Cisco Certification Exam Review:

WRED, LLQ, And Other Queueing Methods

By Chris Bryant, CCIE #12933

The best queueing solution to help protect against TCP Global Synchronization is Weighted Random Early Detect (WRED). WRED does this by not using Tail Drop, but WRED is effective only on TCP packets. Enable WRED with the random-detect command.

Low Latency Queueing (LLQ) is an excellent solution for voice traffic, since LLQ has a strict priority queue. Priority Queueing also allows for giving priority to voice traffic via the High queue.

Custom Queueing has round-robin service; Priority Queueing does not, which makes packet starvation a possible issue with PQ.

Custom Queueing uses Queue Zero, which is used to transport routing update traffic and other high-priority network control traffic.

Weighted Fair Queueing (WFQ) gives priority to low-volume, interactive traffic. WFQ is the only queueing solution that uses the concept of conversations.

Class-Based Weighted Fair Queueing reserves 25% of an interface's bandwidth for routing updates, etc. To change the amount of reservable bandwidth, use the interface-level max-reserved-bandwidth command.

Both RED and WRED are effective only when the traffic in question is TCP-based.

WFQ will not be the default on any interface running tunnels, bridges, virtual interfaces, dialer interfaces, LAPB, or X.25.

To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com