Consider the following situation.
Host 1 :- VLAN7 10.1.7.10 /24 GW :- 10.1.7.254
Host 2 :- VLAN7 10.1.7.20 /24 GW :- 10.1.7.254
Host 3 :- VLAN7 10.1.1.10 / 16 GW :- 10.1.7.254
Host4 :- VLAN7 10.1.1.20 / 16 GW :- 10.1.7.254
Host X :- VLAN 1 10.1.1.100 /24
ALL Hosts are on the same switch, with an uplink (trunk) to a router sub-interface for vlan 1 and 7 for inter vlan routing
interface fa 0/1.1
ip address 10.1.1.254 255.255.255.0
interface fa 0/1.7
ip address 10.1.7.254 255.255.255.0
Host 1 & 2 can ping each other (Obvious)
Host 3 & 4 can ping each other (Obvious)
Host 1 and 2 can ping Host X (via inter-vlan routing fa 0/1.1 and fa 0/1.7)
QUESTION IS :-
Can a packet from Host 3 or 4, reach host X (even if its uni-directional) ???
Beacuse Host 3 is on vlan7, it can only reach the router via sub-interface 7 (Correct ??)
Now what happens when it reaches the sub-interface ??
Will the router DROP it because of a subnet mis-match
Will it say .. ok, you want to go to 10.1.1.x, go out via fa 0/1.1
Return will fail however, because 10.1.1.100/24 will look for 10.1.1.10 in
the local lan and not find it.
>> Can a packet from Host 3 or 4, reach host X (even if its uni-directional) ???
only if Proxy-ARP is enabled on subinterface .7 because host3 and host4 will try to resolve the destination using an ARP request.
And only if you haven't mechanims like unicast RPF on subinterface .7
return packet delivey will be attempted out subinterface .1 and it will fail.
you can check if IP proxy-ARP is enabled with
show ip interface subinterface.7
Hope to help