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How can an end host host get a new IP address when its VLAN is changed ?

Posted on 2015-01-06
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Last Modified: 2015-01-07
Hi,

I am trying to implement NAC (by ForeScout) in our company.

Before we push automatic VLAN change upon user login (by ForeScout NAC device) into our production network, I am testing end-host behavior by changing VLANs manually from a CounterACT Manager Console. However end hosts are not getting a new IP address after I change their VLAN from CounterACT Manager Console. The work around to this problem is that we manually need to disconnect and reconnect Ethernet cable to the end host NIC for IP address change to take effect.

So to debug more into this, I changed VLAN manually from switch itself & observed that end host is still NOT able to get a new IP. Again this time I had to implement same workaround of disconnecting and reconnecting Ethernet cable to the end host for IP address change to take effect.

When observed on Wireshark, it shows that end host sends DHCP Discovery request for new VLAN only when I disconnect & reconnect the Ethernet cable attached to its NIC.

Before carrying out those tests I had made sure that NIC drivers were up-to-date on the end host.

How do we make end-host get a new IP address without disconnecting (& reconnecting) cable attached to its NIC? Has anybody seen this issue when NAC (Cisco or ForeScout)  is implied ?


Thanks !!!
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Question by:CCIE_Universe
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by:giltjr
giltjr earned 500 total points
ID: 40534682
When you use DHCP the end computer will not try and get a new address unless:

1) It sees a physical disconnect
2) The lease  expires
3) It is forced to by issuing ipconfig /release then ipconfig /renew.

Changing the VLAN on a port will not cause any of these situations.  You will have to somehow force something.

What I would try is do a "shut" on the interface, change the VLAN, then do a no shut.  The other option is to run a script that will do the /release & /renew.
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by:CCIE_Universe
ID: 40534744
Thanks for the comment/ answer Giltjr, I agree with you.

But still I am confused that there do exist NAC deployments in a production environment in which IP address gets changed for a host. How do they achieve this?

Thanks Again!
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giltjr earned 500 total points
ID: 40534754
I believe if you have a NAC client installed and the switch ports are configured as trunks, the client will detect the VLAN change and initiate release renew.
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Author Comment

by:CCIE_Universe
ID: 40534888
Thanks Giltir !
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Author Closing Comment

by:CCIE_Universe
ID: 40536147
Thanks for the Help!
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