Question about VLAN 1 communication

[waterwalk]

I have a question about how VLAN 1, native VLAN, communicates with other switches and their VLAN interfaces.

My understanding is that by default, a new out of the box switch, VLAN 1 is the native VLAN, all ports are assigned to this VLAN, and the traffic that comes and goes is untagged.

Because the traffic leaving these ports is untagged by default, then if I plug a port from this new switch into another existing switch whose port is configured for an access VLAN, then the traffic should be tagged only once is accepted into the port of the existing switch and then be tagged for that VLAN.  If this is the case, then if I assign an IP address to VLAN 1 on the new switch, in the correct subnet of the existing switch's VLAN interface, why can't traffic pinging from the new switch ping the layer 3 VLAN interface of an existing switch trunked?

Let me give an example to clarify:

Let's say I have three new 3850's, just for example.

New switch (out of the box) - VLAN 1, IP address 1.1.1.2 255.255.255.0, ip default-gateway 1.1.1.1, all switchport interfaces default config

Existing switch 1 - VLAN 100, int g1/0/1 access VLAN 100, int g1/0/2 trunked to existing switch 2

Existing switch 2 - VLAN 100 interface layer 3 routed, IP address 1.1.1.1 255.255.255.0, interface g1/0/2 trunked to existing switch 1

New switch (int g1/0/1) <--> (int g1/0/1 VLAN 100) existing switch 1 (g1/0/2 trunk) <--> (g1/0/2 trunk) existing switch 2

I plug port g1/0/1 of the new switch into access port g1/0/1 of the existing switch 1.  I try to ping from the new switch to IP address 1.1.1.1 and I don't get any replies from the layer 3 interface of existing switch 2.  Why is this?  If VLAN 1 traffic is untagged, shouldn't it only get tagged once it enters the existing switch then the traffic be put on the correct VLAN 100, then trunked to existing switch 2 then communicate with the correct layer 3 interface?  If I plug a PC for example into this same interface and assign it the same IP address, it can communicate fine, there's only an issue when it's coming from VLAN 1 on the new switch, which should be untagged I thought.

[Reza Sharifi]

If I plug a PC for example into this same interface and assign it the same IP address, it can communicate fine, there's only an issue when it's coming from VLAN 1 on the new switch, which should be untagged I thought.

It maybe because the new switch does not have an IP in the same segment as the existing switch in vlan 1 (sources address).

So, on the new switch, create an SVI for vlan 1 and give it an IP in the same subnet as the existing switch and than try to ping.

HTH

[waterwalk]

Thanks for the reply Reza.

I already have on the new switch VLAN 1 set with an IP address in the same subnet as the VLAN 100 of the existing switch which the port I'm plugged into is assigned. I think this is what you're suggesting. Am I misunderstanding? Isn't the interface VLAN 1 with an assigned IP address the SVI for VLAN 1?

In theory, is it correct that the native VLAN 1 on the new switch should be able to communicate with different VLAN interfaces on other switches? I'm thinking this is correct since the traffic should be untagged for VLAN 1 if it's the native VLAN.

Thanks

[Reza Sharifi · Accepted solution]

Hi,

I think this is what you're suggesting.

Yes, that is correct. I just want to create an SVI for vlan 1 in the new witch with an IP so when you ping from the new switch you have a source IP. 

In theory, is it correct that the native VLAN 1 on the new switch should be able to communicate with different VLAN interfaces on other switches? I'm thinking this is correct since the traffic should be untagged for VLAN 1 if it's the native VLAN.

That is correct as long as both side on the connection is configured as an access port, it should work regardless of the vlan id.

So, in your case the new switch's port is in vlan 1 and the existing switch's port that connects to the new switch is in vlan 100. If that is the case and not working, try and change the vlan from 100 to 1 on the existing switch and test again.

HTH

[waterwalk]

Thanks Reza! I appreciate you clarifying this for me. It seemed to be odd behavior, and I wanted to make sure my understanding of it was correct. I'll implement your suggestion.