Allied Telesis AT-9900 用户手册

Once a fault in the ring or node has been rectified, the transit nodes that span 
the (previously) faulty link section will detect that link connectivity has 
returned. They will then move their appropriate ring port state, from 
“Links-Down” to “Pre-Forwarding,” and await the “Ring-Up-Flush” control 
message from the master node. See 

Once these transit nodes receive the “Ring-Up-Flush” message, they:

■

flush their forward databases for both their ring ports

■

change the state of their ports from blocking to forwarding, which allows 
data to flow through their previously blocked ring ports

Note that the transit nodes do not enter the forward state until they have 
received the “Ring-Up-Flush” message. This is to prevent the possibility of a 
loop condition occurring caused by the transit nodes moving into the 
forwarding state before the master node secondary port is able to return to the 
blocking state. During such a period, the ring would have no ports blocked.

With the link restored, the healthcheck messages that are sent from the primary 
port of the master node now complete the loop and arrive at the master node’s 
secondary port. The master node now takes the following steps to restore 
normal conditions:

■

declares the ring to be in a “complete” state

■

blocks its secondary port for data (non-control) traffic 

■

flushes its forwarding database for its two ring ports

■

sends a “Ring-Up-Flush-FDB” message from its primary port, to all transit 
nodes.