3Com 3824 Implementation Guide - Page 57

Important QoS Considerations 57 ■ Has eight traffic queues, but it is important to note that not all Switches have the same number of priority queues. ■ QoS is about providing a consistent, predictable data delivery service. It should not be used as an alternative to deploying sufficient bandwidth. The recommended configuration for most networks is 10/100 Mbps switching to the desktop, Gigabit connections for servers, and non-blocking Gigabit backbones. ■ QoS requires the support of every network device from end-to-end. All devices in the network should support QoS. If there is just one section in the data path that does not support QoS, it can produce bottlenecks and slowdowns, although a performance improvement will be noticed over the parts of the network that do support QoS. ■ Ensure that all QoS devices are configured the same way. Mismatches will cause the same traffic to be prioritized in one section and not in another. ■ Only use Switches or hardware-based routers in the LAN. Hubs cannot prioritize traffic, and software-based routers can cause bottlenecks. ■ Use Switches and hardware-based routers that understand both the IEEE 802.1D (incorporating IEEE 802.1p) and DSCP marking schemes. ■ Classify traffic as soon as it enters the network. If traffic is not classified until it gets to the WAN router or firewall, end-to-end prioritization cannot be guaranteed. The ideal place for traffic classification is within the Switch. ■ Traffic Marking is performed as a result of classification, and so you should aim to perform the marking only once to reduce the additional requirements that QoS places upon the capabilities of your network infrastructure. ■ As DSCP uses a field in the IP header, it is only possible to use the DSCP in IP packets. It does not apply, for example, to AppleTalk, IPX or NetBEUI. ■ Because DSCP is a redefinition of the use of the TOS byte in the IP header, there are some issues with interaction with IP TOS based networks.