So here's a problem I need to solve and I am having issues researching for an answer.

I have an optical cable that's coming in through SFP and it is forced to be connected to a switch before going to the router. This link is actually a trunk and it holds a few VLANs. The problem is, the switch will take this link and connect to the router with PPPoE, and then the router will have a connection going back into the switch with dot1Q (for VLANs).

So the current scenario is this:

Switch receives an optical trunk link, switch is connected to router (PPPoE), router connects back to switch (dot1Q) in order to distribute to the rest of the network.

So is it possible? If I am not mistaken, the router on hand is the Cisco 1800 router.

And if this is not possible, any suggestions on how to deal with this? The router is not able to take the optical trunk link, which is why this whole problem is created.

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  • You are trying to make a link be both a layer-2 link (trunk), and a layer-3 routed link. You need to have one or the other. Normally PPP is not run on a LAN, and a switch will not have a routed link unless it is a layer-3 switch. You need to give us more information. For instance, what are the device models and the reason you are trying to do this. – Ron Maupin Feb 22 '17 at 14:34
  • As said before I believe it's the 1800 series routers, can't get my hands on the switch models unfortunately. As for why... There's a dorm that needs some work and they are changing their ISP, the new one comes in with an optical cable that has trunks for each floor of the building. Problem is, the router can't take that optical cable, only the switch can. – Chessbrain Feb 22 '17 at 14:37
  • The switch that will receive the link is definitely nothing fancy-ish like a 2960 series etc... it's older than that, but still solid. – Chessbrain Feb 22 '17 at 14:39
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    I still don't understand the reason for PPP. You should be able to run a trunk link. It may be that the router model doesn't have any built-in fiber ports. You could use a distribution switch, and connect it to the router with copper, but I just don't think you want to run PPP. Unfortunately, questions about residential networking are explicitly off-topic here. – Ron Maupin Feb 22 '17 at 14:47
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    voted to reopen. I don't see what's off topic here. It should work provide the PPPOE connection is in a dedicated VLAN (SFP plugged in a port in access mode for the vlan) and the PPP configuration on the router is done under this vlan interface. – JFL Feb 22 '17 at 15:07

Now that your question is a bit more clear, you con connect the switch to the router on one of the copper interfaces. Just configure the link between the router and the switch as a trunk link. If you need PPPoE on one of the VLANs, then set up PPPoE on the subinterface for that VLAN. Just don't run your internal VLANs out to the ISP.

The drawing is a pretty poor way of doing things because you have no firewall, and your network equipment is exposed to the public Internet.

Just because the ISP connects to your switch via fiber, that does not mean that your switch must connect to the router via fiber. The switch really doesn't care which interface is connected to either the ISP, the router, or the hosts. As far as the switch is concerned, the router is just another host, but one connected via a trunk, and you can run a trunk on any of the switch interfaces, as long as the device on the other end of the link understands VLAN tags.

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  • The intention could be to put a firewall "downstream" of the router. – Todd Wilcox Feb 23 '17 at 16:03
  • Thanks a lot for the help! I actually researched a lot that day and came upon a partial solution. With what you said it all makes sense now :D Thanks! – Chessbrain Feb 25 '17 at 14:24

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