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We have a Cisco router from our T-mobile provider, two subnets on two ethernet ports: one subnet for PC data network, one subnet for IP phone network. The subnets have different IP addressing.

As per here, it should be possible to mix two networks on one switch.

We need to connect IP phones in a second building and we consider the following design:

Network design

There are two buildings but only one wifi bridge. The bridge should be shared both for phone and data subnets. We currently use 3Com switches for 172.27.146.xx subnet that are managed and have Radius authentication turned on.

Can we use the existing one wifi bridge or do we need to build an extra bridge for IP phone network?

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It will work. Keep in mind this to learn why:

1 - A LAN is a single broadcast domain and single collision domain.

Collision Domain: It means that a collision will be sensed by all the devices in the network.

Broadcast Domain: A broadcast frame will be heard by all the devices in the network.

2 - A layer 2 switch (also a bridge) is a single broadcast domain and multiple collision domains (one per port).

3 - A Layer 2 switch with VLANs is multiple broadcast domains (one for each VLAN) and multiple collision domains (one per port).

Case 1 is a hub, we will forget about it.

Cases 2 and 3 is a layer 2 switch. Layer 2 switches only check source MAC and destination MAC in the layer 2 frame.

A layer 2 switch doesn't care what is happening in upper layers (IP in this case).

Your network is in case 2, where each broadcast frame is heard by all the devices and is the device who checks the upper layer to see if the packet is aimed to it or simply has to be discarded.

Separating in different subnets (PCs and Voice) is not improving performance or security because:

  • As everything goes through the same level 2 switch, the broadcast is received by every device (PCs and Phones).

  • A mischiveous user could change the subnet of his PC to get into the voice subnet and capture frames, etc.

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    The Ubiquiti bridge doesn't support VLAN trunking, so you can't have both VLANs in both places. – Ron Trunk Nov 2 '17 at 11:49
  • @RonTrunk And should it work in case we do not need to separate to VLANs? Security is probably not the big concern here. – Vojtěch Dohnal Nov 2 '17 at 11:51
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    You are right. I didn't notice that the OP is using only one VLAN. – Ron Trunk Nov 2 '17 at 11:52
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    @VojtěchDohnal Ideally, you would set the Ubiquiti's to router mode, and make the wireless link layer 3. That would reduce broadcast traffic between sites and improve performance. You will have to have different subnets at your other location. – Ron Trunk Nov 2 '17 at 12:47
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    Ubiquity claims it can prioritize voice packets over the wireless link, although the datasheet is short on details. That may affect how you wold redesign your network. – Ron Trunk Nov 2 '17 at 12:48

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