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In our building, we have 2, 6509 core switches, one for our company, and one for another. We have a vlan configured with a /30 which bridges the 2 switches. Both of us are on separate /16 networks. I have various /22 vlans configured within this /16, and so do they. Neither of us can "see" the vlans configured on each of our switches. In other words, I can have vlan 10 and they can have vlan 10 configured.

I have a vlan configured on this switch that I need them to be able to see. Its for a voip system that's being implemented across the two companies. This vlan includes dhcp helper addresses for a specific scope that I created for the voip phones. For us, it was just a matter of adding the "switchport voice vlan XX" to the ports, and the phones pickup the IP's, not so much for them. So, what would be the way to get their network on this vlan? Or do I have them create a new vlan on their switch, I setup a scope with the network they choose, and they add our helper addresses?

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    Looks like you are mixing routing, switching and bridging here... how are the two switches connected? What kind of routing (if any) is taking place? – Massimo Aug 11 '15 at 17:43
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You're mixing layer 2 and layer 3 together and they're two different things. VLAN's "operate" at Layer 2 while subnets "operate" at layer 3. It sounds like you have inter-vlan routing configured and you're using that to route traffic between the subnets between the two companies. When you say that you have a /30 that bridges the two switches what I suspect you mean is that you have a /30 that routes traffic between the subnets between the two companies. Bridging occurs at layer 2, not layer 3. A /30 implies a Layer 3 interface/ip address.

All of that being said, it seems to me that you need to create a "common" VLAN on both switches for the VOIP phones and configure a trunk port between the two switches that allows this VLAN traffic to transit the trunk.

Look at this diagram as an example. Imagine that the 192.168.0.x devices are computers and the 10.x.x.x devices are VOIP phones. All of the 192.168.0.x devices are in VLAN 1 and all of the 10.x.x.x devices are in VLAN 2. All of the 192.168.0.x devices can communicate with each other by virtue of being connected to ports in the same VLAN and having ip addresses in the same subnet. The same applies for the 10.x.x.x devices. The link between the switches is configured as a trunk port on each switch. The trunk ports are configured to forward traffic for all VLAN's. This is roughly what you need to configure, with the exception that you want the trunk to forward traffic ONLY for the VOIP VLAN.

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  • Yes, you're right, it is just a common vlan with a /30 assigned between the two switch, so its a layer 3 connection. – joerockt Aug 11 '15 at 18:36
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If you need the same VLAN to span both switches, you need to define it on both devices and then connect them using a trunk (which also gives you the ability to carry mutiple VLANs on the same physical cable).

Sample configuration (to be reapeated on both switches):

interface GE0/1(or whatever port you want to use)
switchport mode trunk
switchport trunk encapsulation dot1q
switchport trunk allowed vlan XX

Then just physically connect the two ports, and whatever device accesses VLAN XX on one switch will be able to talk to whatever device accesses VLAN XX on the other switch.

If you later need to also carry VLAN YY between the switches, just change the last line to

switchport trunk allowed VLAN XX,YY
  • That makes sense. So they would configure the same vlan with the same network? Or would they just leave out the network info for the vlan interface? For example, I have on this vlan configured a /22 network, would they need to do the same? – joerockt Aug 11 '15 at 18:41
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    Three things need to happen: 1. The ports that the phones are connected to on each switch must all belong to the same VLAN (Layer 2). 2. The VOIP phones must all be in the same subnet (Layer 3). 3. You must connect the two switches and configure these connected ports as trunk ports (on each switch) to carry the VLAN traffic for the VOIP phone VLAN. – joeqwerty Aug 11 '15 at 19:41
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    @joerockt You don't "configure a VLAN with a network"; a VLAN is just a logical segmentation on a switch, all ports assigned to a VLAN behave like if they were connected to a different physical switch than other ports and they can exchange Ethernet frames only between them; but this doesn't have anything to do with the IP configuration of devices connected to that VLAN. – Massimo Aug 11 '15 at 23:45

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