VLAN

[renjithraj007]

Dear All,

I am having a Doubt,Could anyone please help me??

Scenario:

Two switches A and B.

Switch A is having VLAN 10 with 192.168.1.1/24 connected to it.

Switch B is having VLAN 20 with 192.168.1.2/24 connected to it.

And these two switches are interconnected using an Access link not Trunk.

Can 192.168.1.1/24 and 192.168.1.2/24 communicate each other?? If they communicate,how will be the packet flow??

I am having doubt only because the two clients are in same subnet,and the inter switch link is an access.

Please Help

Regards

Renjith Raj 

[Jon Marshall · Accepted solution]

It depends how you configure the access ports connecting the switches.

If switch A access port is configured as vlan 10 and switch B access port as vlan 20 then yes the clients can communicate because there is no vlan tagging on access ports.

You have effectively joined two vlans together.

Note this is fine for a lab test but you shouldn't do this in production.

Jon

[renjithraj007]

Ok thanks for you reply.....

then how will the packet flow from .1 to .2 ? About ARP

could you please tell?

[Jon Marshall]

The packet flow would be the same as any normal packet flow for traffic within the same vlan.

So if the client on switch A in vlan 10 sends an arp it is a broadcast.

The port connecting to switch B is in vlan 10 so switch A sends it out of the port and switch B receives it on a port configured in vlan 20 and so sends it to all ports in vlan 20 which includes the client on switch B.

As I said to Ganesh, this only works with the configuration I initially posted.

Any other configuration and it would not work.

Jon

[renjithraj007]

Thank you for clearing my doubt.

[Joseph W. Doherty]

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Posting

BTW, you can cross connect two ports on the same switch, and join the two VLANs (effectively killing the point of having the two VLANs).

On Cisco switches, with CDP enabled, CDP generally will log messages that the two VLANs don't agree, but it won't stop operation.

[Ganesh Hariharan]

It depends how you configure the access ports connecting the switches.

If switch A access port is configured as vlan 10 and switch B access port as vlan 20 then yes the clients can communicate because there is no vlan tagging on access ports.

You have effectively joined two vlans together.

Note this is fine for a lab test but you shouldn't do this in production.

Jon

Hi Jon,

Just required few clarification on your post, If there is no Vlan tagging happening then is that means switch will blindly send traffic to other switch access port.

what would be the mac table output on switch A and switch B to forward a traffic between the two.

Thanks for clarifying.

-GI

[Jon Marshall · Accepted solution]

Hi Ganesh

Glad to see you back posting.

On switch A the mac address table would say that the mac address of the client on switch B is in vlan 10.

And on switch B the mac address table would say the mac address of the client on switch A is in vlan 20.

But it doesn't matter because of the way the ports connecting the switches are configured.

It only works if the ports connecting the switches are configured as I said before ie. switch A port is in vlan 10 and switch B port is in vlan 20.

If the ports were configured in any other way eg. the other way round to the above or both ports in only one of the vlans then it wouldn't work.

Jon