Cisco Voice Products

Configuring Frame Relay Traffic Shaping

Traffic shaping works on the principle that a router controls outbound traffic to match its data flow to the speed of the remote device. Traffic that fits a particular profile can be "shaped" to meet downstream requirements, eliminating downstream bottlenecks. For example, in Figure 5-5, you can see a partially meshed Frame Relay network.

Figure 5-5. Partially Meshed Frame Relay Network

In this model, the remote sites need IP access to the authorization center. At any given time, there are 300 end-to-end TCP connections from the remote sites to the authorization center. If the link between the authorization center fails and then quickly recovers, a flood of TCP connection requests , followed by application data, quickly saturating the 64-kbps link from the host site to the authorization center. Because the remote sites have a full T1, they send at this speed. They have no information letting them know that the authorization center operates at only 64 kbps. FRTS enables you to control bursts of traffic that can occur during these situations.

Let's recall the terms discussed earlier and define them in terms of relevance toward FRTS:

• Committed burst (Bc) ” The amount of data to send in each Tc interval, measured in bits. Usually set to 1/8 of CIR.

• Excess burst (Be) ” The number of excess bits to transmit during the first interval over and above Bc, only if credit is built up. The Be bit gets set to whatever the burst eligible is set for in the Frame Relay switch. The Be bit is optional. In the field, these parameters are supplied from the WAN provider.

• Committed Information Rate (CIR) ” The average rate that you want to send traffic out. In this case, this is not that same as the CIR provided by the carrier, but it should be set the same as the physical port speed.

• Time Interval (Tc) ” Time interval, which cannot exceed 125ms. Tc = Bc/CIR.

• MinCIR ” The minimum amount of data to be sent during periods of congestion. This value should be set to your actual CIR that the carrier provides you.

• Byte increment ” This value equals Bc/8. It is the amount of data to send during each time interval.

• FECN/BECN ” There are two bits in the address field of each frame for explicit signaling, FECN and BECN. The bits might be set by the frame provider when it detects congestion. The frame provider must not clear the bits if it receives a frame with one or both bits set, thereby providing true signaling to the end user .

  - Backward-explicit congestion notification (BECN) ” This tells the end user that congestion- avoidance procedures should be initiated for traffic destined toward the opposite direction from which the frame was received. Further frames that the user transmits on this VC in this direction might encounter congestion.
  
  

  - Forward-explicit congestion notification (FECN) ” This tells the end user that congestion-avoidance procedures should be initiated for traffic destined toward the same direction from which the frame was received. It also means that this frame on this VC has encountered congestion.
  
  

When traffic shaping is enabled, the router checks to see whether there is a t oken or credit available before sending the packet. The token bucket has tokens put in at a certain rate. The bucket itself also has a predefined capacity. If the bucket is already full of tokens, new tokens cannot be held and made available for future packets. Therefore, at any given time, the burst from the router is about the same size as the token bucket. If there are not enough tokens for the router to send a packet, the packet waits until the bucket has enough tokens, or it is discarded. Before the packet is sent out an interface, it is sent through the queue that has been set up for that VC. The default queue is FIFO, but custom queuing or priority queuing also can be used. For more information on queuing, see Cisco IOS 12.0 Quality of Service , from Cisco Press.

BECN response mode is one form of traffic shaping. With BECN response mode, traffic shaping is enabled. If a router receives any BECNs during the current time interval, it decreases its transmit rate by 25 percent. This rate continues to drop by 25 percent once per Tc interval until the traffic rate gets to MinCIR, where it levels out. When the traffic rate has decremented, it takes 16 time intervals with no BECNs before it starts to increase the traffic rate. Traffic increases by a rate of (Be+Bc)/16 when it starts to increase.

With this background information in mind, you can move on to the specifics of configuring FRTS. Use the following process to configure Frame Relay traffic shaping:

Step 1. Enable Frame Relay traffic shaping on the interface. Apply the frame-relay traffic-shaping command to the serial interface.

Step 2. Create a Frame Relay map class for each VC that you want to apply shaping to. Multiple VCs can use the same map class. Use the frame-relay class class_name_1 [in out] command under each VC. Use the global command map-class frame-relay class_name_1 to define the map class. When configuring the map class, the in/out options are optional. If it is omitted, the value applies to inbound and outbound traffic.

Step 3. After you are in the map class configuration mode, configure the following options:

frame-relay adaptive-shaping[becn foresight] ” You will be using BECN shaping; foresight is available if you are connecting to a Cisco IGX or BPX switch.

frame-relay cir [in out] bps ” Set this speed to be the physical port speed of the circuit.

frame-relay bc [in out] bps ” This is the amount of data sent per interval. A good number to use is 1/8 CIR of the remote circuit.

frame-relay be out bps ” This is the number of extra bits remaining from previous bursts that will be sent on the first interval. This should not exceed the port speed of the remote router.

frame-relay mincir[in out] bps ” This is what the VC will throttle down to when it receives BECNs. Set this value to the actual CIR that the carrier provides to you.

(Optional) frame-relay traffic-rate cir peak _speed ” You can use this command as a shortcut for Steps 2 through 5. For CIR, use the value that the carrier has subscribed on the link. The peak speed should not exceed the physical link speed of the remote router.

Step 4. (Optional) Apply any custom queuing or priority queuing to the map class, not to the interface. FIFO is the default queuing mechanism; apply custom queuing or priority queuing only if you have to. Use the following commands:

  frame-relay custom-queue-list   list_number   frame-relay priority-group   list_number  

Practical Example: Configuring Frame Relay Traffic Shaping

Figure 5-6 shows a Frame Relay point-to-point network.

Figure 5-6. Frame Relay Traffic Shaping

In this model, you want to prevent the marlin router from flooding the glock router's 64-kbps PVC. At the same time, you want to shape traffic to the sig router. The port speed on the marlin and sig router is 1.544 Mbps, and it is 64 Kbps on the glock router. The PVC between the marlin and glock routers has a 32-kbps CIR set by the carrier. The PVC between the marlin router and the sig router is 512 kbps.

Example 5-16 shows the first portion of the configuration for the marlin router.

Example 5-16 FRTS: marlin Router Configuration

 marlin(config)#  int serial 0   marlin(config-if)#    frame-relay traffic-shaping        Enable FRTS   marlin(config-if)#  int s0.1    marlin(config-subif)#frame-relay class 64kb  Set map class   marlin(config-subif)#  exit  marlin(config)#  int s0.2   marlin(config-subif)#    frame-relay class t1  Set other map class   marlin(config-subif)#  ^Z  

Now, you can define two map classes, one called 64kb and the other called t1. Example 5-17 demonstrates the setting of a map classes.

Example 5-17 Setting the Map Clas

 marlin(config)#  map-class frame-relay 64kb   marlin(config-map-class)#    frame-relay adaptive-shaping becn  Enable BENC response     mode    marlin(config-map-class)#    frame-relay cir 1544000  Set to physical port speed    marlin(config-map-class)#    frame-relay bc 8000  set to remote port speed/8    marlin(config-map-class)#    frame-relay be 64000  Initial burst    marlin(config-map-class)#    frame-relay mincir 32000  Carrier enforced CIR   marlin(config-map-class)#  exit  marlin(config)#map-class frame-relay t1  marlin(config-map-class)#  frame-relay adaptive-shaping becn  Enable BENC response     mode    marlin(config-map-class)#  frame-relay cir 1544000  Set to physical port speed    marlin(config-map-class)#  frame-relay bc 8000  set to remote port speed/8    marlin(config-map-class)#  frame-relay be 64000  Initial burst    marlin(config-map-class)#  frame-relay mincir 512000  Carrier enforced CIR   marlin(config-map-class)# 

To verify your configuration, use the show traffic-shape command and add the DLCI number to enhance the show frame-relay pvc dlci_number command. Example 5-18 lists the output of these commands, respectively.

Example 5-18 show traffic-shape and show frame-relay pvc Command Output

 marlin#  show traffic-shape  Access Target    Byte   Sustain   Excess    Interval  Increment Adap t I/F         List   Rate      Limit  bits/int  bits/int  (ms)       (bytes)  Acti ve Se0                56000     7875   56000     56000     125       875       BECN  Se0.1              1544000   10412  8000      64000     12        2412      BECN   Se0.2              1544000   10412  8000      64000     12        2412      BECN  marlin# marlin# marlin#  show frame pvc 120  PVC Statistics for interface Serial0 (Frame Relay DTE) DLCI = 120, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial0.2   input pkts 904           output pkts 3229         in bytes 94596   out bytes 477394         dropped pkts 0           in FECN pkts 0   in BECN pkts 0           out FECN pkts 0          out BECN pkts 0   in DE pkts 0             out DE pkts 0   out bcast pkts 3204       out bcast bytes 474980  Shaping adapts to BECN  pvc create time 19:25:28, last time pvc status changed 19:16:38  cir 1544000   bc 19300     be 64000     limit 10412  interval 12   mincir 512000    byte increment 2412  BECN response yes  pkts 3160      bytes 468932    pkts delayed 0         bytes delayed 0   shaping inactive   Serial0.2 dlci 120 is first come first serve default queueing   Output queue 0/40, 0 drop, 0 dequeued marlin# 

With the aid of these commands, you can verify that you entered the configuration properly. In this example, shaping is listed as inactive because no BECNs have been received from the frame switch.