ap cannot get ip address via dhcp relay on 3com l3 switch

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A Arnold Joseph Ballesteros 3 years 7 months ago
3 5 0

We have a customer with windows 2003 DHCP server and  AP5131  on different access  vlans  connected to dhcp-relay configured  3Com5500-EI Multilayer switch (MLS).     When we connect a windows XP laptop to the AP patch cable, it is able to  gets an IP address  from DHCP server but when we connect the AP5131 or AP7131, the 3Com MLS   does not relay  the "DHCP Discover" messages sent by the AP to the DHCP server. Any help would be greatly appreciated

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5 Replies

M Marcus Kurath

This problem could be related to the spanning tree configuration on the switch port. There is a feature on Cisco swotches called "fast port" which eliminates the spanning tree "Address Table Timer". Some switches do not forward data for the first 30 seconds when the link is connected. This is enough that no sddress gets assigned

M Marcus Kurath

Another possibility is that the 3-Com switch has a "discovery protocol" like Cisco CDP  running on the port. If this is the case it might make sense to disable it

A Alona Gian

What version are you using? Did you contact support?

M Marcus Kurath

I suspect that you have configured multiple WLANS pointing at different VLANS on the AP5131/7131. When you do this the wired LAN port on the AP is automatically configured as an 802.1Q trunk and all the frames are tagged. When you connect an AP to a wired access port, all WLANS must point to the same VLAN. I alwaus use the default -Vlan 1 for this purpose... when a wired switch has a port configured as an access port, it is unaware of VALNS so the AP does not need to be configured as the same VLAN as the port..I could write a few more paragraphs oon 802.1Q, but if you have additional questions please contact me directly

A Arnold Joseph Ballesteros

The AP's are set to default  system configuration out of the box.   The AP is able to get an IP address if it is on the same vlan where there is no dhcp-relay involve. The dhcp relay works fine when the AP is connected to a Juniper or Foundry(Brocade) MLS.  Just have a problem using the 3Com device.  

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