Hello Daniel
In order to answer your question, it is important to understand how and where a directed broadcast is identified and dropped.
When a host sends a packet to a destination that’s a broadcast address of a particular network, none of the devices in the path of that packet know that it is a directed broadcast, because no subnet information is included in the packet. The only device (router) that will be able to identify that destination IP address as a broadcast address is the router that is directly connected to the related subnet. Only that router knows the subnet mask associated with the address space to which the address belongs.
Now having said that, take a look at this diagram again, where R1 sends a packet to 192.168.23.255. Note that there is no way R1 can know if this is a broadcast address because it doesn’t know the subnet mask associated with it:
However, when the packet reaches R2, R2 knows that this is a directed broadcast because the destination address belongs to the subnet connected to its own Fa0/1 interface, and using the subnet mask configured there, it identifies the address as such.
Now imagine you have the following topology:
R1 — R2 — R3 — R4 -------(192.168.44.0/25)--------- R5
Where the network between R4 and R5 is 192.168.44.0/25 as shown. The broadcast address of this network is 192.168.44.127. Now imagine that R1 sends a packet to 192.168.44.127. Note the following:
- R1 does not know that this is a broadcast address since it has no subnet mask information.
- R2 and R3 don’t know it is a broadcast address for the same reason, thus it is forwarded normally.
- Once it gets to R4, only then is it identified as a broadcast address because it belongs to a directly connected subnet.
If R4 is not configured to forward directed broadcasts, it will be dropped.
So getting back to your question, it all depends upon your topology and how many routers exist between the sender of the packet and the destination. The directed broadcast forwarding feature must be configured on the last router in the path, that is the router to which the destination network of the directed broadcast is directly connected. Does that make sense?
I hope this has been helpful!
Laz