Application Analysis and Probe Placement
Ensuring timely access to business-critical application servers is what justifies having a network (and an IT staff) in the first place. Effectively monitoring applications means measuring how well key components of the IT infrastructure are performing over time and identifying problems before they affect your business
Probe placement greatly affects the visibility and accuracy of monitoring networks and applications. Network analyzers are distributed, allowing a single console to monitor multiple switch segments through the use of probes. Probe placement for application visibility in such an environment will largely depend upon the organization’s monitoring objectives. The objective of high-level network visibility can be achieved by placing probes at the core, while closely monitoring the user experience is achieved by placing probes at the edge. Each approach has trade-offs and many organizations use a mixed strategy in determining probe placement.
A probe can only “see” traffic that passes through the network segment where the probe resides. The location of the probe can also affect response-time analysis, because the probe performs the time-stamping function that makes accurate analysis possible.
If the probes are placed at the network edge (for example, by monitoring SPAN sessions obtained from switches at the edge), network managers get a delay reading that more closely reflects user-experienced delay. The tradeoff is that significantly more probes are required to monitor all of the transactions occurring across a multi-segment network.
In placing the probes closer to the application servers (for example, by monitoring a SPAN session of the server ports on the core switch), network managers can see all of the transactions with fewer probes. This visibility is important for tracking and reporting performance across an enterprise. While placing probes closer to application servers may minimally mask how users experience delay, the network manager will be able to determine changes in delay, and identify important application performance and use trends.
An organization should base probe placement upon its monitoring objectives. When monitoring applications that are sensitive to real-time delays or with significant amounts of peer-to-peer traffic like VoIP, it makes sense to place probes closer to the network edge. For larger organizations where enterprise-wide reporting is critical, managers should consider placing probes closer to application servers. Often organizations with multiple monitoring priorities, like VoIP and enterprise-wide reporting, will take a mixed approach of deploying probes at the edge to closely monitor VoIP performance as well as next to core to ensure visibility to all transactions.
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