Class Based Weighted Fair Queuing / Low Latency Queue CBWFQ/LLQ

CBWFQ/LLQ is the best practice for implementing QoS on WAN networks at the core edge routers. The configuration is based on the assumption that you established trust boundaries on the network infrastructure and marked traffic at the access layer according to the following template:

This information concentrates on voice traffic only. All PureConnect voice endpoints can mark their packets. Assuming there are no QoS problems on the LAN side, the issue arises on the edge devices where bandwidth is limited when connecting to a service provider or any other edge device of another network.

Reclassification, queuing, and congestion avoidance using CBWFQ/LLQ with a WRED tail drop policy is the best recommended configuration for such a scenario. PureConnect products do not use WRED extensions such as ECN.

The following table outlines the best practices for reclassification, queuing, and congestion avoidance:

The following example configuration outlines an implementation on the edge router of the network. The voice traffic is given a LLQ of 25% of the available bandwidth.

1. Make class maps to match network traffic on the edge router.

   Start in privileged exec mode:

   configure terminal
   class-map match-any voice 
   match ip dscp ef
   class-map match-any bulk-data
   match ip dscp af11
   match ip dscp af12
   match ip dscp af13
   class-map match-any transactional
   match ip dscp af21
   match ip dscp af22
   match ip dscp af23
   class-map match-any mission-critical
   match ip dscp af31
   match ip dscp af32
   match ip dscp af33
   class-map match-any routing
   match ip dscp cs6
   class-map match-any voice-signaling
   match ip dscp cs3
   match protocol sip
   class-map match-any net-management
   match ip dscp cs2
   exit

2. Make policy-maps that can handle the reclassification, queuing, and dropping.

   Note:
   The class voice-media and voice-signaling together get 25% priority (21+4).

   From the global configuration mode:

   policy-map qos-policy
   class voice-
   priority percent 21 
   class bulk-data
   bandwidth percent remaining 20
   set ip dscp af22
   random-detect dscp-based
   class transactional
   bandwidth percent remaining 15
   set ip dscp af32
   class mission-critical
   bandwidth percent remaining 30
   set ip dscp af32
   class routing
   bandwidth percent remaining 5
   set ip dscp cs6
   class voice-signaling
   priority percent 4
   set ip dscp cs3
   class net-management
   bandwidth percent remaining 5
   set ip dscp af21
   class class-default
   set ip dscp 0
   random-detect dscp-based

3. Associate the policy to the WAN interface.

   This example uses a serial link running at T1 speed.

   From the global configuration mode:

   interface serial 1/0
   bandwidth 1544
   service-policy output qos-policy

Call Admission Control (CAC)

CIC has built-in Call Admission Control on the SIP Line. You can use it in situations where you must limit the number of calls that the system places. Adding just one more call can degrade the voice quality of all the other active calls when there is not sufficient bandwidth to support it. The following example shows that we have limited the CIC server to have a maximum of 10 calls associated with this SIP line. This setting does not account for bandwidth. The person doing the implementation must plan for the codec type and layer 2 overheard required.

Note:
CIC cannot guarantee the bandwidth needed for other activities on the WAN in this configuration.