Frame-Relay and traffic shaping (FRTS)

Frame-Relay congestion control can be done on a DLCI basis. Real flow control is done by upper layer protocols. Remember frame-relay is just a layer 2 protocol. So frame-relay just throws packets away and only notifies about congestion. This could be done, for example, by TCP.

So with frame-relay the mechanisms are called congestion-notification. So if a sender is sending to quick, frame-relay is sending a congestion-notification to the sender to slow down. Frame-relay in the meantime, just throws away packets. If a packet is lost, the receiving device has to request e retransmition.
The following terms are important.

  • FECN – Forward-explicit congestion notification
  • BECN – Backward-explicit congestion notification
  • DE – Discard Eligibility
  • CIR – Committed Information Rate

Both informations FECN and BECN, are carried as single bit in the frame-relay header.

The header also includes a DE bit, which is responsible for dropping and marking no to important traffic in times of congestion. A frame with the DE bit set, has low priority and can be dropped in case of congestion.

CIR is an agreement about the normal transmission rate over a link in a certain time.

BECN – indicates to the receiving device that frames sent in the opposite direction may experience congestion.

FECN – indicates to the receiving device that the frame has experienced congestion.

DE – discard eligible, which mean, when the DE bit is set in a frame, the packet could be selected for discarding, it’s even possible to set this DE bit, for certain protocols

Terms to know about frame-relay traffic shaping and you can configure them on an interface dlci with a map-class command:

  • EIR is the rate that you can exceed CIR before the ingress port will discard your frames.
     
  • CIR – Committed information rate

is the average rate that you can transmit at measured over a time period Tc without having the ingress port on the Frame network flag your frames as over contract by setting the DE bit. If you have multiple connections, the aggregated total of the CIR for all PVC’s should not exceed the physical port speed.

  • BC – committed burst

is the maximum amount of data that a network will transfer in one interval of time (Tc) or in other words is the total number of bits that you can send in period Tc and not have the frames marked with DE.

  • BE – Burst in excess

is the maximum amount of data that the network will transfer above Bc in one interval of time. When transmitting data at excess burst (Be) these frames would either be dropped by the network at ingress or marked discard eligible (DE) and potentially dropped somewhere else in the network if congestion is experienced

or you can say is the total number of bits in addition to Bc that you can send in period Tc that the switch will allow before discarding any excess traffic at the ingress port to the Frame network.

  • MINCIR – minimum committed information rate

The actual guaranteed rate obtained from service provider in bps. This value should be the minimum rate you should drop to in the event of congestion (dropping below this rate implies you are not getting the bandwidth you are paying for). In certain cases the mincir and cir values must be the same. The value of mincir is half of the CIR value in bps by default.

  • TC the default is 1/8 = 0.125 seconds, BC=CIR*TC

For example, if you have a CIR of 64000 Bits per second and want for voice a TC of 10 ms (0,01) or 1/100, than BC would be 640 bits. 10 ms is the smallest value for TC.

Parameters you configure:

Example 1.

1. setup the interface for shaping

interface Serial0/0
ip address 152.2.0.1 255.255.255.0
encapsulation frame-relay
frame-relay traffic-shaping
frame-relay interface-dlci 103
class to-router3

2. define a map class containing shaping parameters

map-class frame-relay to-router3
frame-relay cir 32000

32000 Bits per second is the average amount of data on the agreed rate with the service provider. The default an an interface is 57000 bps. Make sure, that the sum of all cir’s configured for one interface does not exceed the bandwidth of the interface. If your remote router does only 16000 Bits per seconds, this CIR rate would be „frame-relay cir 16000“.

Example 2.

1. Setup the interface for frame-relay traffic-shaping

interface Serial0/0
ip address 152.2.0.1 255.255.255.0
encapsulation frame-relay
frame-relay traffic-shaping
frame-relay interface-dlci 103
class to-router3

2. define a map class containig shaping parameters

map-class frame-relay to-router3
frame-relay cir 32000
frame-relay bc 320

So here i use a TC of 10 ms or 0,01 seconds for the configured BC. The advantage is that the router is more able to prioritize certain traffic, would be needed for voice. Because the shorter choosen intervals per second.

CIR = BC/TC

32000=BC/0,01

=> 320=BC

Example 3.

Your Frame-Relay provider gives you a line with a 512k circuit and a CIR of 384k to your network. You would configure it like this.

interface Serial0/0
ip address 152.2.0.1 255.255.255.0
encapsulation frame-relay
frame-relay traffic-shaping
frame-relay interface-dlci 103
class to-router3

map-class frame-relay to-router3
frame-relay cir 512000
frame-relay mincir 384000

frame-relay bc 64000
frame-relay be 128000

Whitch calculates as follows:

CIR = port speed
mincir = commited rate from prvider
be= port – commited rate
bc = port speed / 8

These parameters are inter-related.

CIR = Bc/Tc

BC = CIR * Tc

TC = BC / CIR

BE = Portspeed – CIR

EIR = Be/Tc

Source:

http://www.groupstudy.com/archives/ccielab/200209/msg01962.html

http://www.cisco.com/warp/public/125/traffic_shaping_6151.html

http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/frame.htm#wp1020620

http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124cg/hwan_c/ch05/h_afrts.htm#wp1015437

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