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New Member

3550 ip connectivity

Hmm, anyone knows why all ip connectivity stops between two cisco 3550

switches when adding them to a network? I am adding 2 switches to a networkm

after going through the basic setup and assigning an ip adress to them. When

i ad the first switch with ip to the existing network and try

to ping it from a server it responds ok. When i insert the second switch

with ip address, neither the 1st or second responds by ping. If

i unplug the 1st, then the 2nd switch starts to respon by its ipadress.

Anyone why this is happening? I havent done anything with these switches

beside going through the basic setup. I tried a 3rd switch with a, but the same problem came. Once i add 2 cisco 3550 , they loses

ip connectivity.

I am trying to wiring them into an existing Netgear gsm7324 Layer2/3 GB


And please dont come to me with a cable issue as a solution. Im experienced,

but not with cisco hardware.


Re: 3550 ip connectivity

I had a similar issue lately. It was caused by incompatible spanning tree protocols. The solution: make the cisco installation a separate spanning tree domain by disabling bpdu's on the link to the rest of the network. I used the following command:

int gi0/x

description Uplink to Netgear

spanning-tree bpdufilter enable


It may not be possible to implement this if it is not acceptable to have two spantree domains. Still you can use this method for troubleshooting, and implement your customized solution afterwards.



New Member

Re: 3550 ip connectivity

The problem is most likely something known as the Dynamic trunking Protocol (DTP). It is a multicast message that is sent between Cisco switches to form a trunk. A non-Cisco device will forward this and it will seem to the Cisco switches plugged into this device that they are directly connected and they will form an ISL trunk by default. When only one switch is plugged up the problem should not occur, but once the second one is attached both ports will start sending and expect to receive frames with ISL headers.

To correct the problem, set the ports attached to the Netgear switch as access ports. This will turn off the DTP protocol on the ports.

The command is as follows.

Interface gig 0/1

switchport mode access

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