Is there an any option in cisco Router we can restrict
MTU size to something smaller? default is
1514 but for experiment i want to allow
512 only and if packet if bigger than 512 discard them.
You can do this with policy based routing, although this is an example of filtering any packet bigger than 512 bytes, it does not make your router fragment packets to 512 bytes.
ip access-list extended Filter permit ip any any router-map Filter match ip address Filter match length 1 512 set interface Null0
On the inbound interface:
ip policy route-map filter
This method will only work on inbound traffic coming in through the interface you have set the policy on
If you want to filter outbound traffic from the router, instead of an interface level command, use the following global command:
ip local policy route-map filter
You can also match only certain traffic by modifying what the access-list matches, or if using the global level command, and you want to filter outbound traffic only on a certain interface, you can do an extra
match <out-bound interface> command in the route map