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New Member

ip helper-address problem


I am having a problem on a 1601 router on a remote site in a frame-relay point to multipoint network. There are currently 9 working remote sites where the routers are forwarding DHCP requests to a central DHCP server except the one 1601. Any ideas why this could be? How does the router on the DHCP site return the DHCP request.



New Member

Re: ip helper-address problem

By default broadcasts are not forwarded by routers; routers breakup broadcast domains. A DHCP request is a BOOTP broadcast. In order for a router to forward these boradcasts to their proper desination, you need to use th ip helper-address command.

Communication Sequence

The client broadcasts (MAC and IP) a BOOTP request. If a Cisco router sees this, has a helper address coded, and is forwarding UDP port 67, it does the following:

1. The router places the IP address of the interface which received the broadcast in the "giaddr" field (meaning Gateway IP Address). This will be the address to which the BOOTP server will send the BOOTP response.

2. The router forwards this packet as an IP unicast to the IP helper address.

3. The BOOTP server gets the packet, finds the client's MAC address in its tables, and sends a reply with the client's IP address and boot file information.

4. The reply is unicast directly to the Cisco router's IP address (giaddr).

5. When the router receives the BOOTP response (UDP port 68), it retrieves the original client's MAC address and IP address from the data portion of the packet and sends the packet as a MAC and IP unicast out whichever interface is directly connected to the client's IP subnet (unless the "broadcast reply" bit was set by the client; in that case, it is a MAC-level broadcast).

That's how it works.

I'd compare confis with the other devices.

Good luck,


New Member

Re: ip helper-address problem