Why does my BLF light keep flashing?
If a message that is supposed to shut down a blinking LED does not reach the phone, it will continue to flash indefinitely. Below are some possible causes for this.
Possible causes of BLF issues
Section titled “Possible causes of BLF issues”- High packet loss: BLF updates may fail if there’s significant packet loss.
- Temporary network drops: If the network temporarily disconnects, the device might not receive SIP NOTIFY messages about call termination or answer.
- Router blocking notifications: Some routers may block SIP NOTIFY messages, especially if a DoS protection mechanism is triggered.
- SIP ALG enabled: SIP ALG (Application Layer Gateway) can interfere with SIP traffic, leading to missed BLF updates.
- Incorrect QoS configuration: If QoS is not properly configured, the firewall may misclassify and delay SIP messages.
- Short UDP timeout: Firewalls with a short UDP timeout may drop SIP messages prematurely.
- Overloaded device hardware: Some devices with limited or slower hardware may struggle to handle a large number of BLF subscriptions, becoming overwhelmed by the volume of SIP messages. Reducing the number of BLF subscriptions can help resolve this.
Example scenario: ring group with 6 phones
Section titled “Example scenario: ring group with 6 phones”For a single inbound call to a ring group of 6 phones, the following SIP traffic is generated:
| Message type | Count |
|---|---|
| SIP INVITE — one to each phone | 6 |
| SIP 180 Ringing — one from each phone, to notify the PBX it’s ringing | 6 |
| SIP NOTIFY — BLF updates, 5 per line across lines 1–6 | 30 |
| Total | 42 |
That’s 42 SIP messages for a single inbound call — all of which are sent at the same time to the router/firewall serving those 6 phones.
When someone picks up, another 40+ messages are sent to notify the PBX and all BLF-enabled phones which line was answered.
If any of these messages are delayed or dropped due to network issues, lack of QoS, bandwidth constraints, or processing delays, BLF updates may fail. This phenomenon is often referred to as a SIP notify message flood.