IP-HELPER command

Unanswered Question
Oct 28th, 2007

Hi everybody

I have two routers (routerA and routeurB)

RouterA has one LAN interface, one serial link towards routerB.

The latter (routerB) has, apart the serial link with routerA, two LAN interfaces : fa0/0 fa0/1.

A DHCP server is on the subnet of fa0/1

My question is about how to use the ip helper command to help people on routerA's LAN interface to go the DHCP server via its serial link.

on routerA's fa0/0 : ip-helper DHCP_IP_ADDRESS.

but before reaching the DHCP server, the UDP broadcasting packet must pass through the serial link.

Does routerB needs no to advertise its serial interface with the ip-helper command also ?

Or routerA encapsulated directly the packet to reach the DHCP server ?

Here's what i think the packet should look like (in my opinion, it won't be a new broadcasting causing routerB use ip helper on its serial int)

IP sender : routerA's IP

IP destination : DHCP'IP

MAC sender : routerA's

MAC destination : routerB's

and the DATA will inform the DHCP server the host which is trying to get an IP by putting it's MAC associated with the IP

I have this problem too.
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bvsnarayana03 Mon, 10/29/2007 - 03:08

Config is ok, assuming that router A knows how to reach the dhcp server on router B, either by routing protocol or static routes.

swmorris Tue, 10/30/2007 - 12:19

You are correct on the IP source/destination part. The MAC's vary on layer2 links, so that would depend on how many hops are between the forwarding router and the DHCP server!

As for the Data part, not necessarily. If you have "ip dhcp smart-relay" turned on then it WILL update the "giaddr" field of the DHCP data for gateway interface address.

Otherwise, the receiving DHCP server will match the scope based on the IP source address of the forwarded packet.



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