Congestion Avoidance

• Tail Drop
  • Default for all queues.
    • When queue full, new packets trying to enter the tail of queue are denied admission.
  • Tail drop treats all packets equally.
    • No classification is performed.
  • Tail drop can result in global TCP synchronization.
    • Simultaneous drops.
    • Looks like shark tooth.
    • Slow start by all senders at same time until they gradually begin going up again at same time. Then restart process.
• Random Early Detection
  • Selectively drop flows from queue before buffer is 100% full.
  • Goal is to send individual senders into slow start, not all senders at once.
  • Result is more even traffic.
• WRED adds weighting to Random Early Detection.
  • Higher weight means less likely to be dropped.
  • Configured as ‘random-detect’
    • Can be combined with other queueing mechanisms.

Configuration Example:

• Create new Class-map

Create new Policy-map and add Class-map ‘SQL’ to the it.

And above we’re adding a bandwidth guarantee and then specifying ‘random-detect’ for WRED.

Note- A bandwidth command is required to enable random-detect.