Basic Routing Qs :- Will interface route packet with incorrect (wider) netmask ?

[shahedvoicerite]

Hi,

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

Now :-

       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

                                   OR

                   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.

Thanks

[Giuseppe Larosa · Accepted solution]

Hello Shahed,

>>  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

Giuseppe

[shahedvoicerite]

Thanks !

So, if Host 3 / 4 were however trying to reach 1.9.x.x for example, they would go to the default gateway (10.1.7.254) correct ?

       Because 10.9.x.x is not in the same subnet ?

Thanks again !

[Giuseppe Larosa]

Hello Shahed,

thanks for your kind remarks

>> So, if Host 3 / 4 were however trying to reach 1.9.x.x for example, they would go to the default gateway (10.1.7.254) correct ?

       Because 10.9.x.x is not in the same subnet ?

yes, your understanding is correct

Hope to help

Giuseppe