2 Data VLANs in one subnet

[Jose Manuel Garcia]

Hi,

Yesterday, I realized that one of my company's client has 2 Data VLANs belonging to the same logical subnet. I mean in the subnet 192.168.100.0/24 there are hosts in VLAN 10 and hosts in VLAN 20. 

How is this possible?

Can this fact be the origin of some kind of troubles?

Thank you in advance

[Reza Sharifi]

Hi,

Not best practice but possible to assign a subnet to multiple vlans.

The end device only knows about the IP and usually does not care about what vlan it is in.

HTH

[Julio E. Moisa]

Hi Jose,

As Reza mentioned, it is not a best practice unless you have isolated network environments and they will never be in communication, or if you want to unify them using fallback bridging or using a NAT.

http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3750/software/release/12-2_55_se/configuration/guide/scg3750/swfallbk.html

Hope it is useful.

:-)

[Jon Marshall]

Just to add, this is a valid configuration when you are using a device such as a firewall or load balancer in transparent mode where the device in effect joins the two different vlans together. It is done this way to avoid an STP loop.

However as both Julio and Reza have said if these are just normal client vlans then it is not a good thing to do.

Jon