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One site ISBC SIP trunk fault analysis and dealing process

Publication Date:  2016-05-20 Views:  2700 Downloads:  0

Issue Description

1. IP topological graph is as following picuture, the two SIP trunks between ISBC and carrier STC are both fault. The two SIP trunk share one same SIP server at carrier side, but they have different gateways and different IP routes.

2.Our call center system can't call out or call in.

Alarm Information

1. “SIP Trunk Fault” alarm from our ISBC Monitoring system:

2.ISBC sends SIP option requests but receive no response from carrier side device.


Handling Process

1.  Query ISIPTG status from device, it feedback “faulty” from both links:

2. Inform carrier engineer about this fault, ask them to troubleshoot at the same time.

3. Switch ISBC group, switch master ISBC from ISBC A to ISBC B,  then SIP trunk is still down

4. Ping from ISBC to carrier gateways, find out that both gateways are not reachable.

5. Ping from ISBC to carrier SIP server, find out that the SIP server is not reachable.

6. Carrier engineer ping from carrier SIP server to carrier gateways, find out that both gateways are reachable.

7.Carrier engineer ping from carrier SIP server to our side ISBC, find out it's not reachable.

8.We check carrier GPON device, discovered the GPON device is not working normally, the traffic light of its interface is not flashing.

9.Reboot the carrier GPON device. After reboot, the GPON device resumed back to normal.

10. SIP trunk resumed back to normal.

Root Cause

The carrier GPON device was in fault, so network route between ISBC and carrier SIP is down.


Check the network and every device in the route bewteen ISBC and carrier SIP server, after reboot the carrier GPON device, the problem is solved.


1.The carrier GPON device is single node, if it's down, the whole SIP trunk will be down, so we suggest customer to install another SIP trunk by other network route. And customer accepted the suggest.

Now the site has one additional SIP trunk using the route by Microwave.

2. The carrier GPON device is too old, it may be in faulty after running days,so need to replace it with a new one.