topic Re: Can two Vlans on the same subnet in Switching

Can two Vlans on the same subnet

dtom — Thu, 07 Mar 2019 19:14:58 GMT

I thought I saw this somewhere but can't seem to find it again...

If two switches are configured with different vlans but on the same subnet:

1) Traffic will pass between the switches untagged

2) Layer 3 communication will work because the two vlans share the same subnet

Am I correct here?

Also, what would happen if two switches on diffrent vlans but on the same subnet are connected to each other?

Can two Vlans on the same subnet

Raju Sekharan — Tue, 22 Jan 2013 23:38:02 GMT

If the link between the switches is access link it will work

PC1---------SW1--------Accesslink-----------------SW2--------------PC2

Think both the ports on SW1 are in Vlan 10 and all the ports on SW2 are i nVLAN 10, and PC1 and PC2 are in same subnet, they can communicate each other

Thanks

Raju

Re: Can two Vlans on the same subnet

rivanfrank — Wed, 23 Jan 2013 10:27:58 GMT

Hi,

We should always use the terminology of network and subnet properly. When we talk about networks we should always remember it is a classful either /8, /16, or /24 ex: 192.168.10.0 ; 172.16.0.0 ; 10.0.0.0

And for the subnets it means a major network is divided into smaller networks.

Example: Network: 192.168.10.0/24

Subnet: 192.168.10.32/27

192.168.10.64/27

192.168.10.96/27 and so on...

So to answer your question, I think you cannot configure two switches with different vlan within same subnet because it will overlap other vlan or the ip address that was already configured.

I made and example for this situation, you can create different vlans in both switches and still communicate if they are in the same vlans. For example in switch1 have vlan 10, 20 and 30 same as in the switch2 and pc1(192.168.10.40) can ping with pc2 in switch2 (192.168.10.41) because both are in the same vlan. In switch1 there are 2 pc in vlan 30 and they ping each other together with the laptop in switch2 (192.168.11.12).

I hope this will help you understand.

Thanks.

Re: Can two Vlans on the same subnet

Raju Sekharan — Wed, 23 Jan 2013 10:33:02 GMT

There was a Typo in my previous message. Correcting that

If the link between the switches is access link it will work

PC1--------(Vlan 10)-SW1-(Vlan 10)-------Accesslink---------------(Vlan 100)--SW2--(Vlan 100)------------PC2

Think both the ports on SW1 are in Vlan 10 and all the ports on SW2 are in VLAN 100, and PC1 and PC2 are in same subnet, they can communicate each other

Thanks

Raju

Re: Can two Vlans on the same subnet

Naren0007 — Tue, 30 Jan 2018 05:47:33 GMT

Can two Vlans on the same subnet comminucate if we are using different vlans.

ie we use vlan 10 on SW1 and 20 on SW2

PC1---------SW1--------Accesslink-----------------SW2--------------PC2

waiting for your reply

Re: Can two Vlans on the same subnet

237MaxB — Tue, 30 Jan 2018 08:15:37 GMT

Hi,
is there trunk between your 2 switches?
If yes, your trunk transport which VLAN ID?

Re: Can two Vlans on the same subnet

Naren0007 — Tue, 30 Jan 2018 08:22:34 GMT

no there is access port only . now im able ping vise-versa. refer attached file

Re: Can two Vlans on the same subnet

237MaxB — Tue, 30 Jan 2018 09:07:49 GMT

It's normal you ping the PC, because the two PC are in the same VLAN and same Subnet.
But it is impossible to have differents VLAN in the Same Subnet, because VLAN is virtuel network, so network

Re: Can two Vlans on the same subnet

Poorya Naji — Tue, 30 Jan 2018 11:46:46 GMT

r u sure u r using different vlans?cause as far as i know, if u use vlan 10 on sw a and vlan 20 on sw b, theres no way that traffic of vlan 20 pass to vlan 10 on layer 2 switchs.

vlan tag is on data link layer and sw will look at that before forwarding the pdu to another switch. ip address is not checked at l2 switchs.

Re: Can two Vlans on the same subnet

237MaxB — Tue, 30 Jan 2018 12:15:19 GMT

Exactly, if your two computer are in different VLAN, you can't ping one to another even if your two computer are on the same subnet.

Verify if Your two PC are on the same VLAN.

Re: Can two Vlans on the same subnet

Joseph W. Doherty — Tue, 30 Jan 2018 13:22:05 GMT

Yes they can, as was also answered in Pulikkal's 2nd response (although he used VLANs 10 and 100 rather than 10 and 20).

Re: Can two Vlans on the same subnet

Joseph W. Doherty — Tue, 30 Jan 2018 13:26:42 GMT

True only if frames are tagged. If you use untagged frames (e.g. access ports), you can jump across VLANs because the switches cannot tell the frames belong to different VLANs. (NB: on Cisco devices, though, if CDP is active, it will note, but not block, access ports that are on different VLANs.)

Also BTW, this can also be done on the same switch, if you cross connect two ports, each on different VLANs, again using untagged frames.

Re: Can two Vlans on the same subnet

Richard Burts — Tue, 30 Jan 2018 17:31:50 GMT

Joseph makes an important point about whether frames are transmitted with tags or without tags. If two switches are connected on an interface which is configured as trunk then the frames will be tagged and if they are tagged then the PCs will NOT be able to communicate. But most of the questions in this discussion have specified that the link connecting the switches is an access port. In this case there are no tags on the frame, each switch believes that the peer switch is in the same vlan (does not know the configured vlan of the peer) and the PCs can communicate successfully.

HTH

Rick

Re: Can two Vlans on the same subnet

Joseph W. Doherty — Tue, 30 Jan 2018 17:35:13 GMT

"If two switches are connected on an interface which is configured as trunk then the frames will be tagged and if they are tagged then the PCs will NOT be able to communicate."

NB: except for Cisco's "native VLAN" on a trunk. Those frames, too, are untagged.

Re: Can two Vlans on the same subnet

Richard Burts — Tue, 30 Jan 2018 18:22:00 GMT

Joseph makes an appropriate technical correction. And frankly it is not important whether an individual frame is tagged or is untagged. What is important is that on the trunk connection the switch is tag aware and that the switch does know what vlan the frame is associated with on the neighbor and therefore knows to what vlan it can forward the frame.

On an access ort there is no knowledge of what is on the neighbor switch and on a trunk there is knowledge of what is on the neighbor switch. And that is what determines whether the PCs can communicate.

HTH

Rick

Re: Can two Vlans on the same subnet

Naren0007 — Tue, 30 Jan 2018 19:37:58 GMT

Yes i have done it on packet tracer

Re: Can two Vlans on the same subnet

Joseph W. Doherty — Tue, 30 Jan 2018 21:10:52 GMT

"And frankly it is not important whether an individual frame is tagged or is untagged. What is important is that on the trunk connection the switch is tag aware and that the switch does know what vlan the frame is associated with on the neighbor and therefore knows to what vlan it can forward the frame."

Rick, I would suggest whether a trunk frame is tagged or not is crucially important, because if the trunk frame is untagged than the switch can only presume (based on port config) the VLAN the frame should be part of (basically the same situation as for an access port).

For tagged frames, we're in 100% agreement.

Re: Can two Vlans on the same subnet

Poorya Naji — Wed, 31 Jan 2018 08:47:15 GMT

Hi,
Now i confused, in switch port configuration, if you make a port as access port shouldn't you config what vlan it belongs? and if you didn't config it, do switch put that port on diffault vlan (vlan 1)?

Re: Can two Vlans on the same subnet

Joseph W. Doherty — Wed, 31 Jan 2018 10:41:37 GMT

Yes and yes.

Re: Can two Vlans on the same subnet

Richard Burts — Wed, 31 Jan 2018 15:51:41 GMT

Joseph

The question we are dealing with in this discussion is whether 2 PCs can communicate with each other if connected on 2 switches and assigned to different vlans on those switches. My point was that it depends on whether the switches are vlan aware. If switch A and switch B are connected using access ports then they are not aware of what vlan a frame came from and switch B will forward any frame that it receives from switch A. If the switches are connected using a trunk then switch B is aware of what vlan a frame was associated with on the other switch. So when switch B receives a frame from switch A it knows what vlan it originated from (and knows that whether the frame is tagged or is untagged).and therefore will only forward the frame to the PC is the vlans match.

If the switch will make the correct decision whether the frame is tagged or is untagged then I believe that it is appropriate for me to have said "And frankly it is not important whether an individual frame is tagged or is untagged".

HTH

Rick