Chapter 12 Frame Relay

Transcription

1 1 Chapter 12 Frame Relay

2 Contents Frame Relay Stand. & Eq. Virtual Circuits LMI Performance Parameters Frame Format Frame Relay Topologies Frame Relay Config. Monitoring Frame Relay Review Questions 2

3 3 Frame Relay Standards and Equipments Frame Relay is a packetswitching and encapsulation technology that function at the Physical and Data Link layers of the OSI model.

4 4 rame Relay Standards and Eq. (cont.) The data rate for the Frame Relay can be from 56 Kbps to Mbps, or higher.

5 5 rame Relay Standards and Eq. (cont.) ITU-T and ANSI define Frame Relay as a connection between the data terminal equipment (DTE) and data communications equipment (DCE).

6 6 rame Relay Standards and Eq. (cont.) DCE is switching equipment supplied by a telecom. provider that serves as a connection to the public data network (PDN).

7 7 rame Relay Standards and Eq. (cont.) DTE is also known as custom premises equipment (CPE), because it is the equipment that belongs to, and is maintained by, the PDN customer.

8 8 rame Relay Standards and Eq. (cont.) For example, if you connect your Cisco router to a Frame Relay switch, the Cisco router is the CPE and the Frame Relay switch is the DCE. (see Fig. 12-1)

9 rame Relay Standards and Eq. (cont.) In Fig. 12-2, you can see a CSU/DSU that is used with a Cisco 2501 router to make the connection. The CSU/DSU is at the customer location. The unit is used for encoding, filtering, and translating communications to and from 9

10 10 rame Relay Standards and Eq. (cont.) Some routers have built-in cards that allow them to make WAN connections. Fig indicates a T1 CSU/DSU cards built into the router.

11 11 rame Relay Standards and Eq. (cont.) A typical example Frame Relay connection was shown in the figure.

12 Contents Frame Relay Stand. & Eq. Virtual Circuits LMI Performance Parameters Frame Format Frame Relay Topologies Frame Relay Config. Monitoring Frame Relay Review Questions 12

13 13 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.

14 14 Virtual Circuits (cont.) Frame Relay separate each data streams into logical (software-maintained) connections called virtual circuits, which carry the data transferred on the connection.

15 15 Virtual Circuits (cont.) Two types of virtual circuits, switched virtual circuits (SVC) and permanent virtual circuits (PVC), connect Frame Relay ports.

16 16 Virtual Circuits (cont.) The SVC software automatically dials the WAN, establishing and terminating the connection as required to transfer data over Frame Relay service.

17 17 Virtual Circuits (cont.) PVCs remain permanently connected to the WAN. The administrator manually defines the PVC; it remains until the network administrator removes it.

18 18 Virtual Circuits (cont.) DLCI Frame Relay connections identify virtual circuits by Data Link Connection Identifier (DLCI) number.

19 Virtual Circuits (cont.) DLCI (cont.) The DLCI number map virtual circuits to layer 3 protocol addresses. For example, a DLCI number associates an IP address with a specific virtual circuits. 19

20 Virtual Circuits (cont.) 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 in RAM. 20

21 Virtual Circuits (cont.) Fig shows a sample Frame Relay configuration. Notice that the Frame Relay switching table is configured to map its ports to the correct DLCI numbers for the virtual connection. 21

22 Virtual Circuits (cont.) The switching table in this example is simplified. Remember that DLCI numbers are only locally significant, so it would be possible both routers use the same DLCI numbers to specify a virtual circuit. 22

23 23 Virtual Circuits (cont.) Subinterfaces A single router serial interface can service multiple PVCs through a single physical serial interface.

24 24 Virtual Circuits (cont.) Subinterfaces (cont.) To allow a single serial interface to support multiple PVCs, the IOS divides the interface into logical subinterface.

25 25 Virtual Circuits (cont.) Subinterfaces (cont.) For example, if Serial 0 (s0) had three subinterfaces, they would be referenced as s0.1, s0.2, and s0.3.

26 26 Virtual Circuits (cont.) Subinterfaces (cont.) Subinterfaces are not real physical interfaces; they are virtual interfaces associated with a physical interface.

27 27 Virtual Circuits (cont.) Subinterfaces (cont.) With subinterfaces, the cost of implementing multiple Frame Relay virtual circuits is reduced because one port is required on the router.

28 Contents Frame Relay Stand. & Eq. Virtual Circuits LMI Performance Parameters Frame Format Frame Relay Topologies Frame Relay Config. Monitoring Frame Relay Review Questions 28

29 29 LMI The Local Management Interface (LMI) is used only locally, between the Frame Relay DTE (e.g., a router) and Frame Relay DCE (e.g., a switch), as shown in the figure.

30 30 LMI (cont.) The LMI basically extend the functionality of Frame Relay by: Making the DLCIs globally significant rather then locally significant

31 31 LMI (cont.) Creating a signaling mechanism between the routers and Frame Relay switch, which could report the status of the link. Supporting multicasting

32 32 LMI (cont.) Providing DLCI numbers that are globally significant makes automatic configuration of Frame Relay map possible.

33 LMI (cont.) LMI uses keepalive packets to verify the Frame Relay link and to ensure the flow of data. The Frame Relay switch in turn provides the status of all VCs and their respective DLCI numbers. 33

34 LMI (cont.) Each virtual circuit, represented by its DLCI number, can have one of three connection states: Active Inactive Deleted 34

35 LMI (cont.) Without LMI, the Frame Relay map must be built statically in the router. By making DLCIs globally significant, LMI facilitates dynamic Frame Relay map tables through the use of inverse ARP protocol. 35

36 36 LMI (cont.) Inverse ARP In multipoint config., routers use the protocol inverse ARP to send a query using DLCI number to find a remote IP address.

37 37 LMI (cont.) Inverse ARP (cont.) As other routers respond to the inverse ARP queries, the local routers can build its Frame Relay map automatically.

38 38 LMI (cont.) Inverse ARP (cont.) To maintain Frame Relay map, routers exchange inverse ARP messages every 60 sec by default. LMI is required for inverse ARP to work.

39 LMI (cont.) Inverse ARP (cont.) Do not confuse inverse ARP with reverse ARP (RARP); RARP is used primarily on LANs to provide hosts that only have MAC address with IP address. 39

40 40 LMI (cont.) Encapsulation Types Different Frame Relay switches, CPE, and Frame Relay equipment connectivity support different types of LMI encapsulation.

41 LMI (cont.) Encapsulation Types (cont.) Cisco routers support theses types of LMI encapsulation: cisco ansi q933a 41

42 42 LMI (cont.) Split Horizon Split horizon is a routing technique that reduces the chance of routing loops on a network.

43 LMI (cont.) Split Horizon (cont.) A split horizon implementation technique preventing routing update information received on one physical interface from being rebroadcast to other devices through that same physical interface. 43

44 LMI (cont.) Split Horizon (cont.) People also refer to this rule as nonbroadcast multiaccess (NBMA). NBMA is a term used to describe WAN networks that use VCs for connectivity. 44

45 45 LMI (cont.) Split Horizon (cont.) Fig shows a split horizon problem. RouterA would not be able to send route updates received from RouterB to RouterC, and vice versa.

46 46 LMI (cont.) Split Horizon (cont.) The best solution is to configure separate pointto-point subinterfaces for each virtual connection. (see Fig. 12-6)

47 47 LMI (cont.) Split Horizon (cont.) Point-to-point connections allow you to divide a single serial interface into multiple subinterfaces, each supporting a separate virtual connection.

48 LMI (cont.) Split Horizon (cont.) In a multipoint configurations, the network administrator can configure a single serial interface or subinterface to support multiple connections to physical or logical interfaces on other routers. 48

49 49 LMI (cont.) Split Horizon (cont.) A single subinterface configured for multiple connection is still subject to the spit horizon rule.

50 50 LMI (cont.) Split Horizon (cont.) The only benefit to the multiple configuration is that it allows you to use a single network for all of your routers, as shown in Fig

51 Contents Frame Relay Stand. & Eq. Virtual Circuits LMI Performance Parameters Frame Format Frame Relay Topologies Frame Relay Config. Monitoring Frame Relay Review Questions 51

52 52 Performance Parameters Terms that appear in the contract may include: Access rate: The speed of the line, which indicates transfer rate. Access rate is also known as the local access rate.

53 53 Performance Parameters (cont.) Committed Information Rate (CIR): The minimum transfer rate that the Frame Relay customer negotiates with Frame Relay service provider.

54 54 Performance Parameters (cont.) CIR (cont.): The service provider agrees to always allow the customer to transfer information at no less than the transfer rate specified by the CIR.

55 55 Performance Parameters (cont.) 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.

56 Performance Parameters (cont.) 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. 56

57 57 Performance Parameters (cont.) Oversubscription: When the sum of the data arriving over all virtual circuits exceeds the access rate, the situation is called oversubscription. Oversubscription results in dropped packets.

58 Performance Parameters (cont.) Two features of oversubscription are: Fewer T1 lines: less infrastructure; Win-win concept: huge saving for both customers and providers. 58

59 59 Performance Parameters (cont.) The figure shows an (excess urst rate) committed burst rate) example of how the Frame Relay traffic parameters affect the data rate of a VC.

60 60 Performance Parameters (cont.) As long as the data rate of VC is below the CIR/B C values, the Frame Relay switch allows the frames into the Frame Relay network.

61 61 Performance Parameters (cont.) However, those frames (4 and 5) that exceed the VC value will have their Discard Eligible bits set, which allows the carrier to drop these frames in times of internal congestion.

62 62 Performance Parameters (cont.) Also, any frames that exceed the B E are dropped, in this example, frames 6 and 7 are dropped.

63 Performance Parameters (cont.) Congestion When the Frame Relay switch recognizes congestion, it send a forward congestion explicit notification (FECN) message to the destination router. 63

64 64 Performance Parameters (cont.) Congestion (cont.) In addition, the switch sends a backward explicit congestion notification (BECN) message to the transmitting, or source router.

65 Contents Frame Relay Stand. & Eq. Virtual Circuits LMI Performance Parameters Frame Format Frame Relay Topologies Frame Relay Config. Monitoring Frame Relay Review Questions 65

66 66 Frame Format Frame Relay devices can use different Frame Relay frame format. Fig shows the Cisco proprietary Frame Relay frame format.

67 Contents Frame Relay Stand. & Eq. Virtual Circuits LMI Performance Parameters Frame Format Frame Relay Topologies Frame Relay Config. Monitoring Frame Relay Review Questions 67

68 68 Frame Relay Topologies Frame Relay can use all of the WAN technologies discussed in Ch. 5: peer (point-to-point), star (huband-spoke), partial mesh, or full mesh physical topology. (see Fig. 12-9),

69 Contents Frame Relay Stand. & Eq. Virtual Circuits LMI Performance Parameters Frame Format Frame Relay Topologies Frame Relay Config. Monitoring Frame Relay Review Questions 69

70 Frame Relay Configuration Basic Multipoint Config. With 2 Routers In Fig , RouterA is a Cisco router running IOS ver. 11.2, so it has ability to autosense LMI type. In addition, it automatically receives the DLCI information by querying the network. 70

71 71 Frame Relay Configuration (cont.) Config. With 2 Routers (cont.) Table 12-1 lists the Cisco router prompts and commands that you need to complete this configuration.

72 72 Frame Relay Configuration (cont.) Config. With 2 Routers (cont.) If RouterB is running IOS release ver. 11.0, the configuration commands for the router are shown in Fig

73 73 Frame Relay Configuration (cont.) Another example of configuring with 2 routers

74 Frame Relay Configuration (cont.) Multipoint Configuration Using Subinterfaces To configure a multipoint subinterfaces, you map it to multiple remote routers using the same subnet mask, but different DLCI number. 74

75 75 Frame Relay Configuration (cont.) Multipoint Configuration Using Subinterfaces (cont.) Table 12-2 outlines the steps to configure RouterA.

76 Frame Relay Configuration (cont.) (config-subif)# frame-relay interface-dlci 102 (config-subif)# frame-relay interface-dlci 103 (config-subif)# frame-relay interface-dlci

77 77 Frame Relay Configuration (cont.) Point-to-Point Configuration Using Subinterfaces In Fig , RouterA will be configured for three point-to-point connections to remote routers B, C, and D.

78 78 Frame Relay Configuration (cont.) Point-to-Point Configuration Using Subinterfaces (cont.) Point-to-point connections Frame Relay configurations do not support Inverse ARP.

79 Frame Relay Configuration (cont.) Point-to-Point Configuration Using Subinterfaces (cont.) You have to configure each subnet separately and use the frame-relay interfacedlci command to associate the DLCI number with a specific subinterface. 79

80 80 Frame Relay Configuration (cont.) Point-to-Point Configuration Using Subinterfaces (cont.) These remote routers configurations will resemble the earlier basic multipoint configuration with two routers.

81 81 Frame Relay Configuration (cont.) Point-to-Point Configuration Using Subinterfaces (cont.) The commands to configure RouterA for the point-to-point example are shown in Fig

82 82 Frame Relay Configuration (cont.) Frame Relay Static Mapping You statically configure your DLCI entries in the following situation: The remote router doesn t support Inverse ARP.

83 83 Frame Relay Configuration (cont.) Frame Relay Static Mapping (cont.) You need to assign specific subinterfaces to a specific DLCI connections. You want to reduce broadcast traffic.

84 84 Frame Relay Configuration (cont.) Frame Relay Static Mapping (cont.) You are configuring OSPF over Frame Relay.

85 Contents Frame Relay Stand. & Eq. Virtual Circuits LMI Performance Parameters Frame Format Frame Relay Topologies Frame Relay Config. Monitoring Frame Relay Review Questions 85

86 86 Monitoring Frame Relay You can check your Frame Relay configurations by using show commands.

87 Monitoring Frame Relay (cont.) The most common show commands for monitoring Frame Relay operations are: show interface show frame-relay pvc show frame-relay map show frame-relay lmi 87

88 88 Monitoring Frame Relay (cont.) Figs ~18 show these commands with their output.

89 Contents Frame Relay Stand. & Eq. Virtual Circuits LMI Performance Parameters Frame Format Frame Relay Topologies Frame Relay Config. Monitoring Frame Relay Review Questions 89

90 Review Questions Which protocol is used to automatically build the Frame Relay map along with LMI? A. ARP B. RARP C. Inverse ARP D. DLCI E. None of the above Ans: C 90

91 Review Questions (cont.) What is the purpose of keepalive packets? A. To reduce data transfer rates B. To keep PVCs active C. To increase data transfer rate D. To negotiate connection speed E. None of the above Ans: B 91

92 Review Questions (cont.) Which of the following layers do WAN specifications typically define? (Choose all that apply.) A. Physical B. Data Link C. Network D. Transport E. Presentation Ans: A, B 92

93 Review Questions (cont.) In Frame Relay, what would be considered the DCE? A. Customer s router B. Terminal adapter C. PPP D. Frame Relay switch E. None of the above Ans: D 93

94 Review Questions (cont.) Which of the following is a type of virtual circuit? (Choose all that apply.) A. MVC B. PVC C. SVC D. QVC E. RVC Ans: B, C 94

95 95 Review Questions (cont.) Which of the following are LMI encapsulation types supported by Cisco routers? (Choose all that apply.) A. LMI 2 B. cisco C. ansi D. q923i E. q923a