These notes are taken from “Cisco IOS Quality of Service Solutions Configuration Guide” Release 12.4T.
In general, edge routers perform the following QoS functions:
- Packet classification and marking
- Admission control
- Configuration management
In general, backbone routers perform the following QoS functions:
- Congestion management
- Congestion avoidance
The order of the QoS process is to act on traffic in this order:
- Congestion Management/Queueing
- Congestion Avoidance
- Police and Shape
Classification — Is the process of organizing traffic into classes or categories based on different criteria. This is important because it is the foundation upon which many QoS features are based.
- IP Precedence
- Precedence bits are a subset of of the DSCP and is comprised of the upper three bits of the ToS byte.
- Precedence bits can define eight classes of service (0-7) but numbers 6 and 7 are reserved for network control.
- Placed as close to the edge as possible to enable more congested aggregation switches to use the classification.
- Precedence can be set by end user devices or devices outside the network’s trust boundary, therefore you should reset precedence of packets upon entry into your network, preventing users from acquiring better service for their traffic.
- Committed Access Rate (CAR)
- CAR can be used to set IP precedence.
- Implements both classification and policing through rate limiting.
Congestion Management — Trying to control congestion after it happens. Here are some of the ways in which you can manage this on Cisco IOS:
- FIFO queueing
- Priority queueing (PQ)
- Custom queueing
- Weighted fair queueing (WFQ)
- Distributed WFQ (DWFQ)
- Low Latency Queueing (LLQ)
- Distributed Low Latency Queueing (DLLQ)
Congestion Avoidance — Attempts to monitor traffic trying to avoid congestion before it becomes a problem. This provides differential treatment to minimize impact to time critical traffic flows.
- Weighted Random Early Detection (WRED)
- Distributed Weighted Random Early Detection (DWRED)
- flow-based WRED
- DiffServ WRED
Traffic Policing — Implemented through the rate-limiting aspects of CAR.
Traffic Shaping —
- Generic Traffic Shaping (GTS)
- Class-Base Shaping
- Frame Relay Traffic Shaping (FRTS)