Protection of Ethernet Services

IntroductionWith the development and deployment of Carrier Ethernet and Ethernet in the first mile (EFM), cus-tomer sites are accessed directly with Ethernet. To make Ethernet a carrier grade transport technology operators need to add high availability (99.999% up-time) and reliability that can compete with SDH ring architectures. Up to now, redundancy in Ethernet networks was achieved by the use of the spanning tree protocols (STP, RSTP, MSTP) and the bund-ling of Ethernet links with link aggregation (LAG). These mechanisms have several disadvantages (e.g. convergence time of xSTP) which resulted in the standardization of two protection mechanisms by the ITU-T: � G.8031 Ethernet Linear Protection Switching � G.8032 Ethernet Ring Protection Switching

Service reliability is one of the most important requirements in today‘s networks. To meet the availa-bility of an Ethernet service (EVC, Ethernet Virtual Connection) defined in a service-level agreement (SLA), service providers must ensure end-to-end protection mechanisms to deal with equipment and/or link failures.

G.8031 Ethernet Linear Protection SwitchingG.8031 is designed to protect a point-to-point VLAN connection, allowing the protection of an EVC from UNI to UNI (User Network Interface). This is achieved by defining both a working path and a protection (standby) path. In case of connec-tivity loss or degraded signal quality on the working path the Ethernet service frames are switched to the protection path.The working and protection paths can be either physically separated or they can be logically se-parated by VLAN ID using the same or partly the same physical path.End-to-end connectivity on both paths is monito-red by exchanging Continuity Check Messages (CCM, defined in G.802.1ag/Y.1731 Connectivity Fault Management CFM) between the two end-points. With CCM sent in intervals of 3.3ms a switchover time to the protection path far below 50ms is achieved.

UNI UNI

End-to-end EVC protection with bothphysically and logically separated paths

Working pathWorking VLANProtection VLAN

Protection path

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Linear protection switching can be operated in uni-directional or bidirectional mode. In unidirectional mode the devices act fully independent, meaning the decision whether to switch over to the pro-tection path is based solely on local information. There is no coordination with the remote device required. Though this operation is simple to set up, the bidirectional operation mode offers more functionality and provides additional information in case of troubleshooting. Devices running in bidi-rectional mode always select a common path for the transmission of Ethernet service frames.For the coordination of the devices G.8031 intro-duced the Automatic Protection Switching (APS) protocol. APS messages use the same frame format as the CFM messages using a dedicated opcode and are transmitted and received by the same Maintenance End Points (MEP) that monitor the connectivity of the protection path.The APS protocol is not only used to signal status information. It also offers the possibility to execute a manual switchover of the Ethernet service for maintenance reasons or troubleshooting.Multiple protection instances can be instantia-ted on the same device even using the same physical Ethernet ports. This load balancing bet-ween the EVCs can be utilized to use as much available bandwidth as possible by configuring different working and protection paths for different instances.The Albis Technologies ULAF+ product family offers linear protection switching on all ACCEED devices. As a unique feature any combination of Ethernet ports can act as working and protection paths e.g. a fiber port configured as working path can be protected by a copper port configured as protection port.Since the protection functionality is fully embed-ded in the ACCEED demarcation device there is no need for the edge and core equipment to be updated.

G.8032 Ethernet Ring Protection SwitchingAs its name implies G.8032 Ethernet Ring Pro-tection Switching is designed and optimized for ring topologies. The main task of a ring protection instance is to avoid a traffic loop in any circum-stance. In case there is no failure condition pre-sent in the ring, a dedicated link is intentionally kept blocked for traffic frames. This link is called Ring Protection Link (RPL) and the device that is responsible for blocking and unblocking the RPL is called RPL Owner.

The ring links are monitored by the same mecha-nisms as used in G.8031 Linear Protection Swit-ching using Connectivity Check Messages. For communication and status reports between all nodes on the ring, the Ring Automated Protection Switching (R-APS) protocol is used. It also uses the CFM frame format and a unique opcode. CCM and R-APS frames use a dedicated VLAN. This R-APS Control VLAN is used in the ring exclusi-vely for link monitoring and R-APS message ex-change.In „Idle“ state the RPL is blocked by the RPL Owner. Continuous transmission of R-APS mes-sages with the „Ring Blocked“ status informs the other nodes in the ring about this condition.

In case of a link failure (whether due to signal loss on the physical layer or due to reported loss of connectivity on the OAM layer) any device that detects the failure condition blocks the failed link and immediately transmits R-APS messages with the status „Signal Failure“. Upon reception of the-se messages the RPL Owner node unblocks the RPL. This allows service frames to reach all nodes in the ring again. All nodes change to „Protection“ state if a failure condition is present in the ring.

Since the topology has changed, the nodes have to flush their forwarding databases to accommo-date to the new topology. As long as the failure condition persists the nodes continue sending R-APS messages with „Signal Failure“ status to prevent the RPL from being blocked.

RPL (blocked)

RPL Owner

R-APS status „Ring Blocked“

R-APS status „Ring Blocked“

VLAN for R-APS and CCMVLAN(s) for traffic

RPL (blocked)

RPL Owner

R-APS status «Signal Failure»

R-APS status «Signal Failure»

VLAN for R-APS and CCMVLAN(s) for traffic

Link Failure

RPL (blocked)

RPL Owner

R-APS status „Ring Blocked“

R-APS status „Ring Blocked“

VLAN for R-APS and CCMVLAN(s) for traffic

RPL (blocked)

RPL Owner

R-APS status «Signal Failure»

R-APS status «Signal Failure»

VLAN for R-APS and CCMVLAN(s) for traffic

Link Failure

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Once the failure condition has been recovered the nodes change the transmitted R-APS status to „No Request“ but keep their links blocked. The RPL is not blocked immediately, instead the RPL Owner is starting the wait-to-restore timer upon reception of R-APS messages with „No Request“ status. While the wait-to-restore timer is running the nodes change to the „Pending“ state. The wait-to-restore timer is used to wait for a stable ring condition and prevents unnecessary topology changes in case of flapping links.Only after expiration of the wait-to-restore timer the RPL is blocked. To restore the original topolo-gy the RPL Owner again starts with the transmis-sion of „Ring Blocked“ R-APS messages. Nodes with a still blocked link will unblock them and all nodes in the ring change back to the „Idle“ state.G.8032 Ring Protection is not limited to a single ring topology. Revision 2 of the specification intro-duced a new type of node called „Interconnection Node“. With interconnection nodes it is possible to connect different rings to create a multi-ring/ladder network, expanding the protected domain.

Similar to G.8031 Linear Protection Switching, multiple instances of a G.8032 Ring Protection process may be active on the same physical ring topology. All protection processes have their dedi-cated logical resources such as the R-APS VLAN or the RPL. Running multiple processes on the same physical topology allows the operator to control the traffic flow for different Ethernet servi-ces. Since every protection instance has its own RPL, load balancing can be achieved between the services and therefore better bandwidth utilization is provided in a stable ring state. A single ring pro-tection instance can monitor and protect multiple Ethernet services.

Summary and ConclusionWith G.8031 Linear Protection and G.8032 Ring Protection all Albis Technologies Ltd. ACCEED Carrier Ethernet Demarcation Devices (EDD) of-fer two sophisticated protection mechanisms for EVCs.G.8031 Linear Protection protects the Ethernet services from UNI to UNI while G.8032 Ring Pro-tection enables Ethernet devices in a ring topology to reach SDH-like availability and reliability. With CCM intervals of 3.3ms shortest possible switcho-ver time in case of failure detection is guaranteed.Full interoperability with devices of other vendors ensures connectivity to existing networks as well as a way to expand protected Ethernet domains.Supporting both protection solutions makes the ACCEED family the first choice for reliable Ether-net services.

RPL RPLInterconnectionNodes RingSubring