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PC Port will not pass DHCP when connected to Cisco Switch
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Bryan Nelson Offline

Posts: 71
Joined: Feb 2013
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Post: #5
RE: PC Port will not pass DHCP when connected to Cisco Switch
I know from experience that Cisco does offer port security for corporate networks that restricts switch ports to offering access to a limited number of mac addresses. The goal being to ensure that employees are not daisy chaining hubs somewhere down the line that may impact overall network performance.

End result is that your switch allows the phone as the single device, but traffic from any other mac address is rejected on that specific port. Try plugging in a plain dumb hub\switch then the phone\computer, and I bet you get the same result - no DHCP\access, as only the hub\switch mac address is allowed.

From Cisco:
"Port Security with IP Phones

Figure 62-1 shows an application in which a device connects to the switch through the data port of an IP phone.

Figure 62-1 Device Connected Through IP Phone

Because the device is not directly connected to the switch, the switch cannot physically detect a loss of port link if the device is disconnected. Later Cisco IP phones send a Cisco Discovery Protocol (CDP) host presence type length value (TLV) to notify the switch of changes in the attached device's port link state. With Cisco IOS Release 12.2(33)SXI and later releases, the switch recognizes the host presence TLV. Upon receiving a host presence TLV notification of a link down on the IP phone's data port, port security removes from the address table all static, sticky, and dynamically learned MAC addresses. The removed addresses are added again only when the addresses are learned dynamically or configured."

Sounds like Cisco phones will automatically work with this port security to allow more devices? I'm not 100% sure, but I bet port security is your issue.

Good Luck
12-04-2014 07:12 AM
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RE: PC Port will not pass DHCP when connected to Cisco Switch - Bryan Nelson - 12-04-2014 07:12 AM

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