ECN Configuration

WRED discards packets and that is one way for the router to indicate a congestion. Routers will be now able to indicate a congestion condition by signaling it with an approach called Explicit Congestion Notification (ECN).

ECN uses the two last bits in the TOS to indicate whether a device is ECN capable , and if so, whether congestion is experienced.

  • If the bit combination is 00 , then the router means that it is not ECN capable .
  • If the bit combination is 01 or 10 , then the router means that it is ECN capable and is not experiencing congestion
  • If the bit combination is 11 , then the router means that it is ECN capable and is currently experiencing congestion

The 7th bit is the ECT bit and the 8th bit is the CE bit.

Cisco routers can use ECN as an extension to WRED and mark packets that exceed a specified value , instead of dropping the packets. So if the queue depth is at or below the WRED minimum threshold, the packets are sent normally,  just as with WRED.

Also if the queue depth is above the WRED maximum threshold, all packets are dropped , just as with WRED.

But if the queue depth is currently in the range between the minimum and the maximum threshold, one of the following action can happen:

  • If both endpoints are ECN capable , ECT and CE bits are set to 1 and sent to the destination, indicating that the transmission rate should be reduced.
  • If neither endpoints support ECN, the normal WRED behaviour occurs.
  • A packet with its ECN and CE bits marked can reach a destination router that has already a full queue. In such an instance, the notification is dropped.

To activate ECN , use the following command under the policy-map : random-detect ecn ( so it is applied under the MQC but you can apply it also under the interface)

Weighted Random Early Detection – WRED

Whereas queuing provides congestion management, mechanisms such as WRED provide congestion avoidance.WRED’s purpose is to prevent an output queue from ever filling to capacity,which would result in packet loss for incoming packets.

To understand what it is WRED ,let’s explain first RED goals, basics

The purposes of RED is to prevent TCP synchronization (all TCP streams in slow start phase) by randomly discarding packets as an interface’s output queue begins to fill.

The agressivity of discarding packets depends on the queue depth and is influenced by the minimum , the maximum threshold and the Mark Probability Denominator(MPD).

So random dropping will happen when you reaches the minimum threshold and it is full when you reach the maximum threshold .

Explanation of RED

Another definition for the MPD is the maximum drop percentage at the maximum threshold.

RED is not supported on Cisco IOS but Cisco has chosen to support CB-WRED which gives you the opportunity to define several RED mechanisms for different DSCP threshold.

Catalyst-Based Queuing – CB-Queuing

Some Cisco Catalystswitches also support a kind of Weighted Round Robin Queuing as the 2950 switch, with 4 queues  in where  you can configure to place frames with specific CoS marking  . You can also assign a specific weight on these queues which will be handled by the round-robin fashion cycle.

On this switch , queue 4 is designated as an “expedite” queue which give priority treatment to frames in that queue.

Please note that if you want to configure this queue as an expedite queue , you must put its weight to 0.

Low Latency Queuing – LLQ

Low Latency Queuing is almost identical to CB-WFQ. However with LLQ , you can instruct one or more class-maps to direct traffic into a priority queue.

Realize that when you place packets in a PQ , you are not only allocation a bandwidth amount for that traffic but you are also performing policing (that is limiting the available bandwidth for that traffic).

Why are we policing ?
This is necessary to prevent higher-priority traffic from starving out lower-priority traffic.

Please also consider that if you give priority to multiple class-maps , the packets assigned to will go into the same queue.Packets which are queued in the priority queue can’t be fragmented , which is a consideration for slower links.

LLQ is the cisco preferred queuing method for latensy-sensitive traffic such as voice and video.

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