When you're configuring quality of service (QoS) on a given link, you should keep the amount of priority traffic relative to the total bandwidth in mind. Many components of a typical QoS scheme can reserve bandwidth on a link. For instance, this could be done via the configuration of Low-Latency Queuing (LLQ) or Class-Based Weighted Fair Queuing (CBWFQ), or it could happen automatically with RSVP.
Obviously, most routers will object to any attempt to configure a reservation with more than 100% of the line, just as they would deny RSVP requests after the line is full. However, unbeknownst to many, they begin these objections and denials much sooner. For instance, in Cisco IOS, by default, you can't reserve more than 75% of the line speed.
Naturally, while this is a good rule of thumb (it protects you by allowing 25% of the line speed for lower-layer headers and traffic that's not classified), there will be times when you might want to change this. As an example, if you are using more headers than normal (e.g. ISL trunking or VPN tunnels) you might want to reduce this number to compensate. Alternately, you may be able to raise this number a little if a particular link is dedicated to high-priority traffic. An example of this might be an interface that connects to a switch that's dedicated to IP Telephony servers.
To adjust this number in Cisco's IOS, use the command "max-reserved-bandwidth," but remember that by adjusting this too high, you can starve lower-priority traffic in some cases.
Thomas Alexander Lancaster IV is a consultant and author with over 15 years experience in the networking industry, focused on Internet infrastructure.
