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Examples of QoS implementation
Figure 85: 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 3.4.1 June 2005
329
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