Related Documentation
Configuring CoS Congestion Management (CLI Procedure)
An effective congestion management mechanism is imperative to ensure smooth flow of traffic in a network and also to ensure minimum packet drops in the network. Class of service (CoS) provides congestion management methods that enable you to define parameters based on which packets can be dropped when the output queue is full. These parameters vary depending on the EX Series switch that you are using in a network.
You can specify parameters for dropping packets at the [edit class-of-service drop-profiles] hierarchy level and reference them in a scheduler configuration. The parameters that you can specify are fill-level and drop-probability. The first parameter defines the delay-buffer bandwidth, which provides packet buffer space to absorb burst traffic up to the specified duration of delay. When the specified delay buffer becomes full, packets with 100 percent drop probability are dropped from the head of the buffer. The second parameter represents a percentage value that correlates to the likelihood that an individual packet is dropped from the network.
Depending on the switch on which you are configuring a drop profile, you can configure either a weighted tail drop (WTD) profile or a weighted random early detection (WRED) profile.
This topic describes:
Configuring a Weighted Tail Drop Profile
A weighted tail drop (WTD) is a congestion management mechanism in which packets are dropped from the tail of the queue when the queue reaches a certain buffer capacity (that is, the fill level), and hence the name weighted tail drop. When that level is reached on EX2200, EX3200, EX3300, EX4200, EX4500, EX4550, and EX6200 switches, packets marked with a packet loss priority (PLP) of high are prevented from entering the queue (that is, they are discarded).
To configure a WTD profile, create a drop profile name and assign a fill level:
[edit class-of-service drop-profiles]
user@switch# set profile-name fill-level percentage Following is a sample WTD profile in which the fill level is set to 80 percent:
[edit class-of-service drop-profiles]
user@switch# set wtd-profile fill-level
80Configuring a Weighted Random Early Detection Drop Profile
A WRED drop profile enables you to define multiple data points for fill level and drop probability so that packets are dropped at various levels of queue fullness and for various drop probabilities. Unlike the WTD drop profile that can be defined only for packets with a PLP of high, WRED can be defined for packets with a PLP of high and also for packets with a PLP of low.
 | Note: The WRED drop profile is supported only on EX8200 standalone switches and EX8200 Virtual Chassis. |
WRED has two implementations: segmented and interpolated. From a high level, segmented WRED is a stair-step-like drop profile, whereas interpolated WRED is a smoother (curved) drop profile. For a graphical representation of both these implementations, see Understanding Congestion Management in CoS. Although the formation of graph lines is different for both these implementations, the application of the profile is the same. When a packet reaches the head of the queue, a random number between 0 and 100 is calculated. This random number is plotted against the drop profile using the current queue fullness of that particular queue. When the random number falls above the graph line, the packet is transmitted. When the number falls below the graph line, the packet is dropped from the network.
Following is the procedure to define a segmented and an interpolated drop profile:
- To configure a segmented drop profile, specify multiple
data points for fill level (l) and drop probability (p) as follows:
[edit class-of-service drop-profiles]
user@switch# set profile-name fill-level percentage-l1 drop-probability percentage-p1
user@switch# set profile-name fill-level percentage-l2 drop-probability percentage-p2
user@switch# set profile-name fill-level percentage-l3 drop-probability percentage-p3
user@switch# set profile-name fill-level percentage-l4 drop-probability percentage-p4The following is an example of the configuration of a segmented drop profile:
[edit class-of-service drop-profiles]
user@switch# set seg-prof fill-level 20 drop-probability 25
user@switch# set seg-prof fill-level 40 drop-probability 50
user@switch# set seg-prof fill-level 60 drop-probability 75
user@switch# set seg-prof fill-level 80 drop-probability 100 - To configure an interpolated drop profile, specify multiple
data points for fill level (l) and drop probability (p) using the interpolate statement as follows:
[edit class-of-service drop-profiles ]
user@switch# set profile-name interpolate fill-level percentage-l1 drop-probability
percentage-l1
user@switch# set profile-name interpolate fill-level percentage-l2 drop-probability percentage-l2
user@switch# set profile-name interpolate fill-level percentage-l3 drop-probability percentage-p3
user@switch# set profile-name interpolate fill-level percentage-l4 drop-probability percentage-p4The following is an example of the configuration of an interpolated drop profile:
[edit class-of-service drop-profiles]
user@switch# set inter-prof interpolate fill-level 20 drop-probability 25
user@switch# set inter-prof interpolate fill-level 40 drop-probability 50
user@switch# set inter-prof interpolate fill-level 60 drop-probability 75
user@switch# set inter-prof interpolate fill-level 80 drop-probability 100
