Summarization question

[bobrosa]

Page 149, Example 3.15: The question is "What range of addresses will be forwarded based on summary route 192.168.144.0/20"

My answer was 192.168.144.1 to 192.168.159.254 (exclude the broadcast address .255).

Todd states "Yes, the broadcast address is forwarded". I thought routers discarded broadcast packets.

Please help me understand why the broadcast address is forwarded.

[lammle]

In a summary router, the router does not know that this is a broadcast address, so it will forward the entire range to the next router. However, the next router won't forward the broadcast address because it is no longer looking for a range, but the valid subnet addresses.
Cheers!
Todd Lammle

[bobrosa]

Just to make sure I understand:

R2 will forward a packet addressed to .255 because it's within the (summarized by R1) range.

R1 will not forward .255, because its routing table has knowledge of the actual subnets being summarized and advertised to R2, and will be able to determine that .255 is a broadcast address.

Right?

So cool to hear directly (and quickly) from the author!:)

[Fuzz]

That's right, any address in a summary will be forwarded to the next hop. The router that has specific knowledge of the subnet will not forward the broadcast.

The router with the summary is not aware that it is a summary, all it knows is the address and mask. Even if it does, it has no information on what subnets are contained in the summary, so it will forward any address in that range.

[bobrosa]

Got it. Thanks, Todd & Fuzz!