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[Cisco] How to limit rate on interface ? PDF Print E-mail
Written by Chris   

The rate-limit interface configuration command used to use to setup committed access rate (CAR) and distributed CAR (DCAR) policies. To remove the rate limit from the configuration, use the no form of this command.
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Weighted RED (WRED)? PDF Print E-mail
Written by Chris   

The WRED algorithm provides congestion avoidance on network interfaces by providing buffer management, and by allowing Transmission Control Protocol (TCP) traffic to throttle back before buffers are exhausted. This helps avoid tail drops and global synchronization issues, maximizing network usage and TCP-based application performance. WRED works by selectively dropping packets before congestion occurs, so it is considered to be a congestion avoidance feature
  • WRED is a congestion avoidance mechanism, based on the adaptive nature of TCP traffic for congestion.
  • WRED allows for differentiated dropping behavior based on either IP precedenceor DSCP.
  • WRED is configurable in a CBWFQ policy-map.
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Random Early Detection (RED)? PDF Print E-mail
Written by Chris   

Random Early Detection (RED) is a congestion avoidance mechanism that takes advantage of TCP's
congestion control mechanism.
When it comes to Quality of Service, there are 2 separate approaches. The first is congestion management, which is setting up queues to ensure that the higher priority traffic gets serviced in times of congestion. The other is congestion avoidance, which works by dropping packets before congestion on the link occurs.
RED takes a proactive approach to congestion. Instead of waiting until the queue is completely filled up, RED starts dropping packets with a non-zero drop probability after the average queue size exceeds a certain minimum threshold. A drop probability ensures that RED randomly drops packets from only a few flows, avoiding global synchronization. A packet drop is meant to signal the TCP source to slow down. Responsive TCP flows slow down after packet loss by going into slow start mode.

Reference: ‘IP Quality of Service’ from, Cisco Press
 
CAR(Committed Access Rate) ? PDF Print E-mail
Written by Chris   

Committed Access Rate (CAR) is used to rate limit traffic. In this example, all ICMP traffic that exceeds the defined level will be dropped. This will prevent an ICMP flood attack from saturating the link.

CAR definition: Rate limiting is one mechanism to use to allow a network to run in a degraded manner, but remain up when it is receiving a stream of Denial of Service (DoS) attack packets as well actual network traffic. Rate limiting can be achieved in a number of methods using Cisco IOS® software. Namely, through Committed Access Rate (CAR), Traffic Shaping, and both Shaping and Policing through Modular Quality of Service Command Line Interface (QoS CLI).
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Basic QoS PDF Print E-mail
Written by CiscoNET   

  • There are 4 priority queues: high, medium, normal, low.
  • The high and medium queues have precedence over the default queue.
  • The classification is configurable via the command ‘priority-list’
  • The default queue is the normal queue, by default.
 
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