IP issue

[Jun007]

Hi, 

Just want to ask few question about problem i have encountered.

I have core-SW 4500 series on this core-sw i have set my DHCP server. so far its working

but on some occasion, the user get incorrect IP.
(example: my local ip is 10.10.10.x sometimes the host pc would get 192.168.X.X)
 by the way after core-sw connection goes directly to access-sw which is a 2960 G series.

Thank you

[Kasun Bandara]

your network should have secondary DHCP server which you dont know yet. can be small home router, DHCP enabled PC, etc. when your user got that 192 range IP, check the DHCP server IP and find out the device

[Jun007]

Some cables from the same access switch was plug in to a wifi router.

my first suspicion was because of the wifi router. something like a loopback. i hoping i would understand the problem and come up with a solution on this.

[MHM Cisco World]

Just to checking, 
do you config dot1x in SW? if yes then check the dynamic VLAN assign by SW/AAA do
show auth session and check the VLAN apply to port. 
check show auth session in two case "PC get the different IP".

[MHM Cisco World]

Do you check if dot1x is config in core switch?
we need more info. link 
Access-SW is L3SW and connect to Core and DHCP is use ip helper?
Access-SW is L2SW and connect to Core and L2 Broadcast domain VLAN "SVI=10.10.10.x" but the VLAN is mismatch between the Router and Core SW.

[balaji.bandi]

Do you have this IP address in your network for any VLAN ?  (would get 192.168.X.X))

User - same user Pc or random ?.

provide some configuration information and vlan

[Jun007]

 i dont, most probably the 192.168.X.X was coming from the wifi router plug in to some cable on the same switch.

i was trying to understand the problem and hopefully come up with a solution to prevent from happening again.

[Mark Elsen]

 - In general , don't use dhcp services on switches, but use a separate platform , in a  network surge or storm , dhcp packets may get lost (e,g.)

 M.

[Richard Burts]

The original post gives us very little detail to work with. I think the question about whether the 192.168.x.x exists in this network is a good question and would like to know the answer. (and I wonder what is the point in trying to disguise the last 2 octets. 192.168 is private addressing - how would knowing the last octets impact your network?) Seeing the config of the switch would be a good place to start in investigating this issue.

I like the suggestion that there may be some other device acting as dhcp server should have some followup. One way to investigate this would be to run debug for dhcp on the switch and watch for any activity for 192.168.

[Jun007]

Hi Rick,

The 192.168.X.X is only to provide example on what is happening..
the vlan configuration was 10.10.20.0/24. however sometimes the windows pc is getting the above mention IP.

i suspect that the problem could be the wifi router plug in to some other cables which belongs to the same switch.

i looking for a way to prevent.. but so far i couldn't do it.

[Jun007]

i would love to have a separate device that handles DHCP, but unfortunately there is none at the moment.

[Joseph W. Doherty]

Is the host PC a Windows host?

If so, I recall, Windows hosts, if they do not receive a DHCP IP quick enough, will revert to a 192.168.x.x IP.

Also if this is what's happening, it might be caused by edge ports, being used by PCs hosts, not having portfast configured.

[Richard Burts]

The typical Windows behavior when a computer is attempting to obtain an IP address using DHCP and does not get the address is to use an address from 169.254

[Jun007]

169.254 is correct when a windows pc failed to get IP from DHCP server.

[balaji.bandi]

169.254 is correct when a windows pc failed to get IP from DHCP server.

Many client operating systems use something called Automatic Private IP Addressing. This process assigns an IP address even in the absence of a DHCP server. If a DISCOVER message is not answered, the client picks a random 16-bit number and prepends it with 169.254.x.x. It performs a gratuitous ARP and assigns that address to itself.