That depends where is the traffic that you are going to filter coming from and which interface of your router/switch you plan to put the filter. What outgoing from one network/device is incoming to another network/device
For example if you are going to filter traffic coming from internet to your network;
- in your router interface facing internet (WAN interface), you should use "access-group in". This is the recommended way of doing it.
- in your router interface facing local area network (LAN interface), you should use "access-group out".
Another example if you are going to filter traffic coming from internet to your network and from your network to internet;
- in your router interface facing internet (WAN interface), you should use "access-group in" to filter incoming traffic from internet to your network.
- in your router interface facing internet (WAN interface), you should use "access-group out" to filter outgoing traffic from your network to internet.
Inbound ----- The packet will not pass through the routing policy. The packets will not be able to communicate with the router through the interface in question.
Outbound ---- The packet would have been processed by the routing policy. The packets can communicate with other interfaces of the router if there is need for that.
