R-APS messages that carry the G.8032 protocol are sent on dedicated protocol VLAN called the Ethernet Ring Protection (ERP) instance. In a revertive case, G.8032 Protocol ensures that one Ring Protection Link (RPL) owner blocks the RPL link. R-APS messages are periodically sent around the ring to inform other nodes in the Ring about the blocked port in the RPL owner node. In non-revertive mode any link may be the RPL. Y.1731 Ethernet OAM CC is the basis of the RAPs messages. Y.1731 CC messages are typically used by nodes in the ring to monitor the health of each link in the ring in both directions. However CC messages are not mandatory. Other link layer mechanisms could be considered – for example Loss Of Signal (LOS) when the nodes are directly connected.
Initially each Ring Node blocks one of its links and notifies other nodes in the ring about theblocked link. When a ring node in the ring learns that another link is blocked, the nodeunblocks its blocked link possibly causing FDB flush in all links of the ring for theaffected service VLANs, controlled by the ring control instance. This procedure resultsin unblocking all links but the one link and the ring normal (or idle) state is reached.In revertive mode the RPL link is the link that is blocked when all links are operableafter the revert time. In non-revertive mode the RPL link is no different than otherring links. Revertive mode offers predictability particularly when there are multiplering instances and the operator can control which links are blocked on the differentinstances. Each time there is a topology change that affects reachability, the nodes mayflush the FDB and MAC learning takes place for the affected service VLANs, allowingforwarding of packets to continue. 0-1 G.8032 ring in the initial state depicts this operationalstate:
When a ring failure occurs, 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 Layer 2 packet forwarding on a ring. 0-1 G.8032 ring in the protecting state depicts the failed link scenario.
When the failed link recovers, the nodes that blocked the link again send the R-APS messagesindicating no failure this time. This in turn triggers RPL owner to block the RPL linkand indicate the blocked RPL link the ring in R-APS message, which when received by thenodes at the recovered link cause them to unblock that link and restore connectivity(again all nodes in the ring perform FDB flush and MAC learning takes place). The ringis back in the normal (or idle) state.
Within each path, Y.1731 Maintenance Entity Group (MEG) Endpoints (MEPs) are used to exchange R-APS specific information (specifically to co-ordinate switchovers) as well as optionally fast Continuity Check Messages (CCM) providing an inherent fault detection mechanism as part of the protocol. Failure detection of a ring path by one of the mechanisms triggers to activate the protection links. Upon failure, reconvergence times are a dependent on the failure detection mechanisms. In the case of Y.1731, the CCM transmit interval determines the response time. The router supports message timers as low as 10 milliseconds (also 100 ms) so the restoration times are comparable to SONET/SDH. Alternatively, 802.3ah (Ethernet in the First Mile) or simple Loss of Signal can act as a trigger for a protection switch where appropriate. In case of direct connectivity between the nodes, there is no need to use Ethernet CC messaging for liveliness detection.
Revertive and non-revertive behaviors are supported. The Ring protection link (RPL) is configured and Ethernet-rings can be configured to revert to the RPL upon recovery.
G.8032 supports multiple data channels (VIDs) or instances per ring control instance (R-APS tag).G.8032 also supports multiple control instances such that each instance can support RPLson different links providing for a load balancing capability. However, after serviceshave been assigned to one instance the rest of the services that need to beinterconnected to those services must be on the same instance. In other words each datainstance is a separate data VLAN on the same physical topology. When there is any onelink failure or any one node failure in the ring, G.8032 protocols are capable ofrestoring traffic between all remaining nodes in these data instances.
Ethernet R-APS can be configured on any port configured for access mode using dot1q, q-in-qencapsulation enabling support for Ethernet R-APS protected services on the service edgetoward the customer site, or within the Ethernet backbone. ELINE services (using PBBEpipes with the B-VPLS configured with Ethernet rings), E-LAN services, and E-Tree dataservices can be afforded Ethernet R-APS protection and, although the Ethernet ringproviding the protection uses a ring for protection the services are configuredindependent of the Ring properties. The intention of this is to cause minimum disruptionto the service during Ethernet R-APS failure detection and recovery.
In the implementation, the Ethernet Ring is built from a VPLS service on each node with VPLS SAPsthat provides Ring path with SAPs. As a result, most of the VPLS SAP features areavailable on Ethernet rings if needed. This results in a fairly feature rich ringservice.
The control tag defined under each Ethernet-ring is used for encapsulating and forwarding theCCMs and the G.8032 messages used for the protection function. If a failure of a link ornode affects an active Ethernet ring segment, the services fail to receive the CCMsexchanged on that segment or receive a fault indication from the Link Layer OAM module.CCMs are optional but MEPs are always configured to provide G.8032 control. Note thatthe forwarding of CCMs and G.8032 R-APS messages continues in the control VPLS even ifthe service or its SAPs are administratively shut down. The Ethernet ring instance canbe shut down if it is needed to stop the operation of the ring on a node.
For fault detection using CCMs three CC messages plus a configurable hold-off timer must bemissed for a fault to be declared on the associated path. The latter mechanism isrequired to accommodate the existence of additional, 50 ms resiliency mechanism in theoptical layer. After it receives the fault indication, the protection module declaresthe associated ring link down and the G.8032 state machine sends the appropriatemessages to open the RPL and flush the learned addresses.
Flushing is triggered by the G.8032 state machine and the router implementation allows flooding of traffic during the flushing interval to expedite traffic recovery.
0-3 ring example illustrates a resilient Ring Service. In the example a PBB ring (solid line) using VID 500 carries 2 service VLANs on I-SID 1000 and 1001 for Service VIDs (Dot1q 100 and QinQ 400.1 respectively.) The RPL for the PBB ring is between A and B where B is the RPL owner. Also illustrated is a QinQ service on the (dotted line) ring that uses Dot1q VID 600 for the ring to connect service VLAN 100.50. The two rings have RPLs on different nodes which allow a form of load balancing. The example serves to illustrate that service encapsulations and ring encapsulation can be mixed in various combinations. Also note that neither of the rings is closed loop. A ring can restore connectivity when any one node or link fails to all remaining nodes within the 50 ms transfer time (signaling time after detection).
Example configuration:
configure eth-ring 1 description "Ring PBB BLUE on Node B" revert-time 100 guard-time 5 ccm-hold-time down 100 up 200 rpl-node owner path a 6/6/1 raps-tag 100 // CC Tag 100 description "To A ring link" rpl-end eth-cfm mep 1 domain 1 association 1 direction down // Control MEP no shutdown exit exit no shutdown // would allow protect switching // in absence of the "force" cmd exit path b 6/6/2 raps-tag 100 //Tag 100 description "to D Ring Link" eth-cfm mep 1 domain 1 association 1 direction down no shutdown exit exit no shutdown no shutdown exit service vpls 10 customer 1 create // Ring APS SAPs description "Ring Control VID 100" sap 6/6/1:100 eth-ring 1 create // TAG for the Control Path a exit sap 6/6/2:100 eth-ring 1 create // TAG for the Control Path b exit no shutdown exit service vpls 40 customer 1 b-vpls create //Data Channel on Ring description "Ethernet Ring 1 VID 500" sap 6/6/1:500 eth-ring 1 create // TAG for the Data Channel Path a exit sap 6/6/2:500 eth-ring 1 create // TAG for the Data Channel Path b exit exit service vpls 1000 i-vpls // CPE traffic sap 3/1/1:100 create // CPE SAP pbb backbone-vpls 40 isid 1000 exit exit no shutdown exit service vpls 1001 i-vpls // CPE traffic sap 3/1/2:400.1 create // CPE SAP pbb backbone-vpls 40 isid 1001 exit exit no shutdown exit