few questions about broadcast scenarios.

[sarahr202]

Hi every body.

Let say we have enabled forwarding of broadcast on our router.  Please consider the following routing table.

R  10.0.0.0/8 via s0

R 10.0.0.0/16 via  s1

R 10.10.10.0/24 via s3

scenario #1

router receives a packet with destination ip 10.0.255.255.

Since that ip address (10.255.255.255) falls with in the range of both 10.0.0.0/8  and 10.0.0.0/16 ranges, router will forward the packet out of s1 and s0.

Is it correct?

thanks and  happy new year to all of you.

[sarahr202]

correction please  10.255.255.255 should be read as 10.0.255.255, the destination ip address.

[speculor_cisco · Accepted solution]

Hello Sarahr202.

With unicast destination address the rule is "longest match".

So the router would forward the packet via s1.

With directed broadcast destination address I presume the rule is the same.

If nobody will confirm, I will try your scenario with the GNS simulator.

[Peter Paluch · Accepted solution]

Hello,

Correct, the router will use the longest prefix match also on a destination address that constitutes a directed broadcast. In fact, the directed broadcast has the true meaning of a broadcast only on the router that is directly connected to the destination network. Routers farther away from the destination network do not make any assumptions whether the address is or can be considered a directed broadcast address - they cannot do a competent decision because their routing tables may contain summarized or superficially subnetted entries. So for all routers, the destination IP, be it a directed broadcast or a unicast destination IP, it is just an IP and will be routed in a completely normal fashion. It is only the last hop router that will encapsulate this packet into a broadcast frame and send it out the destination interface.

Best regards,

Peter

[speculor_cisco]

Hello Peter and thanks for your, as usual, helpful explanation.