How ring breaks are detected and healed

Figure 1 shows ring interface states following a link break. MRP quickly heals the ring and preserves connectivity among the customer networks.

Figure 1. Metro ring - ring break

If a break in the ring occurs, MRP heals the ring by changing the states of some of the ring interfaces:

  • Blocking interface - The Blocking interface on the Master node has a dead timer. If the dead time expires before the interface receives one of its ring RHPs, the interface changes state to Preforwarding. Once the secondary interface changes state to Preforwarding:
    • If the interface receives an RHP, the interface changes back to the Blocking state and resets the dead timer.
    • If the interface does not receive an RHP for its ring before the Preforwarding time expires, the interface changes to the Forwarding state, as shown in Figure 1.
  • Forwarding interfaces - Each member interface remains in the Forwarding state.

When the broken link is repaired, the link interfaces come up in the Preforwarding state, which allows RHPs to travel through the restored interfaces and reach the secondary interface on the Master node:

  • If an RHP reaches the Master node secondary interface, the ring is intact. The secondary interface changes to Blocking. The Master node sets the forwarding bit on in the next RHP. When the restored interfaces receive this RHP, they immediately change state to Forwarding.
  • If an RHP does not reach the Master node secondary interface, the ring is still broken. The Master node does not send an RHP with the forwarding bit on. In this case, the restored interfaces remain in the Preforwarding state until the preforwarding timer expires, then change to the Forwarding state.

If the link between shared interfaces breaks (Figure 2), the secondary interface on Ring 1 master node changes to a preforwarding state. The RHP packet sent by port 1/3/1 on Ring 2 is forwarded through the interfaces on S4, then to S2. The packet is then forwarded through S2 to S3, but not from S2 to S1 since the link between the two nodes is not available. When the packet reaches Ring 1 master node, the packet is forwarded through the secondary interface since it is currently in a preforwarding state. A secondary interface in preforwarding mode ignores any RHP packet that is not from its ring. The secondary interface changes to blocking mode only when the RHP packet forwarded by its primary interface is returned.

The packet then continues around Ring 1, through the interfaces on S1 to Ring 2 until it reaches Ring 2 master node. Port 1/3/2, the secondary interface on Ring 2 changes to blocking mode since it received its own packet, then blocks the packet to prevent a loop.

Figure 2. Flow of RHP packets when a link for shared interfaces breaks

RHP packets follow this flow until the link is restored; then the RHP packet returns to it normal flow as shown in Figure 1.