It is always said that if single vlan wants to communicate with another vlan, inter vlan routing is required. But in the depicted scenario, vlan 10 is able to communicate with vlan 20 without any layer 3 device. The logic behind the working is very simple and it is totally based on tagged and untagged concept. Access ports always tagged the incoming traffic with vlan configured on port and make untagged the outgoing traffic. The same is happening in the given scenario, when the traffic is being forwarded by PC-1 and Switch A access port receives the traffic, mark that traffic with vlan 10 and look forwards for the vlan 10 ports. After that Switch A another port which is part of vlan 10 receives the traffic and forward it towards Switch B. During forwarding Switch A port untagged that vlan and forwards the traffic without any vlan tag. Switch B port receives the incoming traffic and tagged it with vlan 20 and forwards the traffic towards PC-2. This way end to end communication takes place without the use of router or any layer 3 device. So we can say inter vlan routing without any layer 3 device.