the meaning of "ip broadcast-address"

[att-sgcops]

I have 3 routers in the same LAN. and their IP addresses are:

R1 E0: 10.1.1.1 255.255.255.0

R2 E0: 10.1.1.2 255.255.255.0

R3 E0: 10.1.1.3 255.255.255.0

And on the R1, I configured e0:

"ip broadcast-address 10.1.1.2" and want to see what happen.

from R1 I can ping R3, but cannnot ping R2 (10.1.1.2) any more.

can someone tell me why?

by the way, what is the meaning of "ip broadcast-address 0.0.0.0"?

[pkhatri]

Hi,

That command configures the address as the local broadcast address for the segment. The default is 255.255.255.255. (you can view this using 'sh ip int '

When you set the broadcast address on R1 to be 10.1.1.2, it will be sent with an all-ones MAC address (FFFF.FFFF.FFFF) and a destination IP of 10.1.1.2. Since R2 does not recognise 10.1.1.2 as a broadcast address, it will drop that packet. Since the MAC address is a broadcast address, the IP destination address also has to be a broadcast address (RFC1009).

Some older IP implementations used to treat 0.0.0.0 as the broadcast address (not conformant with RFC1009).. setting it to 0.0.0.0 just emulates that behaviour.

Hope that helps - pls rate the post if it does.

Paresh

[pkhatri · Accepted solution]

A bit more.

It does not make sense to use an address for the broadcast address that is the same as that of an actual device. So to test this, I would use something like 10.1.1.10. Then configure 'ip broadcast-address 10.1.1.10' on all three of your routers. Then, when you ping 10.1.1.10 from any of them, you should get two replies back.

Hope that helps - pls rate the post if it does.

Paresh