Spanning Tree

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I have the following setup:

SIte1. Core Switch A - 192.168.1.1
Site2. Core Switch B - 192.168.2.1
Site3. Access Switch C - 192.168.6.1

The core switches are at different sites and are connected to each other on the 172.16.28.xx network.

The access switch has two WAN links, one to switch A the other to switch B and these are connected on a 172.16.24.xx and 172.16.26.xx network. The core is configured as an OSPF area backbone and the access switch is also a member of this area.

I connect to core A from the access switch and everything is ok. I then connect to core B and the link comes up but switch A goes down due to spanning tree blocking ports.

How can spanning tree take effect when broadcasts are not propagated through different networks?
What are the full range of possibilities causing this? How is it even possible that connecting a second WAN link via OSPF cause a spanning tree port block?

 

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Is this a packet tracer configuration i presume? spanning tree is a layer 2 protocol....if you have routed links between your cores and 'access' switch there wouldn't be spanning tree between those.

If you have your .pkt file upload it so i can take a look.

 

STP shouldn't even be running on a routed port and, in normal operation, doesn't cause switches to go down.

OSPF is a routing protocol and isn't used to connect devices across a WAN link.

If you're unable to describe you're seeing, run appropriate "show" commands and then copying and pasting the output to this thread. In this case, perhaps "show ip interface brief", "show vlan brief", and "show ip protocol" on all three devices.