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CCNA

CCNA. Cisco Certified Network Associate. - Frame Relay. Frame Relay: An Efficient and Flexible WAN Technology. Large enterprises, governments, ISPs, and small businesses use Frame Relay, primarily because of its price and flexibility. Leased-line solutions are prohibitively expensive

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  1. CCNA Cisco Certified Network Associate

  2. - Frame Relay

  3. Frame Relay: An Efficient and Flexible WAN Technology Large enterprises, governments, ISPs, and small businesses use Frame Relay, primarily because of its price and flexibility. Leased-line solutions are prohibitively expensive Frame Relay reduces network costs by using less equipment, less complexity, and an easier implementation.

  4. The Frame Relay WAN

  5. Introducing Frame Relay • Frame Relay is a packet-switched, connection-oriented, WAN service. • It operates at the data link layer of the OSI reference model. • Frame Relay uses a subset of the high-level data link control (HDLC) protocol called Link Access Procedure for Frame Relay (LAPF). • Frames carry data between user devices called data terminal equipment (DTE), and the data communications equipment (DCE) at the edge of the WAN.

  6. Frame Relay vs. X.25 • The network providing the Frame Relay service can be either a carrier-provided public network or a privately owned network. • Because it was designed to operate on high-quality digital lines, Frame Relay provides no error recovery mechanism. • If there is an error in a frame it is discarded without notification.

  7. DTE – Data Terminal Equipment • DTEs generally are considered to be terminating equipment for a specific network and typically are located on the premises of the customer. • The customer may also own this equipment. • Examples of DTE devices are routers and Frame Relay Access Devices (FRADs). • A FRAD is a specialized device designed to provide a connection between a LAN and a Frame Relay WAN.

  8. Frame Relay terminology An SVC between the same two DTEs may change. A PVC between the same two DTEs will always be the same. • The connection through the Frame Relay network between two DTEs is called a virtual circuit (VC). • Switched Virtual Circuits (SVCs) are Virtual circuits may be established dynamically by sending signaling messages to the network. • However, SVCs are not very common. • Permanent Virtual Circuits (PVCs) are more common. • PVC are VCs that have been preconfigured by the carrier are used. • The switching information for a VC is stored in the memory of the switch. Path may change. Always same Path.

  9. Virtual Circuits There are two ways to establish VCs: SVCs, switched virtual circuits, are established dynamically by sending signaling messages to the network (CALL SETUP, DATA TRANSFER, IDLE, CALL TERMINATION). PVCs, permanent virtual circuits, are preconfigured by the carrier, and after they are set up, only operate in DATA TRANSFER and IDLE modes.

  10. Access Circuits and Cost Savings • The FRAD or router connected to the Frame Relay network may have multiple virtual circuits connecting it to various end points. • Each end point needs only a single access circuit and interface.

  11. IETF Frame Relay Frame • Cisco routers support two types of Frame Relay headers. • Cisco, which is a 4-byte header (default, Cisco proprietary). • IETF, which is a 2-byte header that conforms to the IETF standards. • Frame Relayfunctionsbydoingthefollowing: • Takes data packetsfrom a networklayerprotocol, such as IP or IPX • Encapsulatesthem as the data portionof a Frame Relay frame • Passes them to thephysicallayer for deliveryonthewire

  12. Frame Relay bandwidth and flow control • Local access rate – This is the clock speed or port speed of the connection or local loop to the Frame Relay cloud. • It is the rate at which data travels into or out of the network, regardless of other settings. • Committed Information Rate (CIR) – This is the rate, in bits per second, at which the Frame Relay switch agrees to transfer data. The first thing we need to do is become familiar with some of the terminology.

  13. Frame Relay bandwidth and flow control • Forward Explicit Congestion Notification (FECN) – When a Frame Relay switch recognizes congestion in the network, it sends an FECN packet to the destination device. • This indicates that congestion has occurred. • Backward Explicit Congestion Notification (BECN) – When a Frame Relay switch recognizes congestion in the network, it sends a BECN packet to the source router. • This instructs the router to reduce the rate at which it is sending packets.

  14. Frame Relay bandwidth and flow control • Discard eligibility (DE) bit – When the router or switch detects network congestion, it can mark the packet "Discard Eligible". • The DE bit is set on the traffic that was received after the CIR was met. • These packets are normally delivered. • However, in periods of congestion, the Frame Relay switch will drop packets with the DE bit set first.

  15. DLCI • A data-link connection identifier (DLCI)identifies the logical VC between the CPE and the Frame Relay switch. • The Frame Relay switch maps the DLCIs between each pair of routers to create a PVC. • DLCIs have local significance, although there some implementations that useglobal DLCIs.

  16. DLCI Mapping to Network Address • Manual • Manual: Administrators use a frame relay map statement. • Dynamic • Inverse Address Resolution Protocol (I-ARP) provides a given DLCI and requests next-hop protocol addresses for a specific connection. • The router then updates its mapping table and uses the information in the table to forward packets on the correct route.

  17. LMI – Local Management Interface • LMI is a signaling standard between the DTE and the Frame Relay switch. • LMI is responsible for managing the connection and maintaining the status between devices. • LMI includes: • A keepalive mechanism, which verifies that data is flowing • Flow Control • The ability to give DLCI’s global significance • VC status mechanism

  18. Inverse ARP – Knows DLCI, needs remote IP • Inverse Address Resolution Protocol (Inverse ARP) was developed to provide a mechanism for dynamic DLCI to Layer 3 address maps. • Inverse ARPworks much the same way Address Resolution Protocol (ARP) works on a LAN. • However, with ARP, the device knows the Layer 3 IP address and needs to know the remote data link MAC address. • With Inverse ARP, the router knows the Layer 2 address which is the DLCI, but needs to know the remote Layer 3 IP address.

  19. Frame Relay Encapsulation Router(config-if)#encapsulation frame-relay {cisco | ietf} • cisco - Default • Use this if connecting to another Cisco router. • Ietf - Select this if connecting to a non-Cisco router. • RFC 1490

  20. Frame Relay LMI Router(config-if)#frame-relay lmi-type {ansi | cisco | q933a} • It is important to remember that the Frame Relay service provider maps the virtual circuit within the Frame Relay network connecting the two remote customer premises equipment (CPE) devices that are typically routers. • Once the CPE device, or router, and the Frame Relay switch are exchanging LMI information, the Frame Relay network has everything it needs to create the virtual circuit with the other remote router. • In a Frame Relay network, before two routers can exchange information, a virtual circuit between them must be set up ahead of time by the Frame Relay service provider.

  21. Minimum Frame Relay Configuration HubCity(config)# interface serial 0 HubCity(config-if)# ip address 172.16.1.2 255.255.255.0 HubCity(config-if)# encapsulation frame-relay Spokane(config)# interface serial 0 Spokane(config-if)# ip address 172.16.1.1 255.255.255.0 Spokane(config-if)# encapsulation frame-relay Spokane(config-if)# frame-relay lmi-type ansi

  22. Inverse ARP HubCity# show frame-relay map Serial0 (up): ip 172.16.1.1 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.

  23. Configuring Frame Relay maps Router(config-if)#frame-relay map protocol protocol-address dlci [broadcast] [ietf | cisco] • If the environment does not support LMI autosensing and Inverse ARP, a Frame Relay map must be manually configured. • 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: • Forwards broadcasts when multicasting is not enabled.

  24. Frame Relay Maps By default, cisco is the default encapsulation Local DLCI Remote IP Address Uses cisco encapsulation for this DLCI (not needed, default)

  25. More on Frame Relay Encapsulation • 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

  26. Verifying Frame Relay interface configuration • The show interfaces serial command displays information regarding the encapsulation and the status of Layer 1 and Layer 2. • It also displays information about the multicast DLCI, the DLCIs used on the Frame Relay-configured serial interface, and the DLCI used for the LMI signaling.

  27. show frame-relay pvc • The show frame-relay pvc command displays the status of each configured connection, as well as traffic statistics. • This command is also useful for viewing the number of Backward Explicit Congestion Notification (BECN) and Forward Explicit Congestion Notification (FECN) packets received by the router. • The command show frame-relay pvc shows the status of all PVCs configured on the router. • If a single PVC is specified, only the status of that PVC is shown.

  28. show frame-relay map • The show frame-relay map command displays the current map entries and information about the connections.

  29. show frame-relay lmi • The show frame-relay lmi command displays LMI traffic statistics showing the number of status messages exchanged between the local router and the Frame Relay switch.

  30. clear frame-relay-inarp • To clear dynamically created Frame Relay maps, which are created using Inverse ARP, use the clear frame-relay-inarp command.

  31. Reachability issues with routing updates • An NBMA network is a multiaccess network, which means more than two nodes can connect to the network. • Ethernet is another example of a multiaccess architecture. • In an Ethernet LAN, all nodes see all broadcast and multicast frames. • However, in a nonbroadcast network such as Frame Relay, nodes cannot see broadcasts of other nodes unless they are directly connected by a virtual circuit. • This means that Branch A cannot directly see the broadcasts from Branch B, because they are connected using a hub and spoke topology. Frame Relay is an NBMA Network

  32. Reachability issues with routing updates • The Central router must receive the broadcast from Branch A and then send its own broadcast to Branch B. • In this example, there are problems with routing protocols because of the split horizon rule.  • A full mesh topology with virtual circuits between every site would solve this problem, but having additional virtual circuits is more costly and does not scale well. Split Horizon prohibits routing updates received on an interface from exiting that same interface.

  33. One Solution: Disable Split Horizon • To remedy this situation, turn off split horizon for IP. • Of course, with split horizon disabled, the protection it affords against routing loops is lost. • Split horizon is only an issue with distance vector routing protocols like RIP, IGRP and EIGRP. • It has no effect on link state routing protocols like OSPF and IS-IS. Router(config-if)#no ip split-horizon Router(config-if)#ip split-horizon

  34. Another Solution for split horizon issue: subinterfaces • To enable the forwarding of broadcast routing updates in a Frame Relay network, configure the router with subinterfaces. • Subinterfaces are logical subdivisions of a physical interface. • In split-horizon routing environments, routing updates received on one subinterface can be sent out on another subinterface. • With subinterface configuration, each PVC can be configured as a point-to-point connection. • This allows each subinterface to act similar to a leased line. • This is because each point-to-point subinterface is treated as a separate physical interface.

  35. A key reason for using subinterfaces is to allow distance vector routing protocols to perform properly in an environment in which split horizon is activated. • There are two types of Frame Relay subinterfaces. • Point-to-point • Multipoint Mulitpoint Point-to-point

  36. Point-to-point subinterfaces: Each subinterface is on its own subnet. Broadcasts and Split Horizon not a problem because each point-to-point connection is its own subnet. • Multipoint subinterfaces: All participating subinterfaces would be in the same subnet.Broadcasts and routing updates are also subject to the Split Horizon Rule and may pose a problem. Mulitpoint Point-to-point

  37. Configuring Frame Relay subinterfaces • Subinterface can be configured after the physical interface has been configured for Frame Relay encapsulation • Subinterface numbers can be specified in interface configuration mode or global configuration mode. • Subinterface number can be between 1 and 4294967295. • The frame-relay interface-dlci command associates the selected subinterface with a DLCI. RTA(config)#interface s0/0 RTA(config-if)#encapsulation frame-relay ietf Router(config-if)#interface serial number subinterface-number {multipoint | point-to-point} Router(config-subif)# frame-relay interface-dlcidlci-number

  38. Configuring Frame Relay subinterfaces • The frame-relay interface-dlci command is required for all point-to-point subinterfaces. • It is also required for multipoint subinterfaces for which inverse ARP is enabled. • It is notrequired for multipoint subinterfaces that are configured with static route maps. • It can notbe used on physical interfaces.

  39. "out" is an LMI status message sent by the router. "in" is a message received from the Frame Relay switch. A full LMI status message is a "type 0" (not shown in the figure). An LMI exchange is a "type 1". "dlci 100, status 0x2" means that the status of DLCI 100 is active (not shown in figure).

  40. Troubleshooting the Frame Relay configuration Use the debug frame-relay lmi command to determine whether the router and the Frame Relay switch are sending and receiving LMI packets properly.

  41. debug frame-relay lmi (continued) The possible values of the status field are as follows: 0x0 – Added/inactive means that the switch has this DLCI programmed but for some reason it is not usable. The reason could possibly be the other end of the PVC is down. 0x2 – Added/active means the Frame Relay switch has the DLCI and everything is operational. 0x4 – Deleted means that the Frame Relay switch does not have this DLCI programmed for the router, but that it was programmed at some point in the past.

  42. Show frame-relay map Point-to-point subinterfaces are listed as a “point-to-point dlci” Router#show frame-relay map Serial0.1 (up): point-to-point dlci, dlci 301 (0xCB, 0x30B0), broadcast status defined, active With multipoint subinterfaces, they are listed as an inverse ARP entry, “dynamic” Router#show frame-relay map Serial0 (up): ip 172.30.2.1 dlci, 301 (0x12D, 0x48D0), dynamic,, broadcast status defined, active

  43. Each subinterface on Hub router requires a separate subnet (or network) • Each subinterface on Hub router is treated like a regular physical point-to-point interface, so split horizon does not need to be disabled. • Interface Serial0 (for all routers) • encapsulation frame-relay • no ip address • HubCity • interface Serial0.301 point-to-point • ip address 172.16.1.1 255.255.255.0 • encapsulation frame-relay • frame-relay interface dlci 301 • interface Serial0.302 point-to-point • ip address 172.16.2.1 255.255.255.0 • encapsulation frame-relay • frame-relay interface dlci 302 • Spokane • interface Serial0.103 point-to-point • ip address 172.16.1.2 255.255.255.0 • frame-relay interface dlci 103 • Spokomo • interface Serial0.203 point-to-point • ip address 172.16.2.2 255.255.255.0 • frame-relay interface dlci 203 Point-to-Point Subinterfaces at the Hub and Spokes Two subnets

  44. Notes • Highly scalable solution • Disable Split Horizon on Hub router when running a distance vector routing protocol • Interface Serial0 (for all routers) • encapsulation frame-relay • no ip address • HubCity • interface Serial0.1 mulitpoint • ip address 172.16.3.3 255.255.255.0 • frame-relay interface-dlci 301 • frame-relay interface-dlci 302 • no ip split-horizon • Spokane • interface Serial0.1 point-to-point • ip address 172.16.3.1 255.255.255.0 • frame-relay interface-dlci 103 • Spokomo • interface Serial0.1 point-to-point • ip address 172.16.3.2 255.255.255.0 • frame-relay interface-dlci 203 Multipoint subinterface at the Hub and Point-to-Point Subinterfaces at the Spokes One subnet

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