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How does the SBC Session Border Controller work?

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David Pearce

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I'm trying to implement a Windows SBC in a remote office we have. It is not working for us. I hope that someone here has one that is working and can help to understand how a working system works.

I've been told the session border controller forms a secure tunnel between the remote office and the 3CX PBX server. Not knowing how this works exactly is making it difficult to diagnose.

q1: If I do an nmap against the SBC (no firewall), no ports are open below 500 (just the usual MS ports). What ports should be open on a working SBC?

q2: There is a configuration file in ProgramData and another set in the registry. Which one is being used?

I am taking a working phone from the main office on the same network as the 3CX server and putting it on the remote LAN. I make no changes to the phone provisioning. Should I be making changes?

Wireshark dumps show the phone trying to work in the remote office like it is working in the main office, i.e. reaching out to the local IP address of the 3CX server which is not accessible in the remote office.

I'm not finding that the 3CX instruction files are very helpful. Can anyone help me?
 
This may help https;//www.3cx.com/docs/3cx-sbc-windows/

You need to open port 5090 (tcp and udp) on the firewall protecting the 3cx server.

On the firewall protecting the sbc server, just make sure port 5090 (tcp and udp) is open outgoing , along with your https 3cx port (normally 5001)

Have you run the firewall check within the 3cx management console, to check for any issues

Yes, sbc phone configure has a totally different setup as a local phone, so the phones needs resetting and provisioning. Info on configuring phones via sbc is under ip configuration guides https://www.3cx.com/sip-phones/
 
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"Yes, sbc phone configure has a totally different setup as a local phone..."

I'm using a SNOM D120. In the provisioning page, it lumps "Local LAN or SBC" into the same instructions group.
 
"Yes, sbc phone configure has a totally different setup as a local phone..."

I'm using a SNOM D120. In the provisioning page, it lumps "Local LAN or SBC" into the same instructions group.

May mistake, it is the same setup. Not used sbc before

just make sure the 3cx sbc server is up and running and communicating to the 3cx server.

If the phone does not work, I would delete the phone configuration under extension provisioning tab, factory default the phone and hopefully you see the phone appear in the console where you can assign it to an extension
 
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I've done this at least five times. What would really help me is some one who can run nmap against an SBC.
 
BTW: I'm using v15.5

Under "Phone Provisioning" is this interesting option. Any one know how to set this up?

upload_2019-1-16_15-52-4.png
 
upload_2019-1-16_16-1-48.png
What should this be set to?
 
View attachment 9369
What should this be set to?

This is the IP address of the sbc server

The sbc server should have a fixed IP address

If you are manually setting up the phone within the 3cx console, you need to factory default the phone log into the web front end of the phone and type the provisioning url into the phone
 
I haven't checked the configs lately but typically the difference in configuration would be this:

- Local LAN the SIP endpoint just has the 3CX FQDN/IP as the SIP server/registrar/proxy
- SBC the SIP endpoint has the same configuration for SIP server/registrar/proxy but then has the SBC as the outbound proxy.

A working SBC should be listening on 5060 UDP by default for connections from local SIP endpoints.
 
Check your wan IP address has not been blacklisted. On the first screen within the console, you willl,see the number of blacklisted IPs - check on this and you will see each ip
 
I can see from the above string that you have checked all the usual (Firewall checker, ports on the 3CX PBX etc) so will not include these - below are some additional points however:

* Check Windows machine locally - Windows Firewall/defender/Anti-virus software.

* Local SBC Network configuration: Check you do not have a firewall on this remote site that blocks traffic both ways by default (such as a Cisco) most are stateful and will allow outbound traffic by default but have known one or two models that don't.

* If using the SBC in DHCP mode switch to static - ensure you do not have multiple DHCP servers on the remote site if in DHCP mode.

* Look at the Tunnel logs in 3CX - are you seeing an inbound connection attempt.
https://www.3cx.com/docs/collecting-logs-for-3cx-support/

You can try using the remote firewall checker (client) it may also give you some hints: https://www.3cx.com/docs/firewall-checker-client/
 
Hello @David Pearce

Setup the SBC using the following guide: https://www.3cx.com/docs/3cx-tunnel-session-border-controller/
Once the SBC is setup, factory reset a phone in the same LAN as the SBC. It should appear in the management console as a New phone under the Phones tab. Assign it to an extension and press OK. Your phone should now be provisioned and setup to work behind the SBC.
A local phone and a phone behind an SBC do not have the same configuration and you should not expect a phone to work just by moving it from the local LAN to a remote location without additional configuration.
A local phone will connect to the PBX via http and the local IP. A phone behind an SBC will connect via Https and the FQDN. If it tries to connect via http it will fail as the 3CX PBX does not accept http requests from remote locations. Also the proxy of a phone behind an SBC is different.

If you reset a supported phone behind the SBC and you do not see it appear as new behind the SBC it could be that either the SBC is not connect to the PBX or that your server is not listening for PnP requests.
 
Hi on a windows 10 machine SBC do i have do do anything to stop in going to sleep in power setting ? many thanks
 
Hi on a windows 10 machine SBC do i have do do anything to stop in going to sleep in power setting ? many thanks

Probably...in Power setting. You'd want to disable.
 
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