Managing Frame Relay Traffic

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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.

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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 to specified transmission rates.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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).

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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

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Configuring Traffic Shaping

• Router(config-if)encapsulation frame-relay.

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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.

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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

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Verifying Frame Relay Traffic

• This section explains the specific show commands
  available for Frame Relay traffic shaping. These
  are show frame-relay pvc dlcishow
  traffic-shapeshow traffic-shape statistics 

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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.

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On Demand Routing

• Further, because only minimal route information
  is traversing the link between the stub and hub
  routers, bandwidth use is minimal.

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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.