question about secondary dhcp pool subnet

yunchouljung · ‎04-09-2012

hello, my name is yunchouljung

I have a problem about IP allocation on secondary DHCP pool subnet

primary DHCP subnet allocate a IP address normally

but secondary DHCP subnet dont allocate a IP address to wireless clients

and show ip dhcp binding command displays 1 mac address mapped a multiple ip addresses

why this situation occured ?

bellow is a configuration and dhcp binding status

thanks in advance

ip dhcp pool VLAN116
network 172.21.69.128 255.255.255.128
network 172.21.88.0 255.255.255.0 secondary
override default-router 172.21.88.1
override utilization low 1
default-router 172.21.69.129
dns-server 10.1.1.5 168.126.63.1 210.117.65.1
domain-name xxxx.xx.xx

lease 0 0 10
update arp
!
interface Vlan116
ip address 172.21.69.129 255.255.255.128
ip address 172.21.88.1 255.255.255.0 secondary
!

G_MAIN_C3750X#sh ip dhcp pool VLAN116

Pool VLAN116 :
Utilization mark (high/low) : 100 / 0
Subnet size (first/next) : 0 / 0
Total addresses : 380
Leased addresses : 200
Excluded addresses : 130
Pending event : none
2 subnets are currently in the pool :
Current index IP address range Leased/Excluded/Total
0.0.0.0 172.21.69.129 - 172.21.69.254 0 / 126 / 126
172.21.88.7 172.21.88.1 - 172.21.88.254 200 / 4 / 254

G_MAIN_C3750X#sh ip dhcp binding | inc 172.21.88.
172.21.88.2 44a7.cf0a.428f May 05 1993 10:24 PM Automatic
172.21.88.5 44a7.cf0a.428f May 05 1993 10:24 PM Automatic
172.21.88.6 44a7.cf0a.428f May 05 1993 10:24 PM Automatic
172.21.88.7 0100.1302.b4ac.0e May 05 1993 10:24 PM Automatic
172.21.88.8 0100.1302.b4ac.0e May 05 1993 10:24 PM Automatic
172.21.88.9 0168.0927.9f36.97 May 05 1993 10:24 PM Automatic
172.21.88.10 0168.0927.9f36.97 May 05 1993 10:24 PM Automatic
172.21.88.11 0168.0927.9f36.97 May 05 1993 10:24 PM Automatic
172.21.88.12 0168.0927.9f36.97 May 05 1993 10:24 PM Automatic
172.21.88.13 0100.1302.b4ac.0e May 05 1993 10:24 PM Automatic
172.21.88.14 0100.1302.b4ac.0e May 05 1993 10:24 PM Automatic
172.21.88.15 0100.1302.b4ac.0e May 05 1993 10:24 PM Automatic
172.21.88.16 0100.1302.b4ac.0e May 05 1993 10:24 PM Automatic
172.21.88.17 0168.0927.9f36.97 May 05 1993 10:24 PM Automatic
172.21.88.18 0168.0927.9f36.97 May 05 1993 10:24 PM Automatic
172.21.88.19 0100.1302.b4ac.0e May 05 1993 10:24 PM Automatic
172.21.88.20 0100.1302.b4ac.0e May 05 1993 10:24 PM Automatic
172.21.88.21 0168.0927.9f36.97 May 05 1993 10:25 PM Automatic
172.21.88.22 0168.0927.9f36.97 May 05 1993 10:25 PM Automatic
172.21.88.23 0198.d6bb.87eb.62 May 05 1993 10:25 PM Automatic
172.21.88.24 0198.d6bb.87eb.62 May 05 1993 10:25 PM Automatic
172.21.88.25 0168.0927.9f36.97 May 05 1993 10:25 PM Automatic
172.21.88.26 0168.0927.9f36.97 May 05 1993 10:25 PM Automatic
172.21.88.27 0100.1302.b4ac.0e May 05 1993 10:25 PM Automatic
172.21.88.28 0100.1302.b4ac.0e May 05 1993 10:25 PM Automatic
172.21.88.29 0198.d6bb.87eb.62 May 05 1993 10:25 PM Automatic
172.21.88.30 0198.d6bb.87eb.62 May 05 1993 10:25 PM Automatic
172.21.88.31 0168.0927.9f36.97 May 05 1993 10:25 PM Automatic
172.21.88.32 0168.0927.9f36.97 May 05 1993 10:25 PM Automatic
172.21.88.33 0168.0927.9f36.97 May 05 1993 10:25 PM Automatic
172.21.88.34 0168.0927.9f36.97 May 05 1993 10:25 PM Automatic

Peter Paluch · ‎04-14-2012

Hello yunchouljung,

The "secondary" subnet in the DHCP configuration is quite a confusing feature, to be honest, and it is not related to secondary IP addressing on an interface. If your wireless clients are directly placed into Vlan116, it is not possible to determine from incoming broadcasted DHCP messages whether the client should be offered an IP address from the primary or secondary network in a DHCP pool. Therefore, your wireless clients are most probably receiving IP address from the primary subnet.

According to the documentation about the DHCP service at

http://www.cisco.com/en/US/docs/ios-xml/ios/ipaddr_dhcp/configuration/12-4t/config-dhcp-server.html#GUID-42535D84-E35E-4FF7-BAA0-E6EAD1B8AC98

the DHCP server allocates the addresses as follows:

When the DHCP server receives an address assignment request, it looks for a free address in the primary subnet.
When the primary subnet is exhausted, the DHCP server automatically looks for a free address in any secondary subnets maintained by the DHCP server (even though the giaddr does not necessarily match the secondary subnet). The server inspects the subnets for address availability in the order in which the subnets were added to the pool.
If the giaddr matches a secondary subnet in the pool, the DHCP server allocates an IP address from that secondary subnet (even if IP addresses are available in the primary subnet and irrespective of the order in which secondary subnets where added).

The concept of secondary subnet in a DHCP pool may suggest a possible relation to a secondary IP address on an interface, but in reality, these are two independent and unrelated things.

Best regards,

Peter

Jeff Van Houten · ‎04-15-2012

I agree with Peter. If you want your wireless clients to be on a different network, you need a separate vlan and a separate dhcp pool.

Sent from Cisco Technical Support iPad App

yunchouljung · ‎04-15-2012

hi, peter

first of all, thanks for your reply.

i read the documentation deelply

as you mentioned,

first wireless clients received a ip address in the dhcp primary subnet if primary dhcp pool is available.

(i agree with you and i am no matter at this point)

but if the primary dhcp subnet is exhausted(primary subnet is not remain a ip address to allcate for the clients)

the secondary dhcp subnet allcate a ip address to clients automatically.

(at this time, the giaddr(Gateway IP address) does not necessarily match the secondary subnet)

now some wireles clients received a secondary dhcp subnet ip address and using a internet very well.

(see my dhcp binding and pool status)

my question point is that why one MAC address obtained multiple ip addresses

and why some clients can not obtain an dhcp ip address in the secondary subnet (not the all of clients)

best regards

yunchoul