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# Flap List - Cable Modems with intermittent connectivity problems

This post is a summary of the flap list feature. It is not necessarily just a Cisco-only feature, even though I will be focussing on Cisco here. There are other vendors such as Casa, Arris, Huawei.. The list goes on.

A flap list keeps track of CMs with connectivity problems (they're flapping back and forth). The flap list can help in deciding between whether there is a problem with the given CM, or whether there is a problem with the upstream or downstream cable.

There are three classes of problems:

  • Reinsertions: A CM re-registers more frequently than a specified insertion time. Too many reinsertions may indicate problems in the downstream cable or that the CM is provisioned wrongly.
  • Hits and misses: A hit occurs when a CM successfully responds to MAC-layer keepalive messages that the CMTS sends out. A miss occurs when the CM does not respond in after a timeout. Too many misses followed by hits may indicate problems in the up/downstream cable.
  • Power adjustments: A CM can adjust their upstream transmission power up to a maximum power level. Too many adjustments may indicate a problem with an amplifier in the upstream direction.

Use cases:

  • If a customer reports a problem, but the flap list does not contain the customer's modem, problems with the cable can be ruled out. The issue is then most likely local.
  • CMs with more than 50 power adjustments a day have a potential issue in their upstream path.
  • CMs with roughly same number of hits and misses and with a lot of insertions have a potential issue in their downstream path.
  • All CMs incrementing their insertion numbers at the same time indicates a problem with the provisioning servers.
  • CMs with high CRC errors have bad upstream paths.
  • Correlating CMs on the same upstream port with similar flap list statistics (same number of hits/misses/insertions) may show cable or node-wide issues.

References: Cisco Flap List pdf (use cases are from this pdf)


KPI (per CM) Description
ccsFlapInsertionFailNum If a CM registered more than once in a certain period (default: 90s), the first registration is considered failed, which increments the insertion fail KPI.
ccsFlapHitsNum CMTS sends request every 10 secs, a successful response within 25ms from CM increases flap hits by one.
ccsFlapMissesNum If the response is completely missing or takes more than 25ms, flap misses increases by one.
ccsFlapPowerAdjustmentNum If upstream power is adjusted more than X dB (default: 1 dB, but they say it often should be more like 6 dB), this KPI is increased.
ccsFlapCreateTime Time when this modem was added to the flap list. After max age (default: 7 days) they get removed again.

If any of the main KPIs (insertion fail, hit/miss, power adjustment) is significantly higher than the others, this is a very important signal.

If hit/miss is highest → the modem keeps going up and down.

If power adjustment highest → "improper transmit power level setting at the modem end" (personal story: I used to have that with M-Net. Internet kept going down completely every few days. They reduced power level (which capped our downstream MBit at a lower level..), and everything was fine)

The point is that these KPIs tell a lot.

Below is an illustration of misses increasing live on some cable modem. The snmpget was executed in the course of an hour.

snmpget -v2c -c 'REDACTED' IP iso.
iso. = Gauge32: 1580

snmpget -v2c -c 'REDACTED' IP iso.
iso. = Gauge32: 1634

snmpget -v2c -c 'REDACTED' IP iso.
iso. = Gauge32: 1650
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