On-Change vs Periodic Telemetry for Device State?

300-420 By islever Editorial Last updated

Describe the impact of model-driven telemetry on the network

Short answer

C An on-change subscription transmits only when the monitored device state changes, limiting shared memory impact while still delivering the required data.

Question

An infrastructure team is concerned about the shared memory utilization of a device, and for this reason, they need to monitor the device state. Which solution
limits impact on the device and provides the required data?
  1. IPFIX
  2. static telemetry
  3. on-change subscription Correct Answer
  4. periodic subscription

Key Takeaways

The test is whether you recognize that operational state data (up/down, adjacency status) changes infrequently, so an on-change subscription keeps device load low, while the trap is choosing a time-based periodic subscription because it feels more 'continuous'.

Cisco model-driven telemetry offers periodic and on-change YANG subscriptions, and this question asks which one lets an infrastructure team monitor a device's state while limiting impact on shared memory. The page establishes that an on-change subscription is the correct answer because it transmits only when the subscribed state value actually changes.

Common mistake: Option D (periodic subscription) is the most common wrong pick — two commentators argue that memory figures 'change very frequently' and that time-interval reporting suits utilization sampling, but the question explicitly targets device state, not a numeric gauge, and periodic pushes data for the entire subscription lifetime regardless of need.

Expert Analysis

Why the Answer Is Correct

An on-change subscription publishes the initial object state once and then sends updates only when a subscribed value changes, so a device whose state is largely static generates almost no telemetry writes or buffer consumption. Because the team's stated concern is shared memory utilization, minimizing the number of pushed updates directly reduces pressure on the device's telemetry buffers and CPU. Operational state leaves (interface up/down, OSPF or BGP neighbor status) are exactly the leaf types designed for on-change detection, so the required data still arrives in near real time. This matches Cisco's model-driven telemetry subscription model, where on-change is the low-impact mechanism for monitoring device state.

Why the Other Options Are Wrong

Option D, a periodic subscription, streams the full subscribed dataset at every configured interval for the lifetime of the subscription, which continuously consumes shared memory and CPU even when nothing has changed — precisely the impact the team wants to limit. Option A, IPFIX, is an IP flow export protocol tied to flow records and sampling, not a YANG subscription for device state, so it neither scopes the data to state nor offers the low-impact delivery described. Option B, static telemetry, refers to polled or fixed-configuration collection approaches rather than an event-driven subscription, and polling repeatedly walks the datastore instead of reacting only to change. None of these three deliver change-triggered, state-specific reporting with the smallest device footprint.

Community Comment Notes

Most commenters converge on C, with goku2020 stating plainly that "C is correct Change-on subscription have less impact" and bogd reducing the logic to "Less impact ==> on-change subscription." akbntc and namibdigger both point to classic on-change use cases such as an interface or OSPF adjacency going down, while namibdigger cautions that this monitors "the amount of memory used which might work with A (in SDA)." The dissenting voices, zzmejce and CKL_SG, argue for D on the grounds that memory is measured in bytes and changes frequently, or that periodic reporting shows utilization "from time to time" — but both readings mistake a numeric gauge for the state data the question actually names. magicrushb76 adds a useful nuance that the first on-change push is an uncontrollable full synchronization, after which only changed rows are sent.

Official Reference

These links open the official vendor documentation in a new tab.

Exam Strategy

Key off the noun in the question: 'state' (up/down, adjacency) points to on-change, while a numeric gauge or rate points to periodic sampling. Whenever a question stresses limited device impact, the event-driven option is almost always the intended answer.

Frequently Asked Questions

Why is a periodic subscription wrong when memory keeps changing?

The question asks to monitor device state (up/down, adjacency), not a numeric gauge. Periodic pushes the full dataset every interval regardless of need, raising shared memory and CPU load.

Could IPFIX be used to monitor shared memory utilization?

No. IPFIX exports sampled IP flow records, not YANG-modeled device state, so it does not provide the targeted, low-impact state data described in the scenario.

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Discussion 19 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 19

  1. goku2020 👍 11
    C is correct Change-on subscription have less impact, more specific to obtain information.
  2. zzmejce 👍 7 Answered D
    Memory usage is measured in bytes and changes very frequently. On-change subscription will send data almost non-stop. In this case periodic change is more suitable.
  3. luisjuradoledesma 👍 4
    C (change-on subscription) looks correct There are two types of subscriptions: periodic and on-change. With periodic subscription, data is streamed out to the destination at the configured interval. It continuously sends data for the lifetime of that subscription. With on-change, data is published only when a change in the data occurs such as when an interface or OSPF neighbor goes down. https://developer.cisco.com/docs/ios-xe/#!streaming-telemetry-quick-start-guide/streaming-telemetry
  4. guerreroa25 👍 3 Answered C
    On-change subscription is the solution that limits the impact on the device and provides the required data. On-change subscription allows for monitoring specific data in real-time, only sending the data when it changes, and not continuously. This can help limit the impact on device resources, such as shared memory utilization.
  5. namibdigger 👍 3 Answered C
    Aggree on C as correct, that's exactly what on-change subscription is meant for (they monitor the state of the device (e.g. operational/non-operational) not the amount of memory used which might work with A (in SDA)
Show 14 more comments
  1. akbntc 👍 3 Answered C
    C (On-Change) is correct solution for monitoring device state, for example, when an interface goes down, or when an OSPF adjacency goes down.
  2. magicrushb76 👍 2
    Answer is C. With on-change subscriptions, the first push update is the entire set of subscribed to data (the initial sychronization as defined in the IETF documents). This is not controllable. Subsequent updates are sent when the data changes, and consist of only the changed data. However, the minimum data resolution for a change is a row. So, if an on-change subscription is to a leaf within a row, if any item in that row changes, an update notification is sent. The exact contents of the update notification depend on the transport protocol. https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/prog/configuration/1612/b_1612_programmability_cg/model_driven_telemetry.html
  3. CKL_SG 👍 2 Answered D
    concerned about the shared memory utilization of a device, and for this reason, they need to monitor the device state question mention they are concerned and need to monitor, thus i believe D which is periodic subscription more suitable as they want to know memory utilization from time to time and For periodic subscriptions the update trigger is specified by time interval and an time for the report to be sent. rather than on-change subscription an update trigger occurs whenever a change in the subscribed information is detected
  4. Xavi07 👍 2
    I agree, C is correct. On change subscription
  5. bogd 👍 2 Answered C
    Less impact ==> on-change subscription
  6. SomKeat 👍 2
    answer is C
  7. G_Singh12 👍 1 Answered D
    1. A periodic (sample-based) subscription places a controlled, static timer (like pulling a snapshot every 30 or 60 seconds) over the data path. This ensures that the hardware can gather and stream the memory utilization statistics without overloading the router's shared memory infrastructure or control plane.
  8. Lungful 👍 1 Answered D
    D is correct. Periodic is more suitable for memory usage. "Periodic publication is perfect for counters or measures that constantly change, such as the current bandwidth utilization of your network interfaces." https://community.dataminer.services/simplify-network-operations-with-telemetry-streaming-data/
  9. PicoOstrava 👍 1 Answered D
    Creating an on-change subscription for a data node value that is known to change constantly would send excess data to the MDT stream, and could be detrimental to the data collection process. It is highly recommended to use periodic subscriptions for data node values that are constantly changing.
  10. drinu89 👍 1 Answered C
    On-change telemetry sends updates only when the monitored value changes, rather than continuously polling or sampling at fixed intervals. This minimizes CPU, memory, and bandwidth consumption while still providing immediate visibility when the state changes.
  11. e14efdb 👍 1 Answered D
    This cisco page (use a translator....) is using memory utilization as an example for periodic updates: https://www.cisco.com/c/ja_jp/td/docs/ios-xml/ios/prog/configuration/1611/b_1611_programmability_cg/model_driven_telemetry.html
  12. PicoOstrava 👍 1 Answered D
    If the main goal is to have comprehensive, real-time updates on memory utilization with less impact, D (periodic subscription) could be more suitable since it ensures regular state data collection
  13. NoHombre 👍 1 Answered D
    It is not recommended to use on-change subscriptions for frequently changing data values such as counters incrementing on an interface.
  14. mithradel 👍 1
    D. periodic -> https://www.cisco.com/c/en/us/products/collateral/switches/catalyst-9300-series-switches/model-driven-telemetry-wp.html

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