3550 Series Switches

3550 Series Switches
The 3550 supports an entirely greater range of QoS options because of the layer 3 capability of
the hardware and the IOS. There is, in fact, a good case for referring to the 3550 as a multiport
router with layer 2 capabilities rather than a switch with layer 3 capabilities.
Essentially, the combined layer 2 and layer 3 QoS functionality means that the switch can
classify traffic using sophisticated access lists, mark at both layers, forward using either DSCP
or 802.1p priority bits, and even translate from one to the other. This combined functionality
involves accepting the default mapping that places DSCP traffic into Ethernet frames with a
closely related CoS (or maps the IP datagram inside an incoming Ethernet with CoS set to the
IP datagram itself as a DSCP). If the defaults are not suitable for your network, you can use an
mls qos-map command to establish your own translation values.
This large range of options means that we have to restrict ourselves a little, because the
subject of QoS as applied by routers and other layer 3 devices is large enough to warrant a
book all by itself. In fact, it is one of the core subjects of a new advanced Cisco certification,
the CCIP (Cisco Certified Internetwork Professional). So, to stay on target, we will concentrate
our efforts on those configurations that are likely to appear on the BCMSN exam.