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| 7.5 |  | Voice Quality Issues |
| 7.5.7 |  | Traffic classification at Layer 3 |

Using the 802.1p bits within the 802.1Q tag provides the desired QoS results at Layer 2. When traffic has to cross a Layer 3 boundary, however, it becomes imperative to implement these mechanisms using Layer 3 parameters, such as the 3 IP precedence bits (commonly referred to as ToS) or the new DSCP parameter, which uses the six most significant bits within the ToS byte of the IP header. Traffic crosses a Layer 3 boundary when packets are routed between subnets by Layer 3 switches or routers. Traffic also crosses a Layer 3 boundary when packets need to go out of the campus network onto the WAN through edge routers. When this happens, Layer 2 classification does not help. Layer 3 classification is needed for achieving the desired level of QoS. All of the QoS techniques employed by the routers (including the very important WAN QoS) rely on Layer 3 classification.

Layer 3 classification can be achieved by using the appropriate platforms in the campus. Beginning with the IP phones, packets are already presented to the switch with CoS = ToS = 5. This Layer 3 classification is preserved even if the packets travel all the way through to the WAN edge router where the Layer 2 header is removed. So, if the trust boundary is at the source (IP phone), voice traffic has the ToS bits set to 5 and is presented to the network devices for appropriate treatment. WAN routers can use this classification to employ any of the queuing techniques. If the trust boundary is not at the source and packets need to be reclassified, then the device performing this function should be capable of doing it at Layer 3 before it can cross a Layer 3 boundary.

Summary recommendations for QoS implementation:

- Create a trust boundary at the network edge in the wiring closet. Make ports trusted on the wiring closet switch where IP phones are attached.
- Reclassify ToS at the edge if devices cannot be trusted.
- Shrink the trust boundary to the distribution layer and reclassify ToS there if reclassification is not possible at the edge.
- Use a priority queue if possible for delay-sensitive traffic.
- Use QoS ACLs for more granular classification of packets using Layer 4 information.
- Use policing if necessary to limit traffic for individual flows as well as aggregate flows.
- Have traffic going to the WAN edge classified at Layer 3 so that the router can use it for advanced WAN queuing mechanisms.
- Use a WAN edge router as the classifier for very small remote site networks where a Layer 3 capable switch is not available.