stp (ieee) stops loops but R-PVST goes flapping?

[John Peterson]

Hi,

Stange problem which I encountered today, I have a Cisco 2960 which is connected to a netgear. The switch started showing itself in CDP and was running STP. I checked the cables physically today and noted 3 uplinks to the netgear, all port on the Cisco active and forwarding and green lights.

The Cisco was running STP, I changed it to R-PVST and the lights on the Cisco went crazy and I got the message port flapping on the switch but the switch did not block any ports (all ports on same vlan).

There after I changed it back to stp and the switch blocked the other up links apart from one.

Sure R-PVST is far superior than STP?

Thanks

[Peter Paluch]

John,

It is difficult to comment on this issue without knowing the configuration of both your 2960 and Netgear. In my opinion, the loop first formed when the switch began seeing itself in CDP. Changing to R-PVST may have aggravated the situation but it does not seem to have caused it in the first place.

Best regards,

Peter

[John Peterson]

Thanks,

There is only one cisco and one netgear, I understand that both cdp and stp are layer 2 protcols, therefore surely when the switch saw it self the bpdu packet should have blocked the port?

The switch has basic out of the box config with all ports on vlan 1 which is given a private ip address for management.

[Peter Paluch]

Hi John,

There is only one cisco and one netgear, I understand that both cdp and  stp are layer 2 protcols, therefore surely when the switch saw it self  the bpdu packet should have blocked the port?

Indeed, it should. But the question is: did the BPDUs really come back to the 2960?

Best regards,

Peter

[John Peterson]

Where else can it have gone?

The cisco switch was dropping connection therefore the bpdu must have been received, and when I switch to r-pvst why it did falp and not just block the look ports?

[Peter Paluch]

Hi John,

Does the NetGear talk STP? If yes, which version?

Do you have a message about the port flapping stored somewhere? I do not see a reason for a port flap in particular. What could have happened is a MAC address flapping rapidly between ports.

Is there any PortFast configured on your switch?

Best regards,

Peter