UNDERSTANDING ETHERNET RING PROTECTION SWITCHING FOR CARRIER NETWORKS

Transcription

1 UNERSTNING ETHERNET RING PROTETION SWITHING FOR RRIER NETWORKS RMSTRONG MTHIYLGN ssistant Vice President, Technology, ricent RJESH KUMR SUNRRJN ssistant Vice President, Product Management, ricent EMREEN F. XVIER Technical Leader, ricent YRLG KISHORE KUMR Principal Systems Engineer, ricent

2 UNERSTNING ETHERNET RING PROTETION SWITHING FOR RRIER NETWORKS Ethernet Ring Protection Switching (ERPS), as defined in the G.8032 recommendation, is an effort from the ITU-T to provide a highly reliable and stable protection mechanism for Ethernet ring networks. It helps transport-network operators design resilient networks with very high quality of service (QoS) characteristics and service level agreements (SLs). Link failures are a common occurrence in networking, so many methods exist to improve network reliability in the event of connectivity loss between network elements. SONET (Synchronous Optical Network) or SH (Synchronous igital Hierarchy) ring is an example of a self-healing network technology that provides a high level of protection in data broadband networks. RPR (Resilient Packet Ring), defined in IEEE , is another technology for the optimized transport of data traffic over optical fiber ring networks that provides a resilience similar to that of SONET/SH rings. With the increasing use of Ethernet in carrier networks, as an alternative to SONET/SH, it is necessary to define similar resiliency mechanisms tailored to Ethernet. The G.8032 recommendation from the ITU-T is a practical and economical mechanism to meet this objective. INTROUTION TO ERPS The ring is a popular topology employed in networks to provide redundant path connectivity between nodes. t the same time, because the ring provides an alternative path between nodes, loops exist in the network. Therefore, the key to using the ring topology in networks has been to harness the redundancy when required while avoiding loops. These very same principles lie at the core of the ITU-T G.8032 Ethernet Ring Protection Switching (ERPS) recommendation. n Ethernet ring consists of ring nodes that form a closed physical loop. Each ring node is connected to two adjacent Ethernet ring nodes via a duplex communications facility. Loop formation in the ring is avoided by ensuring that, at any time, traffic may flow through all but one of the links in the ring. This particular link, through which traffic is normally not allowed to flow, is called the Ring Protection Link (RPL). Under normal conditions, when there are no failures in the ring, the RPL is blocked and therefore does not allow any traffic flow. The node where the RPL is configured is called the RPL owner. The port on the RPL owner, to which the RPL is connected, is called the RPL port. The node, adjacent to the RPL owner node and connected to the RPL owner node through the RPL, is called the RPL neighbor. The RPL owner blocks or unblocks the port connected to the RPL. The RPL neighbor may also participate in blocking or unblocking its end of the RPL. 1

3 Figure 1 shows the converged ring under normal conditions with traffic blocked on the RPL. Figure 1: Loop avoidance by blocking RPL under normal conditions In Figure 1, nodes,,, and form a ring. Node is the RPL owner, and the RPL port is Port of Node, and is blocked to avoid the traffic flow. Figure 2 shows the converged ring under normal conditions with both the RPL owner and the RPL neighbor blocking traffic to the RPL. RPL Neighbor Figure 2: Loop avoidance by RPL owner and RPL neighbor blocking RPL under normal conditions In Figure 2, nodes,,, and form a ring. Node is the RPL owner, and the RPL port in node is. Node is the RPL neighbor and port in Node is the RPL neighbor port. Under normal condition, of node and of Node go to a blocking state. allowing traffic flow into it, upon failure of any of the other links or nodes, constitutes protection switching and provides protection to the traffic between the nodes. This mechanism for protection switching being handled by the nodes with no administrator or operator intervention, like with the techniques described in this paper, is known as utomatic Protection Switching (PS). When applied to a ring, it becomes Ring-PS (R-PS) Each node monitors connectivity to its neighbor through the link connecting them. The messages for monitoring and coordination of the PS actions use a dedicated VLN called the R-PS VLN. node triggers the protection switching when it encounters one of the following conditions: > The node detects loss of connectivity on one of the links connected to it > n administrator issues an explicit command to block another link and to move the traffic to the RPL (there are 2 different variations of this situation, called Forced Switch and Manual Switch, which will be covered later in this whitepaper) > When a previous condition of connectivity loss is corrected and connectivity is re-established PROTETION SWITHING ON ETETION OF ONNETIVITY LOSS The nodes on either end of each link send periodic connectivity check messages to each other. The nodes use the lack of reception of such connectivity check messages to detect loss of connectivity. Such a failure of a link, or node, in the ring, results in traffic being switched (protection switched) into the RPL. The RPL owner is responsible for unblocking the RPL, thereby allowing the RPL to be used for traffic. The failed link is blocked in order to avoid loop formation in the event that the failed link becomes functional at any time. In Figure 3, failure of the link between nodes and results in unblocking of the RPL while the ring converges, as shown below. PROTETION SWITHING In Figure 1 and Figure 2, traffic can flow from any of the nodes to any other node through any of the links other than the RPL. The RPL alone does not have any traffic flow. loop is avoided by preventing traffic flow to the RPL. The RPL is maintained ready to be brought into service if any of the other links fail. In the event that one of the other links fails, or if a node fails and the connectivity between two other nodes is thereby lost, the RPL owner and RPL neighbor, after following a protocol, start allowing traffic flow into the RPL. This action of activating the RPL and Figure 3: In the event of failure, RPL link opened up to provide connectivity 2

4 MINISTRTOR-INITITE PROTETION SWITHING network operator can manually trigger the traffic redirection instead of it being triggered by a connectivity failure. This is done by a forced switch or manual switch command, which is useful in situations like maintenance operations or repair. When the administrator issues a forced switch or manual switch command on a specific port at a given node, the node places the administrator-specified port into a blocked state. ased on the protocol working, the RPL owner then unblocks the RPL to allow it thereby allowing the RPL to be used for traffic flow. pplying a force switch in of Switch results in the unblocking of the RPL and the ring converging as shown below. Non-Revertive Mode In the non-revertive mode of operation, when the failed link recovers, the RPL link remains unblocked and one of the failed ports remains in a blocked state. In situations where there is no advantage in immediately reverting to the normal working transport entities, such a mode is preferred. In this case, a second traffic interruption is avoided by not reverting the protection switching. Figure 5: Protection switching on signal recovery in non-revertive mode In the figure above, Nodes,,, and form a ring. Node is the RPL owner and the RPL port in node is. When the link between and fails and then recovers, one of the failed ports (the highest priority port) remains in a blocked state. The RPL port remains in an unblocked state. Figure 4: On initiation of force switch by operator, RPL link opens up to provide connectivity In the figure above, Nodes,,, and form a ring. Node is the RPL owner, and the RPL port in node is. When the forced switch is applied on port of Node, the port is moved to a blocked state and the RPL port ( of Node ) moves to an unblocked state. PROTETION SWITHING ON REOVERY FROM LOSS OF ONNETIVITY fter a protection switching action, when the failed link has been repaired, there is once again a potential loop in the network. This is avoided by the nodes detecting the recovery, and blocking either the repaired link or the RPL, resulting in two possible modes of operation for the ring: > Revertive Mode > Non-Revertive Mode Revertive Mode In the revertive mode of operation, when a failed link recovers, the RPL is blocked and the failed link is unblocked to start carrying traffic. espite it causing an additional momentary traffic interruption, the revertive mode may be desirable in situations where the working transport entity resources can be more optimized. ERPS IN SUTENE (INTERONNETE) RINGS ERPS also supports the protection of services that traverse through interconnected rings. Interconnected rings can be formed using single or dual ring nodes, or a multi-ring/ladder network that consists of conjoined Ethernet rings. For interconnected rings, the protection mechanism ensures that no super loop is formed when there is a link failure between the ring nodes. The protection mechanism specified in the G.8032/Y.1344 standard protects interconnected rings according to the following principles: > R-PS VLNs are not shared across ring interconnections > Traffic and R-PS channel of each link should be controlled (for blocking or flushing) by the ERP control process of only one ring > Each ring or sub-ring must have its own RPL The following figure represents an example of a topology composed of interconnected rings: Ring 1 and Ring 2. The link between the two interconnected nodes is under the control of the ERP control processes of the ring that it is configured to be a part of. In the example below, the ring link between nodes and is under the control of Ring 1. Failure of the link between the interconnected nodes triggers the protection switching event on the ring that contains this link. The sub-ring is not aware of the failure. 3

5 E P3 Ring 1 Ring 2 P3 F Figure 6: In the figure above, Nodes,,, and form the main ring. Node is the RPL owner and the RPL port in node is. Ring 2 is the sub-ring and consists of ring links ->E->F->. The RPL port of Ring 2 is of Node E and Node E is the RPL owner. ETHERNET RING PROTETION USING RING INSTNES FOR LO LNING Multiple logical ERP ring instances may be supported over a single physical ring. For example, traffic belonging to one VLN may be routed in one direction while traffic belonging to a second VLN may be routed in the opposite direction. When multiple ring instances are configured in a ring, some traffic can pass through one path while other traffic can choose a different path. This division of ring traffic supports load balancing in the system. When ring instances are configured for the ring, each ring instance should have its own RPL owner, RPL neighbor, and R-PS VLN. ERPS VERSION 1 N VERSION 2 The ITU-T first standardized the G.8032 in Today it is known as v1 or version 1 of the standard. Subsequently, more facilities have been incorporated based on feedback from network operators and designers. n updated version of the G.8032 was standardized in 2010, which is now known as v2 or version 2 of the standard. Key improvements and enhancements in ERPS version 2 from version 1 include: > Non-revertive mode of operation (version 1 specifies only the revertive mode of operation) > Forced Switch command for administrators > Manual Switch command for administrators > Increased support for ERP instances to protect multiple logical rings > dditional methods for minimizing segmentation of interconnected rings ERPS N RSTP The Rapid Spanning Tree Protocol (RSTP) and its multipleinstance version Multiple Spanning Tree Protocol (MSTP) are similar protocols to the ERPS and have been serving enterprise networks satisfactorily for many years now. From a protocol perspective, at their core, RSTP and MSTP are based on the same underlying principles as ERPS: (a) providing alternative redundant paths in a network and (b) loop avoidance. So how necessary is ERPS, and can RSTP or MSTP be used instead because they are already widely deployed? oth ERPS and STP are loop-avoidance protocols, but the protection switching performance of ERPS is much better when compared to STP. RSTP and MSTP were developed for a more generic topology than a simple ring. The protocol, therefore, has overheads to deal with complex topologies, and are unnecessary in a simple ring topology. For these reasons, RSTP or MSTP need more time to re-build network topology because they each use various parameters to re-calculate alternate paths. ecause the G.8032 does away with these overheads, and is specifically optimized for ring topologies, the ERPS protocol provides better protection switching performance and much greater levels of availability in carrier networks. ERPS can efficiently and predictably deliver sub- 50-millisecond protection switching, which is not the case with RSTP or MSTP. RIENT ERPS s part of its comprehensive portfolio of networking products, ricent offers a licensable software implementation of the G.8032 specification for ERPS. ricent ERPS is available for licensing as an individual component that can be easily integrated into networking products. It is also available as an integrated part of ricent s industry leading Intelligent 4

6 Element Management Redundancy Framework onnectivity/ Fault Management Example Multi-board System Framework ricent ERPS OS bstraction Layer Hardware bstraction Layer OS Switching Silicon river or ata Path Figure 7: ricent s ERPS in a Switch Stack rchitecture Switching Solution (ISS) for a variety of networking products catering to arrier Ethernet and Metro Ethernet applications. The software is implemented with clearly defined interfaces to other components in the switch, including abstraction layers to the operating system and switching silicon interfaces, allowing a developer or system integrator to integrate it easily. In addition to the mechanism in the G.8032 standard, ricent ERPS implements multiple additional features and extensions to build highly scalable, resilient, and fault-tolerant networks. These include: > Extensive support for configuration and management > Extensions to the protocol for highly available redundancy > Extensions for working on systems composed of multiple individual switching units > Extensions for functioning in a distributed environment, harnessing processing power from multiple PUs View for an overview of ricent ERPS. Network equipment manufacturers can use proven and tested components like ricent ERPS to reduce technology complexity and to optimize product development cycles, thereby accelerating time to market with reduced costs, ONLUSION Implementation of protection switching allows carrier Ethernet networks to meet higher levels of fault tolerance, resilience, and service-level agreement satisfaction. Protection switching in Ethernet networks with ring topologies can be efficiently implemented based on the ITU-T G.8032 specification. 5

7 RMSTRONG MTHIYLGN is ssistant Vice President, Technology, for ata ommunication products at ricent, focusing on routing and switching solutions, including ricent s ISS. He has over 18 years of experience in architecting software for switching, routing, arrier and Metro Ethernet. armstrong.m@aricent.com EMREEN F. XVIER is a Technical Leader for ata ommunication products at ricent, focusing on ricent s ISS. She has over 6 years of experience in the datacom domain including development and implementation of networking protocols. emreen.xavier@aricent.com RJESH KUMR SUNRRJN is ssistant Vice President for ata ommunication products at ricent, focusing on routing and switching solutions including ricent s ISS. He has over 16 years of industry experience in strategizing and managing software for communications. YRLG KISHORE KUMR is a Principle Systems Engineer for ata ommunication products at ricent, focusing on routing and switching solutions including ricent s ISS. He has over 11 years of experience in developing software for switching, routing, security, arrier and Metro Ethernet. yarlagadda.kumar@aricent.com 6

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