3Com 7757 Configuration Manual page 905

The primary port of the master node transmits the loop detection packet, and the
secondary port of the master node receives the loop detection packet.
When an Ethernet ring is in the healthy state, the secondary port of the master
node allows only RRPP packets to pass, but logically blocks data packets in data
VLANs.
When the Ethernet ring is in the broken state, the secondary port of the master
node stops blocking the data VLAN and begins to forwards data packets in data
VLANs.
The primary and secondary ports of a transit node are functionally the same.
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The node roles are determined by user configuration. As shown in Figure 250,
Switch A is the master node of Ring 1. Port 1 and Port 2 of Switch A are the
primary port and secondary port respectively. Switch B, Switch C and Switch D are
the transit nodes on ring 1, and their respective port 1 and port 2 are the primary
port and secondary port on ring 1.
Subring protocol messages are processed as data packets in the primary ring.
Thus, when the secondary port on the master node of the primary ring, or the
RRPP ports (including the primary and secondary ports) on the transit node are
blocked, both data packets and subring protocol messages cannot pass through
the port. After the ports are unblocked, these packets or messages can pass
through the ports.
Common port and edge port
Of the two ports connecting an edge node (or assistant edge node) to a subring,
one is the common port and the other is the edge port of the node. The common
port connects the edge node to the primary ring and a subring at the same time.
An edge port is connected only with a subring.
Conceptually, a common port is not treated as a port on the subring. Instead, it is
a part of the primary ring. In another word, the common link is a link on the
primary ring instead of the subring. Status change of a public link is only reported
to the master node of the primary ring and the master node of the subring needs
not know about the change.
The node roles are determined by user configuration. As shown in Figure 250,
Switch B and Switch C are on ring 1 and ring 2 at the same time. Port 2 of Switch
B and Port 1 of Switch C connect the primary and a subring, so they are common
ports. Port 3 of Switch B and Port 3 of Switch C connect only the subring, so they
are edge ports.
MAC address FDB
The Layer 2 forwarding database (FDB) on a switch is updated through the source
MAC address auto-learning function of the switch.
Timer
Two timers, Hello timer and Fail timer, are involved when the master node sends
and receives RRPP packets.
Hello timer: Defines the time interval at which the primary port of the master
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node sends the health detection packet.
RRPP Overview 905