From: André Bersvendsen (an-bersv@xxxxxxxxx)
Date: Mon Jul 22 2002 - 07:46:22 GMT-3
Hi,
It is no problem with two or more DHCP servers on the same segment if
they follow the DHCP spesifications.
If you read how DHCP works then you will see that it is no problem with
two or more DHCP servers.
Let's say that you turn on a PC with a DHCP client. This will be the
sequence:
1. The client sends out a DHCPDISCOVER message (BROADCAST).
2a. DHCP server #1 find a IP address that it want to use. This address
is pinged to ensure that it is not in use. Then a DHCPOFFER message is
sent with a 32-bit transaction ID (BROADCAST).
2b. DHCP server #2 do the same as DHCP server #1 at the same time. And
it will send out a DHCPOFFER message with a 32-bit transaction ID that
is not the same as server #1 use (BROADCAST).
3. The DHCP client receive two DHCPOFFER messages. Let's say that it got
the message from DHCP server #1 first.
4. Since the message from DHCP server #1 was received first the message
from DHCP server #2 is ignored.
5. The DHCP client send out a DHCPREQUEST to tell the DHCP servers that
it will use the offered IP address (and other information given in DHCP)
from DHCP server #1. The 32-bit transaction ID received from server #1
is in the message (BROADCAST).
6a. The DHCP server #1 see that the client accept the information and
send out a DHCPACK with the 32-bit transaction ID (BROADCAST).
6b. DHCP server #2 ignore this packet because of the 32-bit transaction
ID that is not known by this server.
7a. DHCP server #1 update the DHCP database.
7b. The client can start using IP unicast.
A good reference for DHCP is "The DHCP Handbook" by Droms and Lemon
(ISBN 1-57870-137-6).
>This looks like it could be troublesome; like if to routers respond at the
>same time with the same address, which one will go into the database?
>
>I'd stick to the split subnet solutions others provided. That's the
>standard way to do it in the industry until the DHCP spec is updated and
>standardized.
>
>-----Original Message-----
>From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
>Andre Bersvendsen
>Sent: Sunday, July 21, 2002 8:11 AM
>To: Mike Thompson; ccielab@groupstudy.com
>Subject: Re: DHCP back up
>
>
>Hi,
>
>Set up both of the routers with identical DHCP configuration and use a
>TFTP server to store the "DHCP database".
>The TFTP server can be set up on one or both of the routers. I use a
>TFTP server on a SUN machine.
>
>My DHCP configuration:
>
>!
>ip dhcp database tftp://192.168.13.70/dhcp-database-FRONTLAN
>ip dhcp excluded-address 192.168.13.0 192.168.13.100
>!
>ip dhcp pool FRONTLAN
> network 192.168.13.0 255.255.255.0
> default-router 192.168.13.1
> dns-server 130.67.15.194 130.67.60.70 62.148.39.11 62.148.39.10
>193.212.1.11 193.212.1.10
> domain-name online.no
> option 150 ip 192.168.13.1
> lease 2
>!
>
>
>
>>Hi,
>>I'm trying to configure 2 routers has a primary and secondary DHCP. I
>>mean is the primary is down the secondary will be the DHCP server. I
>>tried the cisco website I couldn't find much
>>
>>Can anyone help me out?
>>
>>Thanks in advanced.
>>Mike
>>
>>
>>
This archive was generated by hypermail 2.1.4 : Sat Sep 07 2002 - 19:36:38 GMT-3