I have several small sites that are part of an MPLS network. Most of the sites have Cisco 1921 routers that are handing out DHCP for about 6 machines. Looking to add VOIP at these sites and thought about doing VLANS, but don't want to spend money on a layer 3 switch since we are going to do POE.

What would be the proper way to do this for those smaller sites?

  • How bout a layer 2 switch with PoE connected to your Cisco 1921?
    – user36472
    Mar 15 '18 at 13:31
  • This is my thought, I'm unsure of the VLAN config on the 1921 though. Mar 15 '18 at 14:08
  • You can use sub interfaces on your router.
    – user36472
    Mar 15 '18 at 14:11
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin
    Apr 1 '18 at 23:03

If your MPLS solution includes 802.1q-tagged traffic and there's only a single VoIP gateway anyway there's no need for L3 switching - you can do all the L3 switching or routing near the VoIP gateway.

However, you should implement QoS on each site to avoid congestion on the uplinks and check with your MPLS provider if QoS is possible on the interconnects. Without provider support, QoS will require you to statically reserve some bandwidth for VoIP.

Without tagged inter-site traffic you'll need to route/L3 switch on each site. The 1921s should be able to handle that pretty easily though.

  • It's a VOIP cloud solution. Mar 15 '18 at 14:06
  • OK - then let the 1921s handle the VLAN routing.
    – Zac67
    Mar 15 '18 at 15:40

I see a couple ways to do this:

  1. Don't do multiple vlans: If you're just going to have a couple phones at a small site (you mention about 6 machines), you don't have to do multiple vlans. Having a voice vlan is highly recommended, but not actually required. Voice and computer devices can coexist on a vlan (and, therefore, subnet) without causing any issues. But what you really need to do is make sure that you have QoS configured correctly to give the voice traffic the proper treatment. (You will need this regardless of which option here that you choose.)
  2. Have the router trunk two vlans to the switch. The switch port that connects the router would be configured as a trunk, carrying 2 vlans. The router would have two subinterfaces configured, one for computers and one for phones. Don't forget the QoS.
  3. Just configure a Layer 3 switch. I know you said you don't really want to spend money on it, but most PoE switches these days support routing, anyways! So there's a really good chance that whatever switches you're going to buy can do it with no additional cost. And QoS, too.

If it were me, I'd just go with option 1, unless you have some security reasons to do option 2 or 3. But if you do have security reasons, then make sure you're configuring port-security, DHCP snooping, vlan ACLs, and whatever else to prove that you care about security. And have I mentioned QoS yet?

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