Exam Essentials

Understand what Quality of Service is. QoS is a combination of processes and procedures for trying to enhance the service usually allocated to a frame or packet delivered by a best efforts network. This involves identifying the data, marking it, and then using that marking as a key for how the data will be managed inside queues across a network.

Understand why some applications benefit from QoS. Not every application benefits greatly from applying priorities to its data. Many legacy applications are built to run as store-and-forward flows, and are satisfied with the simple reliability that they get from TCP. E-mail, FTP, and so on do not have the same urgency as mission-critical data with a delay limit. In addition, some applications place considerable demands across the network because although the bandwidth needs may be small, they cannot manage jitter. Defined as the variation in latency between successive frames or packets, jitter spells the death knell for multimedia applications.

Understand what QoS features Cisco switches can support. Not every Cisco switch can support every QoS feature. This is true largely because QoS can be applied at either layer 2 or layer 3. Some Cisco switches are simple layer 2 devices, whereas others have so much layer 3 capability that they could easily be called routers. Obviously, layer 2 switches cannot support layer 3 QoS. Layer 2 QoS is limited to setting and responding to the TOS bits inside the 802.1p extension to 802.1Q. Layer 3 QoS uses either the TOS bits from legacy IPv4, or the newer DSCP implementation of the same field. Both can be mapped to the layer 2 TOS at a device supporting both layers.

Understand how to configure QoS on Cisco switches. There are still different versions of operating systems on Cisco switches. From the 4000 and 6500 running CatOS, to the same switches running IOS, to the 3550 running a full IOS and the 2950 running IOS in either the standard or enhanced image options, many differences occur. Using the basic information regarding layer 2 and layer 3 QoS, you need to be able to configure any of these switches for QoS. Remember that the BCMSN exam has simulations and is new, so there may be more in the future. Simulations carry several extra marks, so make sure you are familiar with all the commands in this book.

Understand how redundancy is achieved using Cisco switches and routers. Redundancy can be applied to many places in the network, but this course, focusing as it does upon the campus network, exposes the fact that most PCs use a default gateway to get off-LAN. This critical device can be a single point of failure. Cisco's HSRP and less commonly VRRP can be used to provide that redundancy. By creating a virtual router IP address and using that as the default gateway, we can configure more than one router to be prepared to forward data sent to the group, with options for prioritized selection and preempting of control. Both SLB and GLBP can be used to load share the cross-router traffic.