Two PC on two different Vlan but same subnet . But ping is working

[Abhinav16]

PC1 (10.1.1.1 255.255.255.2) --------(Vlan 10 ) SW1  (Vlan 10) ----------------(Vlan 20) SW2  (Vlan 20) ------ PC2 (10.1.1.2 255.255.255.0) 

How ping is working ?

[MHM Cisco World]

I see this issue before the native VLAN in SW1 is 10 
and native VLAN in SW2 is 20 
the SW1 send untag traffic to SW2 and connect happened. 

[Joseph W. Doherty]

If you have VLAN 10 physically connected to VLAN 20 (there are a couple of Cisco ways to do this, one way mentioned by @MHM Cisco World ), you no longer have two VLANs, but one VLAN.  This would explain why hosts can ping each other being on the same subnet.

BTW, on Cisco equipment, if CDP is active, it likely will throw log messages noting two different VLANs (i.e. defined with different VLAN numbers) are interconnected.

[Spooster IT Services]

Hi Abhinav16, 

This issue can arise in two scenarios 1 ). Native Vlan Mismatch ( Native Vlan's send untagged Traffic , they're used for backward compatibility  with devices like Hub )  

                                                        2).  Link Between SW1 & SW2 configured as Access ports. (The access ports switch check the VLAN membership of ingress port and forward packet to the same VLAN membership port as untagged.) 

By default the Native Vlan On switches is VLAN 1 ,  so start with checking ,  if you made the link between SW 1 & SW 2 as Trunk . If it is trunk then check for the native vlan mismatch .

[Richard Burts]

We do not know whether the link between the switches is configured as a trunk or as access ports. And that makes a difference. If the link is configured as a trunk then there are 2 vlans and if they communicate then it would be because of a native vlan mismatch, as previous responses have explained. If the link is configured as access ports then we have 2 vlan names but in reality only a single vlan. It may help if we remember the definition of a vlan - a vlan defines a broadcast domain. Any 2 devices that can broadcast to each other (as these devices can) are in a single vlan.