In the initial state, MSTP is not enabled on the RNC-109 device, but MSTP is enabled on routers 110 and 111. The RNC-109 device transmits MSTP packets of routers transparently, and STP is generated through the calculation. The calculated result is that 1/0/23 on router 110 is blocked.
When a fault occurs on the link between the RNC-109 device and router 111, STP packets of router 111 cannot be sent to 1/0/3 of router 110 through 2/0/0. Router 110, however, cannot detect a link fault. Before a root interface ages, an interface remains as the root interface. When 1/0/3 ages, STP configures the root interface of 1/0/3 as a designated interface, 1/0/3 remains blocked. The peer of interface 1/0/3 cannot respond to the STP packets. The interface waits for the forward-delay (by default, the forward-delay is 15s) and then enters the learning state
After waiting for another forward-delay, the interface enters the forwarding state and the traffic is transmitted again. As a result, services are interrupted for 30s.