Understanding the default behaviour of the WRED and Tail drops on Juniper routers.

Overview

By default, Juniper devices utilize Weighted Random Early Detection (WRED) on queues to manage traffic and prevent congestion. WRED proactively drops packets before the queue reaches full capacity, which helps maintain better queue stability and reduces the likelihood of tail drop.

What is WRED

WRED is a queuing mechanism that monitors the average queue size. As the queue size increases and approaches its configured threshold, the probability of dropping packets also increases. When the queue reaches a fill level of approximately 98.4% (due to Trio ASIC granularity on MX routers), WRED will result in a 100% drop rate to prevent congestion. This mechanism is particularly effective for managing TCP traffic, as it selectively drops packets to trigger congestion avoidance mechanisms.

 Key Points about WRED:

- Proactive Congestion Management: WRED helps avoid congestion by dropping packets before the queue is full.

- Threshold Monitoring: The drop probability increases as the queue size approaches its threshold.

- TCP Traffic Optimization: By dropping packets selectively, WRED facilitates better traffic flow and stability.

The behaviour of WRED and tail drop can vary based on the line cards used in both MX and PTX series routers.

 PTX Series Routers

- WRED Configuration: On PTX series routers, WRED can be explicitly configured for congestion avoidance, although it is typically not enabled by default.

- Tail Drop Behavior: By default, tail drop is the primary method for dropping packets based on Class of Service (COS) configurations when the queue is full. If configured, RED will drop packets based on the average queue length.

 MX Series Routers

- Default WRED: WRED is enabled by default on MX series routers.

- Tail Drop Activation: Tail drop occurs when WRED reaches its maximum drop probability or when the queue is full.

 Summary

In summary, WRED is an effective tool for managing congestion on Juniper devices, particularly on MX series routers, which are enabled by default. For PTX series routers, while WRED can be configured, tail drop typically serves as the default method of packet dropping. Understanding the behaviour of these mechanisms is crucial for optimizing network performance and ensuring efficient traffic management.

 Recommendations

- For MX Routers: Leverage WRED to maintain queue stability and optimize TCP traffic flow.

- For PTX Routers: Consider configuring WRED or RED based on your network's specific needs for proactive congestion management.

Below are the KB links for your reference to achieve the tail drops:

https://supportportal.juniper.net/s/article/Tail-Dropped-PacketsObserved-on-interface

https://supportportal.juniper.net/s/article/MX--Achieving-and-Verifying-Tail-Drops-on-the-MX-Device