single dhcp server with multiple subnets

Nikolay P nikolay.p at cos.flag.org
Fri Jul 25 23:33:17 UTC 2014


As Patric said the server should know what subnet a request came from. Then it will be able to assign address from proper pool. 
Show us DHCP daemon log records containing those requests. 
In my setup when such a request comes in it looks like this: 

DHCPREQUEST for 10.0.0.1 from xx:xx:xx:xx:xx:xx via 10.0.0.254 

10.0.0.254 in this case is a Cisco switch VLAN interface (which is a router for this VLAN) address. 
Server knows that request came from 10.0.0.254 network, thus it will assign proper address. 

So, show us some of your DHCP server logs where the requests can be seen. 


----- Original Message ----- 
From: "Mike Senko" <Mike.Senko at seattle.gov> 
To: dhcp-users at lists.isc.org 
Sent: Friday, July 25, 2014 6:15:09 PM GMT -05:00 US/Canada Eastern 
Subject: single dhcp server with multiple subnets 




I’ve gone through the archives, but haven’t found a solution to using a single dhcp server 

to service multiple subnets. 



The subnets are set up on a router (non-cisco) that uses dhcp-relay to pass dhcp requests 

through the configured switch interfaces. The switches are layer 2 and also are configured 

for dhcp relay. 



The dhcp request packets show up at the server. 



The dhcp server then assigns and address and sends it to the router’s address. 



The problem I’ve run into is the server is always sending the first dhcp address in the first configured 

subnet regardless of the subnet the request comes from. 



In other words, dhcp ip address assigned is always 10.1.6.200 even if the request comes from 10.1.8.0 subnet. 



If the dhcp request comes from the 10.1.6.0 subnet, all is fine. 



I think the problem is in the dhcp server configuration: 



option domain-name "TestLab.NSC"; 



default-lease-time 600; 



max-lease-time 7200; 



authoritative; 



log-facility local7; 



subnet 10.1.6.0 netmask 255.255.255.0 { 



option routers 10.1.6.2; 



range 10.1.6.200 10.1.6.254; 



option subnet-mask 255.255.255.0; 



} 



subnet 10.1.7.0 netmask 255.255.255.0 { 



option routers 10.1.6.2; 



range 10.1.7.200 10.1.7.254; 



option subnet-mask 255.255.255.0; 



} 



subnet 10.1.8.0 netmask 255.255.255.0 { 



option routers 10.1.6.2; 



range 10.1.8.200 10.1.8.254; 



option subnet-mask 255.255.255.0; 



} 



subnet 10.1.9.0 netmask 255.255.255.0 { 



option routers 10.1.6.2; 



range 10.1.9.200 10.1.9.254; 



option subnet-mask 255.255.255.0; 



} 



subnet 10.1.5.0 netmask 255.255.255.0 { 



option routers 10.1.6.2; 



range 10.1.5.200 10.1.5.254; 



option subnet-mask 255.255.255.0; 



} 



This is the entire configuration, there must be something missing. I’m not using dns, just trying to get addresses assigned 

to the right vlan/subnet. The server is attached to 10.1.6.0/24. 



Thanks, ms 
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