Quality of service enabling on Cisco routers |
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EXPERT RESPONSE FROM: Greg _Ferro
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QUESTION POSED ON: 30 April 2001
Which Cisco series routers have QoS enabling?
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This is a good question. The trick here is to understand that there are several different types of QoS and that not all features are available on all Cisco platforms. Let me attempt to explain a pretty complicated area. I can't possibly explain the in and outs of QoS here, but I'm willing to give it a shot...
Types of Traffic Shaping
There are seven basic QoS tools in Cisco routers:
- Weighted Fair Queueing (WFQ)
- Weighted Random Early Discard (WRED)
- Priority Queuing (PQ)
- Custom Queueing (CQ)
- Class Based Weighted Fair Queueing (CB/WFQ)
- Traffic Shaping -- Generic and Frame Relay
- Committed Access Rate
Now, one and two are buffer management techniques, as such, they only work when the buffer is full, i.e. you are overloading the interface buffer.
So WFQ and WRED are mechanisms for handling congestion. Specifically, WFQ does some fancy math on the buffer contents and "weights" certain types of traffic to get preference. This is the default for all interfaces in Cisco routers up to 2Megabits of bandwidth.
WRED attempts to slow down the senders by intelligently dropping packets. By dropping packets early and for certain conversations, the overall buffer performance improves. When you enable WRED, WFQ is disabled. This is because WRED tries to avoid congestion rather than manage congestion.
PQ and CQ are mechanisms for policing and allocation.
Priority Queueing is designed to give strict priority to important traffic. PQ ensures that important traffic gets the fastest handling at each point where PQ is used. The problem? If there is too much high priority traffic, the low priority gets chucked out.
Custom Queueing reserves a percentage of the available bandwidth of an interface for each selected traffic type. If a particular type of traffic is not using the bandwidth reserved for it, then other traffic types may use the remaining reserved bandwidth. The problem? There is no way of using WFQ or WRED inside a Custom Queue. So when the bandwidth overloads, you can't intelligently manage the buffer.
Class-based WFQ (CBWFQ) extends the standard WFQ functionality to provide support for user-defined traffic classes. It allows you to specify the exact amount of bandwidth to be allocated for a specific class of traffic. Taking into account available bandwidth on the interface, you can configure up to 64 classes and control distribution among them. Now this is basically Custom Queueing with Weight Fair Queueing added in. Just to complicate things, there are a number of knobs to handle special traffic, like Low Latency Queueing for voice, Priority Queues and other really hardcore stuff.
Now this is all really good, but for some people, you need to be able to say I want Y amount of bandwidth for this, and X amount of bandwidth for that. This is known as policing and we use both Committed Access Rate and Traffic Shaping to do this.
GTS provides a mechanism to control the flow of outbound traffic on a particular interface. FRTS provides parameters that are useful for managing network traffic congestion. Now, both GTS and FRTS are pretty good, but they have some limitations. They don't classify traffic well and they use some inefficient mechanisms.
So Cisco came up with Committed Access Rate. Basically, CAR is the Swiss Army knife of traffic shaping.
Which Traffic Shaping is Where?
Here are some pretty rough and ready rules about what runs where. A Cisco router is software and hardware. Some of these tools are straight software, so things like WFQ, WRED, PQ, CQ, GTS and FRTS are all software based. They are in every model of Cisco router that runs IOS. This is the "older" generation of tools.
The new tools, CAR and CBWFQ, are linked to hardware within the router. This hardware was only introduced in the recent Cisco routers like 26xx, 36xx and the more recent 7200VXR and 7500 VIP modules.
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