverse Address Resolution Protocol (IARP). You can also create a mapping between a DLCI and next- hop IP address manually using frame-relay map ip command. Encapsulation Cisco IOS uses Cisco encapsulation for Frame Relay and if you are using only Cisco routers, this default setting works fine without any additional configuration.

Configuring Frame Relay Here is your step-by-step guide to configuring Frame Relay: The first step should always be to configure the physical interface to use Frame Relay encapsulation using the command encapsulation frame-relay in interface configuration mode. Configure an IP address on the interfaces or sub-interface using the ip address command.

Configuring Frame Relay Optionally, configure the LMI type of each physical interface using the frame-relay lmi-type command. The default is to use the Inverse ARP (IARP) to map the DLCI to the IP address of next-hop router. However, you can also configure static mapping using the frame-relay map ip ip-address dlci broadcast command. There are two ways to associate one DLCI to point-to- point or multiple DLCIs to multipoint interfaces. The first involves using the frame-relay interface-dlci dlci sub-interface command. The second involves using the frame-relay map ip ip-address dlci broadcast sub- interface command.