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Intermittent Connectivity

Symptom: internet works sometimes, drops for seconds-to-minutes, then comes back. Not a hard failure; specific times of day, or on specific apps.

Most Likely Causes

  1. WAN link quality — your ISP is the problem; intermittent loss.
  2. SD-WAN flapping — SLA thresholds too tight; member oscillates in/out.
  3. DHCP renewal — WAN gets DHCP from ISP; renewal hiccup.
  4. DNS resolver hiccups — primary DNS slow / intermittent.
  5. Firewall session timeouts — long-idle sessions evicted, app reconnects.
  6. PoE / hardware — overloaded switch starves devices.

Diagnose

# Long-running ping with stats:
execute ping-options repeat-count 1000
execute ping 8.8.8.8

# Check for ISP issues over time — sustained ping from FortiGate:
execute ping-options view-settings
execute ping-options interval 1
execute ping-options repeat-count 600
execute ping 8.8.8.8

Lost packets in batches = ISP / WAN issue.

Check SD-WAN flap history

diagnose sys sdwan health-check
diagnose sys sdwan service

Frequent state changes = SLA tuning needed.

Check interface drops

get system interface physical
# Look for drop counts and CRC errors growing.

diagnose hardware deviceinfo nic <interface>
# Detailed NIC stats: collisions, runt frames, misalignment.

CRC errors / runt frames = bad cable, bad SFP, or speed/duplex mismatch.

Check system log for events

Log & Report → System Events — filter for "link," "interface," "sdwan."

Fixes by Cause

  • Document the loss (ping logs).
  • Call ISP with the data. They usually need objective measurement.
  • Use SD-WAN with second WAN as backup.

SD-WAN flap

DHCP renewal

# Force renewal:
execute interface dhcpclient-renew <interface>

Set a static IP if your ISP allows.

DNS issues

Configure two reliable resolvers (Cloudflare + Google), not the ISP's defaults.

Hardware

Replace cable, swap SFP, force-set speed instead of auto-negotiate.