Avaya 555-245-600 manual Assumptions for Example, High-quality service across a congested WAN link

Examples of QoS implementation

Figure 92: High-quality service across a congested WAN link

CB-WFQ/LLQ is a priority-aware queuing strategy that has a strict priority queue for voice packets, and does round-robin queuing for other types of traffic. Non-prioritized traffic is still forwarded, however, so this should not interfere with a customer’s data network. Use weighted random early detect to manage the fair queue.

The actual router configuration used for this testing follows. First, set the endpoints to tag interesting traffic as DSCP 46. Cisco routers support DiffServ in IOS 12.0 and later. Next, set up a class map to match traffic that is marked with DSCP 46. Once traffic is defined by the class map, set policies for it using a policy map. For the policy map to take effect, it has to be applied to an interface. Queue packets on the outgoing interface. In the sample configuration, 768 K of bandwidth is reserved for RTP. This value should be set at or above the maximum bandwidth to be used for IP Telephony. In our case, 768 K supports 9 calls using G.711, or 31 calls using G.729. This example should work well in most cases using Cisco routers with point-to-point WAN links. Networks that use Frame Relay might need additional steps.

Assumptions for Example 1

Suppose all endpoints are capable of tagging with DSCP 46, which is the default for audio. This would be true in a Communication Manager system with TN799DP C-LAN circuit packs running firmware v5 or later. Previous firmware versions and the TN799C circuit pack cannot tag at Layer 2 or Layer 3. A matching set of configurations is applied to both routers.

Issue 6 January 2008 333