Important things to keep in mind:
- Some switches strip the ToS/QoS tags. If this is the case, then the 3448(s) will not match any IP Precedence-tagged packets. You can show qos map interface eth 0/X (or switchport interface if that's what you're using) to see if matches are incrementing properly.
- Is the uplink speed 10M at both sites? This is an important part. For example, if my Site A has a 10M/2M Internet connection, I need to configure that site's 3448 so that it knows that it has 2M to work with. The 3448 probably links to your ISP modem/whatever at 100M. Unless you specify a traffic-shape rate on that interface, it thinks you have 100M bandwidth. The AOS unit will not take QoS action until it sees that the link has become saturated, so it's important to make it aware that only 2M (in this example) is available. At the interface prompt, use command traffic-shape rate 2M (or whatever matches your actual upload speed). This value may be different at each site. There's a straightforward GUI item for this too.
- QoS is important, but can have diminished benefit when the WAN/Internet connections are used for a mix of low-latency and best-effort traffic. Even when configured properly and functioning at its best, heavy utilization can get in the way. QoS is an egress feature, but traffic sent by the remote site arrives ingress to my site in competition with other download traffic. In cases where Internet use is heavy, there may be little you can do from a QoS configuration standpoint to mitigate poor quality. Private WAN links such as point-to-point T1, MPLS or Metro E provide a way to guarantee toll quality for your calls. A second (less preferred) option is to use separate Internet connections for voice calls between sites, so that they are not congested with general Internet download traffic. In all cases, ISP over-subscription and other factors may cause increased latency or packet loss.
- Voice Quality Monitoring (VQM) is included in the AOS Enhanced Feature Pack (EFP), which you're running to support VPN. VQM is a great way to see what aspects are causing the most trouble and also quantify the frequency.
- Quality of Service in AOS general information; Enhanced Ethernet QoS is also an excellent guide
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