We have a network of four routers which, when an ISDN line is activated, form a physical loop. I.e. each router connects to two others.
Two of the routers are Cisco 2600 and the other two are Cisco 7206. The ISDN connects a Cisco 7206 > Cisco 2600
A forwarding loop is forming after approx 90secs, due to the Dialer interface on the Cisco 7206, transiting from the ‘Blocking’ to the ‘Forwarding’ state.
Debug spanning-tree events on the 7206 outputs the message:
ST: New root is 5 0090.b120.501c on port Dialer 1 cost 2928
ST: Dialer 1 Listening
StateChange STANDBY E1/0 Group0 state Standby Active
ST: Dialer 1 Learning
ST: Sent Topology Change Notice on Dialer 1
ST: Dialer 1 Forwarding
Any suggestions as to the likely cause?
