Nokia 7302 - Figure 5 Ring in the Initial State

FD 100/320Gbps NT and FX NT IHub Services Guide Services Overview
Issue: 13 3HH-11985-AAAA-TQZZA 65
R-APS messages are periodically sent around in both directions to tell other nodes
in the Ring about the blocked port in the RPL owner node. In non-revertive mode any
link may be the RPLY link.1731 Ethernet OAM CC is the basis of the RAPS
messages used by nodes in the ring to monitor the health of each link in the ring in
both directions. However CCM messages are not mandatory. Other link layer
mechanisms could be considered - for example LOS (Loss of Signal) when the
nodes are directly connected.
Initially each Ring Node blocks one of its links and notifies other nodes in the ring
about the blocked link. Once a ring node in the ring learns that another link is blocked,
the node unblocks its blocked link possibly causing FDB flush in all links of the ring
for the affected service VLANs, controlled by the ring control instance. This
procedure results in unblocking all links but the one link and the ring normal (or idle)
state is reached. In revertive mode the RPL link will be the link that is blocked when
all links are operable after the revert time. In non-revertive mode the RPL link is no
different that other ring links. Revertive mode offers predictability particularly when
there are multiple ring instances and the operator can control which links are block
on the different instances. Each time there is a topology change that affects
Reachability, the nodes may flush the FDB and MAC learning takes place for the
affected service VLANs, allowing forwarding of packets to continue. Figure 5 shows
this operational state.
Figure 5 Ring in the initial state
Upon a ring link's failure, a node or nodes detecting the failure (enabled by Y.1731
OAM CC monitoring) send R-APS message in both directions. This allows the nodes
at both ends of the failed link to block forwarding to the failed link preventing it from
becoming active. In revertive mode, the RPL Owner then unblocks the previously
blocked RPL and triggers FDB flush for all nodes for the affected service instances.
The ring is now in protecting state and full ring connectivity is restored. MAC learning
takes place to allow L2 packet forwarding on a ring. Figure 6 shows the failed link
scenario.