If switch 1 has vlan10, switch 3 has vlan 10 but switch 2 which is in the middle of switch 1 switch 3 that does not have vlan 10, so in such case switch 1 will communicate with switch 3?

[pradeepkumarsingh.ece]

ans

[glen.grant]

  No it will not talk to switch 3  without  creating vlan 10 on switch 2 and trunk  from both switch 1 and 3 into switch 2 carrying vlan 10 on those trunks. 

[Richard Burts]

From a perspective of Best Practice and of How Things Work in Ideal circumstances then I agree with Glen.

But the reality is a bit different. Let us consider this scenario:
Switch 1 has fasteth0/1 in VLAN 10 as an access port
This connects to fasteth0/1 in switch 2
Switch 2 has fasteth0/1 in VLAN 1 as an access port
Switch 2 has fasteth0/2 in VLAN 1 as an access port
This connects to fasteth0/2 in switch 3
Switch 3 has fasteth0/2 in VLAN 10 as an access port

In this scenario switch 1 will successfully communicate with switch 3.
The basic principle here is that on switch to switch communication on access ports there is no VLAN identification.

Glen might very well say that this is a misconfiguration. And I would agree that it seems not logical to have the middle switch in a different VLAN. But the question was about whether this would work. And the answer would be yes.

HTH

Rick

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[Joseph W. Doherty]

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Posting

Both Glen (no) and Rick (yes) are correct.  Glen is positing switches are interconnected via VLAN trunks while Rick is positing switches are interconnected via single VLAN access ports.

Oh, and there are even other variations (or scenarios) if you start to play with trunking and native VLANs.

What it boils down to, is whether you're trying to preserve the VLAN 10 isolation through switch 2, as in Glen's response.  What Rick explained mixes VLAN 10 traffic, from switches 1 and 3, with switch 2's VLAN 1 traffic.

[Stuart Gall]

Out of the box it won't. The switch will not forward a vlan over a trunk that it does not know about.
It won't even do it if the vlan exists but has a different name.
This is a security feature.

I think it is possible to force it through.
If you turn off CDP
Turn on BPDU filter
Set VTP mode to transparent

You might be able to persuade the middle switch to pass the traffic.
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[Richard Burts]

Stuart seems to be making the assumption that the switches are connected by trunk ports. In that case the switches will not communicate. But I do not see anything in the original post that says there are trunk ports.

HTH

Rick

[Stuart Gall]

You are correct. I did make this assumption.
If the ports are access ports there is no issue because the LAN packets won't be tagged, so it would work just fine.
You can get issues with STP and root switch election. Which might mess up other communication across the VLANS.
So you should use BPDU filter to prevent this on the access ports.

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