IP Helper

[DanielBowen]

When using IP Helper for say DHCP b/casts, you configure the IP address of the DHCP server on the interface that doesn't have one locally. Then, when the router recieves the b/cast on that interface, say FE1/1, it searches its interfaces for one configured for the network of the address you used, say FE1/2 and then forwards it out of that. But what if the b/cast must traverse a WAN link? For example, if the router has an ATM card in it connected to a 34mb ATM ring, and the subnet configured resides somewhere out of that. Does the address then need to be configured ont the WAN interface as well?

Thanks for your help

Daniel,

 

[wybnormal]

LAN--IPHELPER/E0--router-S0--WAN--S1-router--E0/IPHELPER---LAN

The first iphelper takes the broadcast and *converts*(better word is?) it to an unicast packet until it gets to the target IP. Keep in mind that you can specify a single IP or a subnet as the target.

192.168.1.1 or 192.168.1.255... either works. Also, while I showed IP helper on both ends, it's not required in all cases to be on both ends.

This is pretty easy to mock up in a lab and to play with. In fact, it is recommended to do just that since the explantions in the books tend to be a bit obscure about it.

MikeS Find me at

http://www.packetattack.com

"The trouble with giving up civil rights is that you never get them back"

 

[svermill]

Mike,

Very good discussion on helper address. I think if you specify .255, you need to add 'ip directed-broadcast' at the destination interface.

Regards,

Scott

 

[wybnormal]

Good point Scott.. you did not used to have to do that but the newer IOS load add a default statement that says "no ip directed broadcast" as a security measure. I found that out by forgetting during an upgrade and the next all my DHCP users went away.. far away ;-)

MikeS Find me at

http://www.packetattack.com

"The trouble with giving up civil rights is that you never get them back"