Only fast mode and active mode use IP Service Level Agreement (SLA) probes. Cisco Performance Routing (PfR) enhances traditional routing methods by dynamically selecting the best path for applications based on network performance. The following three monitoring modes are used by PfR:
- Passive mode
- Active mode
- Fast mode

Active mode relies on IP SLA probes that generate traffic to capture performance metrics. Metrics used by active mode include delay, jitter, mean opinion score (MOS), and reachability. Shortterm monitoring uses the last five probe results; longterm monitoring uses the last 60 probe results.

Fast mode is similar to active mode. Active mode generates IP SLA probes only for the active exit path. By contrast, fast mode continuously generates IP SLA probes for all possible exit paths, not just the active exit path. Fast mode allows route changes to be made within three seconds. However, the performance benefits of fast mode require significant processor overhead; therefore, Cisco recommends that you use fast mode only for performancesensitive traffic, such as Voice over IP (VoIP) or video traffic.

Passive mode does not use IP SLA probes. Instead, passive mode relies on NetFlow to capture performance metrics. Metrics used by passive mode include delay, packet loss, reachability, and throughput. Throughput can be measured for all traffic flows. Delay, packet loss, and reachability can be measured only for Transmission Control Protocol (TCP) flows.