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Desktop cannot access vlan, but laptop can

Posted on 2005-05-04
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Last Modified: 2008-02-01
I have Cisco 4506 core, 3550 edge switches.  I have a cable modem setup in a different building.  I created a vlan for this cable modem, plugged it into my edge switched, trunked the switches, and ran it back to my office.  If I plug my laptop into the vlan port on my 4506, I get the cable modem ip address and can access the internet perfectly.  When I take the exact cable and plug it into any of my two desktops, I get a 169.x.x.x invlaid address.  I have tried different nics, different desktops, nothing works on the destkops but works perfect on the laptop.  

Since it does work on the laptop I can assume that the VLAN is setup correctly.  My laptop uses a Intel PRO/1000MT adpater.  The desktops I tried to connect it to had a NETGEAR FA311 Rev c1, Intel Pro/1000 CT, and a Linksys LNE100TX.  If I plug in a cable that is on my normal network vlan, I get a normal ip address of my internal network and I can access my network.  Only when I plug this cable from the cable modem vlan I get a invalid IP on my desktops, but works perfect on my laptop.

Any ideas?


My OS on all machines is XP sp2.  No firewalls are enabled.  
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Question by:teecee33
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pseudocyber earned 2000 total points
ID: 13928430
Assuming you're not restricting access to the VLAN by mac address ... my only other thought is to make sure the PC's NICs are configured the same as your laptop - they should be autonegotiate if your switch is, or 100 Full if your switch is.  They should match your switch ports.  I've seen problems with DHCP when there was a duplex mismatch.
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by:teecee33
ID: 13928478
I have matched all of the settings on my laptop intel NIC with my desktop intel NIC with no luck.  I have tried to manuall configure my IP to match the cable modem settings with no luck.  I have plugged into my desktop, went to the other building, unplugged the power of the cable modem, waiting a minute, and plugged the power back on and not luck with desktop but laptop still works fine.
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by:pseudocyber
ID: 13928504
Are the other ports in the vlan?
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by:pseudocyber
ID: 13928509
And when youu say cable modem - do you mean router?  How are your other machines (more than one) supposed to get multiple IP's from one cable modem?
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by:teecee33
ID: 13928524
I have one port setup for the cable modem vlan and all the other ports are my network vlans.  When I say cable modem i mean cable modem.  It is only plugged into one computer at a time.  I am not trying to use more than 1 computer at a time on the cable modem.
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by:Seelan Naidoo
ID: 13928562
Is your DHCP Server and gateway addresses for your desktops clients explicitly defined in TCP/IP properties ?
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by:harbor235
ID: 13928565
If you connect both desktops to the the switch in the same vlan as the cable modem can the desktops see/talk to each other? Try assigning static IPs to see if it works. This way you can rule out switch/vlan/cable/nic problems. pseudocyber has a goog point, do you have port security on?

harbor235
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by:pseudocyber
ID: 13928566
I'm sorry, I'm getting confused.  The ports where you plug your cable modem and your laptop or PC are in the SAME vlan, right?
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Author Comment

by:teecee33
ID: 13928769
Yes pseud, same vlan.  IF IF IF I plug this same NIC card into a different port, it gets all more normal internal network info and works perfect.  


I did fix the problem.  Got my MAC from my desktop NIC.  Unplug desktop.  Change my laptop nic MAC to the MAC from my desktop.  Turn off cable modem.  Plug ethernet into laptop DRIECTLY, no vlan or switch, and power on cable.  Get a ip and can hit the internet.  Then unplug laptop and plug cable modem back into switch which is vlaned to my desktop NIC.  BAM.  Works perfect.  

Looks like cable modem was grabbing the first MAC it could get and storing that so it was probably seeing the mac of my switch and assigning the ip to that.  Anything else it saw it was ignoring.

The reason it has been working on my laptop this entire time was before I started doing anything I plugged my laptop directly into the cable modem just to make sure the service still worked....
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by:pseudocyber
ID: 13929113
Ah, I forgot about that.  Glad you figured it out.
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by:teecee33
ID: 13929776
Thanks for the help though.  I dont think I can offically hand out points for this but I will try and if the admin needs to correct it he can.
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by:pseudocyber
ID: 13929802
I think you can if you want, or if you don't want to - I can see where you should get a refund as I didn't see anyone think of the fact the cable modem was "bonding" to the MAC address.  It's an unusual setup, a cable modem on "professional" gear.
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