Two subnets in the same ethernet cable?

[brwainer]

10 hours ago, LAwLz said:

You got this entire thing backwards (the bold part). VLANs are configured on a port-by-port basis. It is when you use VLAN that using the correct port becomes important. If his fiber modem did not tag the traffic then it would not matter which port he connected to.

Untagged fiber converter = doesn't matter which port he uses.

Tagged fiber converter = it matters which port he uses, because otherwise the wrong VLANs will be used.

I hear what you are saying, but rather it is you who have it backwards. An "untagged" port on a switch means that the port is a member of a VLAN, but traffic leaving out that port will not be tagged (it will not have the VLAN header inserted into it). Data entering that "untagged" port will be treated as data for the VLAN the port is "untagged" for, but isn't actually tagged with that VLAN unless it leaves the switch through a tagged port. Here's an example, taken directly from a switch at work (explanations added):

 

vlan 1 (default vlan)
int 1-24

no untagged (this removes these ports from vlan 1, allowing them to be set as untagged for a different VLAN)

int 25-28

untagged

 

vlan 101

int 1-12

untagged

int 25-28

tagged

 

vlan 102

int 13-24

untagged

int 25-28

tagged

 

In this case ports 1-12 are untagged for VLAN 101, ports 13-24 are untagged for VLAN 102, and ports 25-28 are the trunk ports (uplink/downlink). Any device plugged into a port 1-24 will be a part of it's respective VLAN, but won't know it at all because the traffic it sends gets tagged by the switch on the way up to the core, and traffic coming to it from the core has the tag removed. So "untagged" and "tagged" from the perspective of a switch refers to the traffic leaving the switch, NOT what type of traffic is expected to be received on the port.

 

10 hours ago, LAwLz said:

If he plugs in his STB into the port meant for his router then his STB will get the wrong DHCP config, and he will not be able to watch TV. Why? Because the traffic to and from his STB will not be tagged as IPTV traffic.

Correct, because from the modem's perspective, that port is untagged, it is NOT TAGGED with a VLAN. From a device's perspective, you might say that the port is tagged, because traffic entering it gets tagged. However that is not the definition used by engineers designing a device, because traffic gets tagged as it leaves not as it enters (it is psuedo-tagged for switching purposes inside the switch/router/modem/etc, but this isn't technically an 802.1Q tag)

 

10 hours ago, LAwLz said:

What you two are saying makes no sense, because if the fiber converter did not tag the traffic then the OP could connect his TV box to any port on the fiber converter and it would work, but clearly there are specific ports he has to use in order to get his Internet and TV service to work... Because the fiber converter is configured so that specific ports uses specific VLAN tags. So for example LAN 1 and LAN 2 might be VLAN 1001 (regular traffic) and LAN 3 and LAN 4 might be VLAN 2002 (IPTV traffic).

That's how it works...

From the perspective of a STB trying to get to the ISP, it expects untagged traffic to reach it, and for it's traffic to get tagged. From the perspective of the ISP returning data to the STB, it sends out tagged traffic and expects the tag to get stripped. Therefore, the uplink port to the ISP (the fiber) is a Tagged port for all VLANs (which is called a Trunk port), and all downlink ports to STB or LAN are untagged, for the proper VLANs.

 

9 hours ago, leadeater said:

The Untagged converter is what I meant by if you yourself had to tag your traffic yourself then you could use any port on the ISP device as shown. The Tagged converter is the type of setup used almost everywhere else and each port labeled for each service is set to an untagged specific VLAN that matches the service, which is why the port matters. If you used wireshark to do a sniff of traffic coming out of those ports no Ethernet frame would have a VLAN header set, those ports are still specifically tied to the VLAN for that service (Internet/TV) but no VLAN configuration is required for any network equipment plugged in to those ports.

Leadeater gets what I'm saying and his explanation is functionally the same as mine. Untagged and Tagged are defined from the perspective of the switch chip inside a device, which only makes the decision to add or not add a VLAN tag after the outgoing port is determined.

 

9 hours ago, LAwLz said:

I think you misunderstood what I said. Maybe I wasn't clear enough. The reason why I told the OP to tag the traffic himself is so that the switch will know which ports on the fiber converter the traffic should go through. If he just connects two dumb switches like in the diagram in the picture then the switch will have no way of knowing if the traffic from the STB should be sent through the "modem TV-port" or the "modem LAN-port". It can't use something like a default gateway to determine it since 1) it is a layer-2 switch and 2) not even the STB knows what default gateway it should use since it hasn't connected to the right DHCP server yet (the one that will give it a 10.X.X.X IP).

My initial comments that a dumb switch would work as long as one of the subnets did not use DHCP was based on the assumption that VLANs were not being used. Which so far, from a resident perspective, they are not. Because all downlink ports on the modem are untagged, for different VLANs, to a device connected to them there is no discernable difference. SO in this case, yse of course VLANs would have to be used. HOWEVER, the ports connected to the modem and to the downlink devices should be untagged, and the trunk port between the two switches should be tagged. This is because all devices expect there to be no VLAN tags for traffic coming into them. The VLAN IDs used by the user's switches at either side of the trunk DO NOT MATTER as long as they are consistent between the two switches. No device will ever seen the VLAN IDs that the user selects except the two switches.

 

9 hours ago, LAwLz said:

Also, customers do not have to worry about VLANs generally. It is only a problem for OP because he does not want to plug everything in according to the instructions (TV box straight to the port labeled TV, and router straight to the port labeled broadband).

The OP would only have to worry about his own VLANs, not the ISP's.

 

EDIT: That really came out angrier than I intended. However, it is my 666th post, so please excuse. I will try to calm down more in the future.

[LAwLz]

5 hours ago, brwainer said:

I hear what you are saying, but rather it is you who have it backwards. An "untagged" port on a switch means that the port is a member of a VLAN, but traffic leaving out that port will not be tagged (it will not have the VLAN header inserted into it). Data entering that "untagged" port will be treated as data for the VLAN the port is "untagged" for, but isn't actually tagged with that VLAN unless it leaves the switch through a tagged port. Here's an example, taken directly from a switch at work (explanations added):

-snip-

Okay I am sorry. I have messed up when translating the terminology. When I said untagged I meant what the Tele2 documentation I linked before refers to as "otaggad". Thinking back, calling it untagged was wrong and I should have said "not tagged" instead. So yeah, I fucked up.

You're right, I had the terminology backwards.

I think that my posts will make a lot more sense if you reread them with that in mind.