CCNP - IP Directed Broadcasts

Author

IP Directed Broadcasts

firechicken

2001-10-30, 8:40 am

I'm a little confused about IP directed broadcasts.

If a router receives a broadcast on an interface with IP directed broadcast enabled, must all the outbound interfaces also have IP directed broadcasts enabled to forward the broadcast?

Thanks.

depamo

2001-10-30, 12:57 pm

What I do know is that you have to turn it off on all routers to deny directed broadcasts in a network. From what I have seen, if you have a router with directed broadcasts enabled (default on 12.0 and higher is disabled) when it receives a directed broadcast it will broadcast the packet to all host on the segment. Those routers that it travels through will not stop it if they do see the packet as it is a unicast packet destined for a network that they don't have directly connected.

Pretty sure that last part is right, someone might want to check it. The first part is accurate since this is how you keep from getting a 'smurf' DoS attack on your network.

MadChef

2001-10-31, 3:47 am

It's how you prevent yourself from being an amplifier, not exactly the victim, of a smurf attack.
no ip directed-broadcast prevents a router from converting a layer 3 broadcast address to a layer 2 broadcast. Say you have the network 192.168.1.0 /24. With ip directed broadcast enable, someone could send a packet to 192.168.1.255 and this would be routed across your network (the unicast as depamo mentioned) but when when it got to your router, it would encapsulate the packet into the appropriate L2 broadcast frame (say an ethernet frame destined to FFFF.FFFF.FFFF) so all hosts on that segment would see it. So it's not really receiving a broadcast on an interface with this enabled so much as it is creating a broadcast on that interface.

Make sense?

MadChef

firechicken

2001-10-31, 8:15 pm

(Light comes on) Duh...of course!

Thanks for your help again, guys.

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