Managing Frame Relay Traffic

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Managing Frame Relay Traffic. Unlike a leased line, which provides a fixed amount of bandwidth at all times, a packet-switched network can provide multiple levels of bandwidth and service.In a Frame Relay network, customers can control, or "shape", traffic so that certain protocols and VCs conform

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Managing Frame Relay Traffic

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1. Managing Frame Relay Traffic The architecture of packet-switched networks affords providers and their customers a great deal of control over how traffic is managed. Providers can raise or lower the rate at which customer data flows by reconfiguring their switching equipment. Frame Relay switches are typically configured to drop traffic under certain circumstances, or prioritize traffic in other cases.

2. Managing Frame Relay Traffic Unlike a leased line, which provides a fixed amount of bandwidth at all times, a packet-switched network can provide multiple levels of bandwidth and service. In a Frame Relay network, customers can control, or "shape", traffic so that certain protocols and VCs conform to specified transmission rates.

3. Managing Frame Relay Traffic Customers may use traffic shaping because of a policy or an application. An application may require that a given interface should not exceed a certain data rate even though the physical line is capable of higher transmission speeds. Customers also shape traffic to avoid having a high-capacity link overwhelm a branch office router that has a low speed connection.

4. Managing Frame Relay Traffic Frame Relay traffic shaping relies on parameters that are useful for managing network traffic congestion. These include committed information rate (CIR), forward and backward explicit congestion notification (FECN/BECN), and the discard eligibility (DE) bit.

5. Managing Frame Relay Traffic This chapter explores Frame Relay traffic shaping from the customer's perspective, including rate enforcement, rate adaptation, and queuing. Finally, this chapter explores on-demand routing, an alternative to configuring and managing the routing process over Frame Relay hub-and-spoke networks.

6. Frame Relay Terminology Local 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, regardless of other settings.

7. Frame Relay Terminology Committed Information Rate (CIR) - The rate, in bits per second, at which the Frame Relay switch agrees to transfer data. The rate is usually averaged over a period of time, referred to as the committed rate measurement interval (Tc). In general, the duration of Tc is proportional to the "burstiness" of the traffic.

8. Frame Relay Terminology Oversubscription - Oversubscription is when the sum of the CIRs on all the VCs exceeds the access line speed. Oversubscription can also occur when the access line can support the sum of CIRs purchased, but not of the CIRs plus the bursting capacities of the VCs. Oversubscription increases the likelihood that packets will be dropped.

9. Frame Relay Terminology Committed Burst (Bc) - The maximum number of bits that the switch agrees to transfer during any Tc. The higher the Bc-to-CIR ratio, the longer the switch can handle a sustained burst. For example, if the Tc is 2 seconds and the CIR is 32 kbps, the Bc is 64 kbps. The Tc calculation is Tc = Bc/CIR.

10. Frame Relay Terminology Excess Burst (Be) - The maximum number of uncommitted bits that the Frame Relay switch attempts to transfer beyond the CIR. Be is dependent on the service offerings available from your vendor, but it is typically limited to the port speed of the local access loop.

11. Frame Relay Terminology Excess Information Rate (EIR) - Defines the maximum bandwidth available to the customer, that is, the CIR plus the Be. Typically, the EIR is set to the local access rate. In the event the provider sets the EIR to be lower than the local access rate, all frames beyond that maximum can be discarded automatically, even if there is no congestion.

12. Frame Relay Terminology Forward Explicit Congestion Notification (FECN) - When a Frame Relay switch recognizes congestion in the network, it sends an FECN packet to the destination device, indicating that congestion has occurred.

13. Frame Relay Terminology Backward Explicit Congestion Notification (BECN) - 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. With Cisco IOS Release 11.2 or later, Cisco routers can respond to BECN notifications.

14. Frame Relay Terminology 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, but in periods of congestion, the Frame Relay switch will drop packets with the DE bit set first.

15. Overview Several factors determine the rate at which a customer can send data on a Frame Relay network. Foremost in limiting the maximum transmission rate is the capacity of the local loop to the provider. If the local loop is a T1, you cannot send more than 1.544 Mbps. The provider typically provides a clocking signal, which determines the speed of the local loop.

16. Overview In Frame Relay terminology, the speed of the local loop is called the local access rate. Typically, the higher the CIR, the higher the cost of service. Customers can choose the CIR that's most appropriate to their bandwidth needs, as long as the CIR is less than or equal to the local access rate.

17. Types of Frame Relay Traffic Management The traffic shaping over Frame Relay feature provides the following capabilities: Rate enforcement on a per-virtual-circuit basis - You can configure a peak rate to limit outbound traffic to either the CIR or some other defined value, such as the excess information rate (EIR).

18. Types of Frame Relay Traffic Management Generalized BECN support on a per-VC basis - The router can monitor BECNs and throttle traffic based on BECN-marked packet feedback from the Frame Relay network. Priority/Custom/Weighted Fair Queuing (PQ/CQ/WFQ) support at the VC level - This allows for finer granularity in the prioritization and queuing of traffic, thus giving you more control over the traffic flow on an individual VC.

19. Configuring Traffic Shaping The traffic shaping over Frame Relay feature can be used in the following typical situations: When you have a Frame Relay network topology that consists of a high-speed (T1 line speed or greater) connection at the central site and low-speed (56 kbps or less) connections at the branch sites.

20. Configuring Traffic Shaping When you have a Frame Relay network that is constructed with many VCs to different locations on a single physical line into the network. If you notice that your Frame Relay connections occasionally get congested.

21. Configuring Traffic Shaping When you have several different types of traffic (IP, Systems Network Architecture [SNA], or Internetwork packet Exchange [IPX]) to transmit on the same Frame Relay VC, and want to ensure that the different traffic types receive a certain amount of bandwidth.

22. Configuring Traffic Shaping This section describes the general procedure for configuring traffic shaping using a Frame Relay map class. A map class defines a set of configuration parameters that can be used by more than one interface or subinterface. You configure Frame Relay traffic shaping parameters for the map class and then apply the map class to one or more Frame Relay interfaces.

23. Configuring Traffic Shaping Frame Relay traffic shaping parameters cannot be configured directly on the interface. To enable Frame Relay traffic shaping, perform the following steps. Specify a map class. Defined with the map-class frame-relay command: -. Router(config)#map-class frame-relay map-class-name.

24. Configuring Traffic Shaping Configure the map class. When you define a map class for Frame Relay, you can do the following: Define the average and peak rates (in bits per second) that are allowed on VCs associated with the map class. Specify that the router dynamically fluctuates the rate at which it sends packets, depending on the BECNs it receives.

25. Configuring Traffic Shaping Specify either a custom queue list or a priority queue group to use on VCs associated with the map class. Enable Frame Relay on an interface. After you have defined a map class with queuing and traffic shaping parameters, enter interface configuration mode and enable Frame Relay encapsulation on an interface with the encapsulation frame-relay command:

26. Configuring Traffic Shaping Router(config-if)#encapsulation frame-relay.

27. Configuring Traffic Shaping Enable Frame Relay traffic shaping on an interface. Enabling Frame Relay traffic shaping on an interface, using the frame-relay traffic-shaping command, enables both traffic shaping and per-VC queuing on all the Permanent Virtual Circuits (PVCs) and Switched Virtual Circuits (SVCs) on the interface. Traffic shaping enables the router to control the circuit output rate and react to congestion notification information.

28. Configuring Traffic Shaping Add the map class to VCs on the interface. Add a map class to all VCs on the interface with the frame-relay class map-class-name command. The map-class-name argument must match the map-class-name of the map class you configured: Router(config-if)#frame-relay class map-class-name

32. Verifying Frame Relay Traffic This section explains the specific show commands available for Frame Relay traffic shaping. These are: show frame-relay pvc dlci show traffic-shape show traffic-shape statistics 

33. On Demand Routing With ODR, a hub router can automatically discover stub networks while the stub routers still use a default route to the hub. ODR utilizes CDP to provide address prefixes (the network portion of the IP address) for the routing table entries. The network portion does not have to be strictly classful. VLSM is supported.

34. On Demand Routing Further, because only minimal route information is traversing the link between the stub and hub routers, bandwidth use is minimal.

35. On Demand Routing It is important to note that ODR is not a true routing protocol. It discovers information about stub networks, but does not provide any routing information to the stub routers. The link information is conveyed by a data link protocol and, therefore, does not go further than from the stub router to the hub router. However, ODR-discovered routes can be redistributed into dynamic routing protocols.

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