PBX unable to find some network phones

[Twearden]

Hi all,

 

This might be a stupid question, or may be the wrong place to post it - forgive me I'm a newbie to both the site and the role of IT Technician.

 

We recently switched our phone system onto a new, separate internet line, with a new firewall in front of it. Ever since, around half of the network-based phones are unable to find the PBX.

 

I believe I need a way of routing the IP address for the PBX x.x.x.254 via the new firewall x.x.x.1 - we're running DHCP through Microsoft Server 2016 on the school server.

 

Any help would be much appreciated on this!

 

Tim

[Steve21]

You're going to have to expand on the explanation a bit there, if it's an internal PBX with internal phones the firewall shouldn't come into play at all.

 

Are you using hosted pbx? Is the firewall on a different subnet? Are you using DHCP options for the phones discovery etc

 

Steve

[mavhc]

You can set DHCP to give different results based on the type of device connecting https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol#Client_vendor_identification

[Twearden]

It's an internal PBX and internal phones, but I believe the network is also hooked up through the firewall. Could very well be wrong!

 

It's a physical PBX rather than a virtual/hosted one (if that's what you were asking?) The firewall is on the same subnet. The addresses for the PBX and firewall are excluded in DHCP, so I'm guessing that means we're possibly not using it for the phones.

 

Sorry I do realise I'm doing a terrible job of describing the issue!

 

T

[Davit2005]

I imagine this should be a different vlan for the phones, it is best to have phones on a different vlan. Can you check the IP address, subnet mask and gateway received on one compared to another.

 

Do the MAC addresses of the phones show up on the correct DCHP scope on the DHCP server. You would probably have some options set on the phone vlan scope so the phones can get the downloads of the settings so t will be important for the phones to be getting an IP address from correct range.

Edited February 4, 2021 by Davit2005

[Twearden]

Hi guys,

 

Thanks for your helpful replies - I've revisited this issue in advance of the children and teachers returning next week and have more info.

 

We have two servers here, both running DHCP for different ranges, but over the same network.

 

IP Phones with DHCP from Server1 work, IP Phones with DHCP from Server2 don't.

 

Am I right in thinking that having two Servers running DHCP in one network is a BAD idea? I didn't set it up and this is the first network I've managed, but it doesn't seem right to me.

 

Anyone got any ideas that could help me, both short term and long term?

 

Tim

[Steve21]

Multiple DHCP servers is standard really (assuming they're setup correctly), but it might be something silly like one has a TFTP boot option set in DHCP and the other doesn't so they'll only boot when getting the info from one. Have a look and see what options are set on each server and if any match the PBX

 

But again, that shouldn't have been affected by an internet change

 

Steve

[Davit2005]

As @Steve21 mentions.

 

Check options, probably tftp options on the DHCP scopes. But could be vendor options such as option 43 depending on PBX.

[mavhc]

If you have 2 dhcp servers then a) they need to have exactly the same settings, including any extra ones added manually for the pbx, and b) not overlap in the ranges they're offering

[FN-GM]

not overlap in the ranges they're offering

 

Unless you’re using windows failover. It does actually spilt the range between them but it’s automatic.

 

Also check both servers have the correct Default Gateway and DNS.

[Twearden]

Should my two servers both have the same subnet mask?

[FN-GM]

Should my two servers both have the same subnet mask?

 

Yes - they should have the same mask.

[Twearden]

Thanks! So just to clarify, is the fact that my two servers are currently using different subnet masks, likely to be causing the main issue with my VoIP phones?

[Davit2005]

Thanks! So just to clarify, is the fact that my two servers are currently using different subnet masks, likely to be causing the main issue with my VoIP phones?

 

Quite possible.

 

Without knowing a bit more about the setup i.e. Diagram switch configs, dhcp scopes etc. It is hard be sure.

 

Have a look at how subnetting, masks etc. work.

 

But basically if you had wrong mask say it was 255.255.255.0 following scenario.

 

DHCP scope setup to give out 192.168.0.1 to 192.168.3.254 with subnet mask of 255.255.252.0, client gets IP address 192.168.2.5 and wrong subnet of 255.255.255.0 instead of correct one 255.255.252.0. This means that client will be in wrong logical network and not be able to get to gateway because it is logically on a different network. No access to gateway, no access to Internet or anything outside of the same subnet.

Edited March 5, 2021 by Davit2005