Same subnet in different Layer 2 vlans

[ranjipremandrewp]

Assume that there are two switches A & B that are connected via a trunk. Switch A has a PC in vlan 10 with IP address 10.0.0.1/24 and Switch B has a PC with IP address 10.0.0.2/24 in Vlan 20. Will the two hosts ping? If not, what is the configuration changes needed to make them ping and all vlans are allowed on the trunk interface. I have tried this is in packet tracer and nothing seems to work for me. Please help me on this.

[Kasun Bandara]

Hi,

if you are using layer 2 devices (layer 2 switches) with VLANs, then you cannot do inter VLAN communication between hosts. trunk port will just pass the allowed VLANs through trunk link. this can setup when configuring trunk mode in switchport and setting allowed VLANs  using below command. by default some switches are enabled all VLANs through trunk.

#switchport trunk allowed vlan

according to your scenario below steps you can use to enable communication between two computers.

1. put both computers in same VLAN

2. use layer 3 device (router or layer 3 switch) to do the inter vlan routing. in this case you cannot have same network ip range between two hosts.

Regards,

Kasun

[ranjipremandrewp]

Thanks for the reply. I had this question in one of the interviews. The interviewer said to configure switchport trunk native vlan 10 on Switch A and then switchport trunk native vlan 20 on Switch B.

I tried what he suggested but I got 'Native vlan mismatch' error messages on the console.

[Kasun Bandara]

yes. if you done that so, console messages will be there with that error. thats not best practise.

[ranjipremandrewp]

I wasn't able to ping the hosts in Packet tracer. Is it possible to ping the hosts when we use real switches and clients?

[Kasun Bandara]

can you upload file here?

[ranjipremandrewp]

Please find the file attached. Thanks for the help anyway

[Kasun Bandara]

Hi there,

as i explained you cannot do inter vlan communication using only layer 2 devices. you need a layer 3 device like a router to do inter vlan communication. 

[pwwiddicombe]

From the IP address, the 2 PC's are in the same subnet.

Effectively, as the interviewer said, by using the "native vlan" command and effectively faking the the trunk to link the 2 vlans together.  With a common subnet addressing and native vlan passing frames untagged, the 2 PC's SHOULD be able to talk.

Oh, and with CDP enabled, the switches will complain.

[Joseph W. Doherty]

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Posting

It depends on whether hosts have a GW defined.

If you intermingle two different subnets on the same broadcast domain, if the hosts have a GW IP, they will direct any traffic, not on their network, to that IP.  If they don't have a GW, they will ARP for IPs both on and off their network.