Very basic question about 3550 switchport behavior

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Jan 9th, 2009
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I have an old 3550 at my desk here, IOS 12.1(13), newly restored to default configuration. When I connect it to my LAN, which is a port on another 3550 running 12.1(22), the other 3550 immediately shuts down its port with a SPANTREE-7-RECV_1Q_NON_TRUNK and SPANTREE-7-BLOCK_PORT_TYPE error.


The port on my LAN switch is configured in "switchport access" mode, with "switchport access vlan 120", so I understand why this would happen if it was connected to a trunk port on my deskside switch.


However, I thought the default switchport mode for my deskside switch was "dynamic desirable"? In which case the port does not trunk unless the other side tells it to?


Obviously my deskside switch is sending out BPDUs for VLAN 1, and the other side is VLAN 120, but I get a separate CDP-4-NATIVE_VLAN_MISMATCH error for that, and I don't think that's the condition shutting the LAN port down.


My question is - it this the way "dynamic desirable" is supposed to work?


Of course I can fix this by setting both ports to access mode and VLAN 120 but I'm curious about the root cause. Thanks.


(I am not trying to trunk between the switches, I'm just trying to set up deskside to upgrade the IOS.)



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Jon Marshall Fri, 01/09/2009 - 14:36
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Wiley


"However, I thought the default switchport mode for my deskside switch was "dynamic desirable"? In which case the port does not trunk unless the other side tells it to?"


The above description applies to a port set to auto mode. Dynamic desirable means the port will actively try and form a trunk -


http://www.cisco.com/en/US/products/hw/switches/ps663/products_tech_note09186a0080094713.shtml#dtp


Jon

wsanders1 Wed, 01/14/2009 - 13:50
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FWIW, on the 3550 your choices of switchport modes are: access, dot1q-tunnel, trunk, dynamic auto, and dynamic desirable.


A newly cleared config, at least in 12.2, has "switchport mode dynamic desirable" in every interface subconfig.


Anyway, setting "switchport mode access" and setting up the correct vlan on the port prevents any problems, it's just interesting that tossing a newly cleared box on your network can bring it down.

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