ip-helper set to network address - what happens?

[dinfantino]

Does anyone know the behavior of an ip helper address configured as the network address?  For example 192.168.1.0 ??  Does this behave in the same fashion as the broadcast address??  I cannot setup a sniffer or configure a lab currently to test.

[Peter Paluch]

Hello,

Interesting question To my best knowledge, it should behave in the same manner as specifying the network's broadcast address, i.e. using a directed broadcast. Such a packet will be routed towards the destination network. On the router that is physically connected to this network, this directed broadcast will either be forwarded to the network - if the interface is configured with the ip directed-broadcast command - or it will be dropped.

Best regards,

Peter

[Fabio Francisco]

Hi Peter,

That's very interesting... I thought the Ip-helper would convert the broadcast message into a unicast message and send it to the specified ip address.

[Peter Paluch]

Fabio,

The ip helper-address command merely repackages selected UDP broadcasts to the IP address you have provided in the command, but if you provided a network or broadcast address, the UDP helper is not going to argue with you, and instead, do just what you ask it to do. If you use the address of the entire network in this command, the router is not going to speculate whether the address you specified is a network address. It simply takes the UDP broadcasts, replaces their destination IP address with the address you have specified, and sends the packet along its route. So at this point, there is no determination what the destination is - whether it can be considered a unicast or broadcast.

It is simply a convention that sending packets to the IP address of the entire network will result in packaging this packet into a broadcast frame in this network, i.e. the same treatment as with broadcast IP addresses.

In most cases, using the ip helper-address this way is not necessary nor common, but some deployments may benefit from that.

Best regards,

Peter

[Joseph W. Doherty]

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Posting

What Peter has described, I believe is what's supposed to happen.  This would somewhat mimic having DHCP server(s) on the same subnet, i.e. you're not tied to specific IPs.