dhcp release message

Unanswered Question
Jan 30th, 2009

Hi every body!

If dhcp server receives a dhcprelease message, it will will return the ip address to the pool. But not all the time, a client would be able send dhcp release. for example, a note book moving from one one cell to another cell assuming each cell is using different subnet. Will the dhcp server keep the ip address reserved because notbook did not send any dhcp release packet ? In real life does mobile client send dhcp release message when move from one cell( one subnet) to another cell( different subnet?

I understand usually cells are not mapped to different subnest. In fact several cells can use the same subnet.

thanks

I have this problem too.
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sarahr202 Fri, 01/30/2009 - 19:19

Well I did find the answer . I am posting it for any one who may have the similiar question in mind.

Once a host has been assigned an ip adress in a cell and it moves to another cell, it  will still keep  the old address. traffic destined to the host, is directed by special tunnel in the foreign cell.

Lets take a different case. We turned off  our lap top and walked to the coverage area of another cell and then turned on our lap top. ( assuming this cell has different subnet)

1)Accrding to dhcp specification, once the laptop turned on,  and it finds it still has the valid lease for the ip address, it will send DHCPREQUEST message which includes chaddr feild (client harware adress), the  current ip address in "REQUESTED IP ADDRESS DHCP " option.

This message will be broadcasted.

2) Since client is in different subnet,  the broadcast would be received by dhcp relay agent which depending upon what ip address is pointed under ip helper command, will forward the the message to the dhcp server.  Now there are two possibilities:

1)the ip helper  command pointed  to the ip address  which belongs to the same dhcp server which previously allocated this ip address to host,

2) The ip address pointed at in  ip helper command  belongs to some other dhcp server.

 

In first case, dhcp server receives the messages and concludes  by lookiing at GIAddr that host has moved to different subnet and its current ip address thus can not be renewed. It sends dhcp nak message which would be unicast to dhcp relay agent  that received the dhcp request message.

2) in second case, the dhcp server won't respond at all.

 

 

3) if client receives the DHCPNAK message, it will go back to  dhcp discovery stage. ie it would send dhcp discovery message and eventually , a new ip address from the current subnet , the host is currently in, will be assigned to the host.

 

4) if  host does not receive any reply, it will continue to transmit "dhcprequest message".

if  no reply is received after a period of time, host will conclude no server has any info about its lease and move to "init" state  and  statrs sending dhcpdiscover message" The host will eventually get the  new ip address after few exchanges of dhcp messages.

Thanks a lot!

 

 

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