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.
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