Wired and Wireless NICs on same subnet using DHCP

[Guest]

Hi,

I have a laptop with a wired nic(fast ethernet) , and a wireless
nic(802.11g). The wireless nic uses WPA to connect.

Both nics connect to the same subnet. Both nics use DHCP to get an IP
address fom a Windows 2003 DHCP Server in that subnet.

When you connect to the network with ONLY the wireless nic, it pulls and IP
address and eveything works. When you connect to the network with ONLY the
wired nic, it pulls and IP address and everythig works.

BUT, it you are already connected via the wireless nic, and then plug in the
wired cable, then the wired nic won't be able to get an IP from the DHCP
server.

I captured the DHCP traffic in this last situation and found out that after
the wired nic gets an IP address from the DHCP server, it performs a Gratious
ARP (nothing bad here, that's good). But then the wireless nic performs a
Gratious ARP for the IP assigned to the wired nic!. The wired nic hears that
gratious ARP and conclude it cannot use the IP given by the server.

Is there any configuration in Windows XP that can prevent this from
happening? This hasn't always happened. I'm not sure when it started. I have
come to believe it may have been an update that started this problem. Is this
something well known?

Thanks for any advice.

 

[Steve Winograd [MVP]]

Hi,

I have a laptop with a wired nic(fast ethernet) , and a wireless
nic(802.11g). The wireless nic uses WPA to connect.

Both nics connect to the same subnet. Both nics use DHCP to get an IP
address fom a Windows 2003 DHCP Server in that subnet.

When you connect to the network with ONLY the wireless nic, it pulls and IP
address and eveything works. When you connect to the network with ONLY the
wired nic, it pulls and IP address and everythig works.

BUT, it you are already connected via the wireless nic, and then plug in the
wired cable, then the wired nic won't be able to get an IP from the DHCP
server.

I captured the DHCP traffic in this last situation and found out that after
the wired nic gets an IP address from the DHCP server, it performs a Gratious
ARP (nothing bad here, that's good). But then the wireless nic performs a
Gratious ARP for the IP assigned to the wired nic!. The wired nic hears that
gratious ARP and conclude it cannot use the IP given by the server.

Is there any configuration in Windows XP that can prevent this from
happening? This hasn't always happened. I'm not sure when it started. I have
come to believe it may have been an update that started this problem. Is this
something well known?

Thanks for any advice.

You can disable the wireless network connection when you want to use
the wired network connection.

I don't see how an ARP can cause network problems. What exactly
happens that tells you that the wired NIC concludes that it cannot use
the IP?
--
Best Wishes,
Steve Winograd, MS-MVP (Windows Networking)

Please post any reply as a follow-up message in the news group
for everyone to see. I'm sorry, but I don't answer questions
addressed directly to me in E-mail or news groups.

Microsoft Most Valuable Professional Program
http://mvp.support.microsoft.com

 

[Guest]

You can disable the wireless network connection when you want to use
the wired network connection.

I don't see how an ARP can cause network problems. What exactly
happens that tells you that the wired NIC concludes that it cannot use
the IP?
--
Best Wishes,
Steve Winograd, MS-MVP (Windows Networking)

Please post any reply as a follow-up message in the news group
for everyone to see. I'm sorry, but I don't answer questions
addressed directly to me in E-mail or news groups.

Microsoft Most Valuable Professional Program
http://mvp.support.microsoft.com

Thanks for the answer Steve.

Here's how the ARP causes a problem: After a DHCP client gets an IP from a
DHCP server, it broadcast an ARP asking "Who has my IP"? (An ARP that ask
for the IP of the ARP sender is called a Gratious ARP). The client expects no
answer. If noone asnwers, this confirms that the IP assigned by the server to
the client is not currently in use by another PC. But if someone else answer
to the ARP, then the client will not use the IP assigned by the server. That
is what I see happening. The "other" PC who answers (via sending another
Gratious ARP) is actually the same PC, but using the wireless NIC MAC. This
is not suppose to happen.

You are right, if I disable the wireless NIC, then plug in the wired cable,
the wired nic will get the IP with no problems. And if after that I enable
the wireless nic again, it will also work. But, if I don't disable the
wireless nic, then the events that I described in the previous paragraph will
occur, and the wired nic will say "limited or no connectivity".

I guess for now disabling the wireless nic would be the workaround.

If someone knows the solution to the problem, let me know (so I don't have
to explain the workaround to all my users).

Btw, the wireless NIC is a Intel Pro/Wireless 3945ABG. Maybe the problem is
the driver, although I have the latest one.

Thanks for your help.