DHCP

[WEERAKOO69BA]

Dear all,

I have one question which has not clear to me.

If I put following  command ,in the router

ip dhcp excluded-address 192.168.80.1 192.168.80.20  

and if I give an ip as 192.168.80.80,or 81 to a pc as a static ip(that means If I configure the pc's ip as 192.168.80.80 or 81) ..what would happen??Will there is a tendency to a loop occuring??Or otherwise nothing will happen because other addresses after 192.168.80.20 - 255 will automatically flooded except 80 and 81 ??

Can someone explain properly??

Thanks

[Latchum Naidu · Accepted solution]

Hi,

When you statically / manually set addressing on a device (DHCP off) you run the risk of the DHCP server reassigning that same IP address to another device when the DHCP server considers the temporary lease it initially issued as expired.

Then you have a IP conflict and one or more devices ends up being dropped from the network until it's resolved.

Hope this clear you.

Please rate the helpfull posts.

Regards,

Naidu.

[WEERAKOO69BA]

Hi  Naidu,

To avoide the flapping i have given the command as " ip dhcp exclude addres 192.168.80.80 192.168.80.81" in the dhcp server( in this case dhcp server=core1).But I have noticed stil the flappling is there on 80 and 81 ips.So I issuesd the command as clear ip dhcp binding 192.168.80.80 and 81.But i got the message as--"% Adddress 192.168.80.80 is not in the database.".That means tis flapping will come back again.Problem did not solve.What kindof solution you can recommand me ??

Thanks

[cadet alain · Accepted solution]

Hi,

Before giving an IP to a host the DHCP server should ping this IP and if it receives an answer it won't offer it to the dhcp client but

for this the host with static IP must be up when the server tries to offer the same address.

So you should  put the static addresses in the excluded addresses list to avoid  duplicate IPs which would lead to strange behaviours in your network.

Regards.

Alain.

[WEERAKOO69BA]

Thanks a lot for both of

you.

I have some other quirres also..

1) In the same sinario,Suppose someone has given a static ip as 192.168.80.81 without the knowledge of others,when it is flapping,how we can find out the cause of the problem ..what are the symptoms of this problem.

2)how we can find which addres is flapping(sh dhcp ip conflict??)

3)how we can  findout  where they have asign  that problamatic ip .what is the procedure.

In short..how to findout/solve this type of ip flapping problem

Many tanks

[cadet alain]

Hi,

What do you mean by flapping?

If the dhcp server gives the same IP to another host then is it flapping for you?

sh ip dhcp conflict--->  will tell you the IP the server tried to offer but didn't because when he pinged he received a reply.

To solve the problem, you mean detect the problem?

the solution for me is to use only static for servers, printers,routers, and so on  and leave all hosts as DHCP.

Regards.

Alain.

[Latchum Naidu]

Hi,

First of all I would like to say thanks for your rating.

1) In the same sinario,Suppose someone has given a static ip as 192.168.80.81 without the knowledge of others,when

it is flapping,how we can find out the cause of the problem ..what are the symptoms of this problem.
You will get the below error messages pop ups on the system.

The system detected an address conflict for IP address with the system having hardware address . The local interface is being disabled.

2)how we can find which addres is flapping(sh dhcp ip conflict??)
You need to use IP Conflict finder to find out the the conflict one, you can get free download in web.

3)how we can  findout  where they have asign  that problamatic ip .what is the procedure.
Once you get the conflit IP, then you need to trace where it is connected by using "sh arp" command in your switch

or router.

Hope this is clear for you.

Please rate the helpfull posts.

Regards,
Naidu.

[WEERAKOO69BA]

Thanks a lot for the reply,

I have one more query also.ie  suppose if I connected my pc to a switch and if I give "sh arp" on the same switch,whose mac address is shown?Is it switch's port mac address or the pc mac address??

why I am asking all these questions you know I just want to know the way of finding of a pertacular ip address to a perticular port.

Thanks

[cadet alain]

Hi,

why I am asking all these questions you know I just want to know the way
 of finding of a pertacular ip address to a perticular port

if you want to link an ip address to a port, you could do use traceroute  mac ip command (if it is supported on your platform/IOS)

http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SXF/native/configuration/guide/l2trace.html

sh arp will only give you ip-mac mapping but then you'll have to look at the CAM table to see the port where this mac was learnt and if the the machine never used the SVI on that switch then your sh arp will be of no use, you'll have to ping from another machine then look at arp cache and trace the mac in the CAM tables.

Regards.

Alain.

[WEERAKOO69BA]

HI Thanks for the quick reply,

In my case switch is an access switch,

#sh arp

#Internet  192.168.60.60           9   0026.9914.e63f  ARPA   Vlan150

...................................................................................................

........................................................................................................

no i need to know the port associated with 192.168.60.60

#Ground_SW1#sh mac address-table address 0026.9914.e63f
          Mac Address Table
-------------------------------------------

Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
  20    0026.9914.e63f    DYNAMIC     Te0/1

31    0026.9914.e63f    DYNAMIC     Te0/1
  33    0026.9914.e63f    DYNAMIC     Te0/1
  50    0026.9914.e63f    DYNAMIC     Te0/1
  60    0026.9914.e63f    DYNAMIC     Te0/2
  70    0026.9914.e63f    DYNAMIC     Te0/1
150    0026.9914.e63f    DYNAMIC     Te0/1
Total Mac Addresses for this criterion: 7

Actually this Te0/1 is the uplink going to the core not the pc which has the abbove ip.

[cadet alain]

Hi,

Then you'll have to do sh cdp nei detail to get IP address of the other switch and then telnet to it and do a show mac address-table for this mac.

Regards.

Alain.