RE: DHCP packet exchange when using ip helper-address

From: Joseph L. Brunner (joe@affirmedsystems.com)
Date: Mon Mar 02 2009 - 04:04:15 ARST

All you need to remember is DORA!

Discover BROADCAST, send by client seeking lease
Offer UNICAST, sent back from server
Reply BROADCAST, send by client saying he'll take the address offered
Ack UNICAST, sent by server telling the client lease params

Guys try the Wikipedia article

-Joe

-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of Sadiq Yakasai
Sent: Sunday, March 01, 2009 6:23 PM
To: C Chan
Cc: Cisco certification
Subject: Re: DHCP packet exchange when using ip helper-address

Hi Chan,

The DHCP Server sends a unicast to the IP address of the interface where the
IP helper address is configured. Its is not a directed broadcast. A good way
to easily test and verify this is to configured the IP helper address on any
router interface, and turn off directed broadcast on the interface (which is
off by default on Cisco routers anyway), and see DHCP working find.

I have not been able to get you a document for the packet flow, but you
should find good info here:

http://www.cisco.com/en/US/docs/ios/ipaddr/configuration/guide/iad_dhcp_rly_agt_ps6350_TSD_Products_Configuration_Guide_Chapter.html

Sadiq

On Sun, Mar 1, 2009 at 11:02 PM, C Chan <cch.ccie@gmail.com> wrote:

> Hi Expert,
>
> Whenever the DHCP server (eg 172.16.1.1) is located remotely, we ususally
> use "ip helper-address 172.16.1.1" in local router interface (eg
> 192.168.1.254) in order to grant IP address to local user. As I know, the
> local router will unicast the DHCP packet (src 192.168.1.254 udp 67, dest
> 172.16.1.1 udp 68) to DHCP server. How will the DHCP server reply? Does it
> unicast back the DHCP packet to router's interface (ie 192.168.1.254)? or
> DHCP will reply to a directed-broadcast address of request LAN (ie
> 192.168.1.*255*)?
>
> Where can I find a cisco official doc to describe the actual L3 packet
> exchange?
>
> Chan
>
>
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CCIE #19963

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