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SPT and Forwarding Loop Issue

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?

  • Other Network Infrastructure Subjects
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