IP-Helper Address

[andrewrocks]

Hello,

According to the documentation the IP Helper Address can either be a single IP address for a server or a network address.

I am a bit unclear which "network address" it's refering to, for example in a 172.20.10.0/24 network would the "network address" in question be "172.20.10.0" or "172.20.10.255".

Thanks

Andrew.

[Jon Marshall · Accepted solution]

Andrew

It would be the subnet broadcast address ie. in your example it would 172.20.10.255.

Jon

[andrewrocks]

Brilliant, thanks Jon.

I've had success with both so wanted to know which one was correct.

[Peter Paluch]

Andrew,

To add to Jon's perfect reply, using the so-called directed broadcast address (i.e. the exact broadcast address of a particular remote network) in the ip helper-address requires also that the final router before the destination network is configured with the command ip directed-broadcast on its interface inside that network. Otherwise, the directed broadcasts would be dropped.

Best regards,

Peter

[andrewrocks]

Hi Peter,

Can I just clarify the setup we have as I'm not sure if this applies to us?

We are using a 3560 with 7 VLANs and a seperate firewall as the default route (none-cisco). The DHCP server is in a VLAN with the other Active Directory servers all conencted to the same 3560.

Do I need the ip directed-broadcast, and if so, which VLAN do I apply it to?

Thanks for your help

Andrew.    

[Peter Paluch]

Hi Andrew,

Oh, I probably created more confusion than helped with this... I am sorry.

If you are using the ip helper-address with a particular IP address of your DHCP server and not with the entire broadcast address of the network where the DHCP server resides then you do not need that ip directed-broadcast stuff at all.

You would need to use that command only if you were relaying the DHCP client requests towards the entire broadcast address of the network with the DHCP server. In your case, the ip directed-broadcast command would be applied on the SVI for the VLAN in which the DHCP server and other Active Directory servers are located.

In any case, if it is working for you now, there's no need to change anything.

Best regards,

Peter

[Joseph W. Doherty]

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Posting

"Do I need the ip directed-broadcast, and if so, which VLAN do I apply it to?"

Normally, if you know the IP of the DHCP server(s), you use that.  (This avoids the need of having directed broadcasts enabled, which can be considered a security issue.)

You would apply helpers on all the VLANs where a DHCP server is not on the same VLAN.

[andrewrocks]

Thanks both, I understand much more clearly now.