can't get IP address from DHCP

jerryjia1 · ‎12-14-2014

I have cisco 1941 as broadband router, configured as a dhcp server, the ip scope is 192.168.0.0/23. the problem is some time the client can't get ip from the router. and i have to telent to the router and issue the command: clear ip dhcp binding * everytime, it will get better for a little while, but will failed to get ip again maybe after 1 hour.

when the problem happens, i checked the cpu and memory utilization, it's not high. so i confused where the root problem is?

or is the NAT transloation session limit issue ?

i have attched everyting i think useful for you, if you need anything else please let me know, and thanks in advance for finding the root cause for me.

johnlloyd_13 · ‎12-14-2014

hi,

your router DHCP config looks fine. it shouldn't be a NAT issue since a client would need to get an IP first.

could you share your sanitized config to further check?

you should capture the problematic client's ipconfig /all output and see if it get's a 169.x.x.x address or gets another IP from a rogue DHCP/router.

jerryjia1 · ‎12-14-2014

below is my configuration.

and for the client, i am sure it gets 169.X.X.X address, thanks

===============================================

Guest#show run
Building configuration...

Current configuration : 2824 bytes
!
! Last configuration change at 11:10:45 Beijing Mon Dec 15 2014 by admin
version 15.3
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname Guest
!
boot-start-marker
boot system flash c1900-universalk9_npe-mz.SPA.153-1.T.bin
boot-end-marker
!
!
enable password password
!
no aaa new-model
clock timezone Beijing 8 0
!
!
!
!
ip dhcp excluded-address 192.168.1.1 192.168.1.100
ip dhcp excluded-address 192.168.1.254
!
ip dhcp pool Wifi_GUESTS
import all
network 192.168.0.0 255.255.254.0
default-router 192.168.1.1
dns-server 202.96.209.5 202.96.199.133 202.96.209.133
!
!
!
ip cef
ipv6 multicast rpf use-bgp
no ipv6 cef
multilink bundle-name authenticated
!
vpdn enable
!
vpdn-group pppoe
request-dialin
protocol l2tp
!
!
!
license udi pid CISCO1941/K9 sn FGL153523F1
!
!
!
!
csdb tcp synwait-time 30
csdb tcp idle-time 3600
csdb tcp finwait-time 5
csdb tcp reassembly max-memory 1024
csdb tcp reassembly max-queue-length 16
csdb udp idle-time 30
csdb icmp idle-time 10
csdb session max-session 65535
!
!
!
!
interface Embedded-Service-Engine0/0
no ip address
shutdown
!
interface GigabitEthernet0/0
ip address 192.168.100.1 255.255.255.0
ip nat inside
ip virtual-reassembly in
duplex auto
speed auto
!
interface GigabitEthernet0/1
no ip address
ip nat outside
ip virtual-reassembly in
duplex auto
speed auto
pppoe enable group global
pppoe-client dial-pool-number 1
no cdp enable
!
interface FastEthernet0/1/0
switchport access vlan 14
no ip address
!
interface FastEthernet0/1/1
switchport access vlan 14
no ip address
!
interface FastEthernet0/1/2
switchport access vlan 14
no ip address

interface FastEthernet0/1/3
switchport access vlan 14
no ip address
!
interface Vlan1
no ip address
!
interface Vlan14
ip address 192.168.1.1 255.255.254.0
ip nat inside
ip virtual-reassembly in
!
interface Dialer1
ip address negotiated
ip mtu 1492
ip nat outside
ip virtual-reassembly in max-reassemblies 1024
encapsulation ppp
dialer pool 1
dialer idle-timeout 0
ppp authentication pap callin
ppp chap hostname username
ppp chap password 0 password
ppp pap sent-username username password 0 password
no cdp enable
!
ip forward-protocol nd
!
no ip http server
no ip http secure-server
!
ip nat inside source list 1 interface Dialer1 overload
ip route 0.0.0.0 0.0.0.0 Dialer1
ip route 192.168.2.0 255.255.255.0 192.168.1.30
!
!
!
access-list 1 permit any
!
control-plane
!
!
!
line con 0
line aux 0
line 2
no activation-character
no exec
transport preferred none
transport input all
transport output pad telnet rlogin lapb-ta mop udptn v120 ssh
stopbits 1
line vty 0 4
password password
login local
transport input all
!
scheduler allocate 20000 1000
!
end

========================================================

johnlloyd_13 · ‎12-14-2014

hi,

check the client if TCP/IP setting is set to auto obtain IP.

is the AP/wifi router connected to the 1941 switch module and configured for VLAN 14?

jerryjia1 · ‎12-14-2014

Dear johnlloyd_13,

i am sure the client TCP/IP setting is no problem, because it can be connected to other wifi successfully.

and i have configured VLAN 14 for this wifi LAN. becuase we have about 20 APs locates in 4 different floors.

everytime when this porblem happens, i just issue: clear ip dhcp binding *, the client can get ip address.

mohseen patel · ‎12-15-2014

Hi Jerry,

Would suggest to capture debug logs debug dhcp detail when this situation occurs if it permits.

Logs will give better idea of issue.

Br.

Mohseen Patel

jerryjia1 · ‎12-15-2014

Dear mohseen patel,

thanks for your suggestion, i will debug it when it happen again next, maybe tomorrow will provide the debug detail information, thanks

jerryjia1 · ‎12-16-2014

Dear mohseen patel,

this morning, the problem happened again, and i issue debug command on this router, found the pool exhausted which caused the problem. but it's weird that when i issue show ip dhcp pool command, it shows only few ip address were assigned to client.

and i also issue the command, show ip dhcp conflict, there are many conflict ip address in this table, maybe this is the root cause, but i don't know why this happened. why there are so many conflict ip address exist ? BTW, i set to the leased time to 1 day.

tprivett · ‎12-16-2014

The reason you are seeing such a high number of conflicts I believe may be from you clearing your binding pool so often. If a client has already obtained an address, and you clear the binding pool, that address gets reassigned. Once the conflict is detected the router will put that address in the conflict pool, and not reassign it.

Instead of clearing your binding pool the next time this happens try to clear your conflicts. It may take some time for you get back to a stable state with clients not trying to request the same address. Also, if this is primarily a wireless client base it may not be a bad idea to reduce your default DHCP Lease time so addresses return to the pool more frequently when users disconnect.

Here is some documentation on configuring the lease time, if you are interested.

http://www.cisco.com/c/en/us/td/docs/ios/12_2/ip/configuration/guide/fipr_c/1cfdhcp.html#wp1001095