Abstract:
A method for communication over a bi-directional ring network includes provisioning a virtual private local area network service (VPLS) over the bi-directional ring network. The VPLS includes connection termination points provisioned respectively on a plurality of the nodes so as to connect each of the plurality of the nodes to a second network external to the ring network. As long as the nodes and spans are fully operational, one or more of the connection termination points are maintained in a deactivated state, so that no more than one of the connection termination points to the second network is active. The nodes exchange messages indicative of a failure associated with the bi-directional ring network, causing at least one of the deactivated connection termination points to be activated so as to maintain connectivity among the users of the VPLS without creating a loop in the VPLS via the second network.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application is related to U.S. patent application Ser. No. 10/993,882, filed Nov. 19, 2004, which is assigned to the assignee of the present patent application and whose disclosure is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to communication networks, and specifically to methods and systems for providing virtual private LAN services (VPLS).  
       BACKGROUND OF THE INVENTION  
       [0003]     Bi-directional network ring topologies are gaining in popularity, particularly in Internet Protocol (IP) networks. Such networks provide efficient bandwidth utilization by enabling data to be transferred between any pair of nodes in either direction around the ring, while maintaining fast protection against faults. The two opposing traffic directions are commonly referred to as an inner ringlet and an outer ringlet, or ringlet  0  and ringlet  1 . It will be understood, however, that in the context of the present patent application and in the claims, the terms “inner” and “outer,” as well as other terms such as “east” and “west” or “right” and “left,” are used arbitrarily to distinguish between the two opposing directions of packet flow in a ring network. These terms are chosen solely for convenience of explanation, and do not necessarily bear any relation to the physical characteristics of the network.  
         [0004]     The leading bi-directional protocol for high-speed packet rings is the Resilient Packet Ring (RPR) protocol, which has been approved as IEEE standard 802.17, “Part 17: Resilient Packet Ring (RPR) Access Method &amp; Physical Layer Specifications,” which is incorporated herein by reference. (The 802.17 standard, as well as other IEEE standards cited herein, is available at standards.ieee.org/catalog/.) Using the RPR protocol, each node (commonly referred to as a “station”) in a ring network has a RPR Medium Access Control (MAC) address and can communicate directly with all other nodes through either ringlet. Each packet sent over either of the ringlets carries a header indicating its RPR MAC destination address. The receiving node recognizes its address in the header and strips the packet from the ring. All other nodes pass the packet onward transparently around the ring.  
         [0005]     Nodes in a RPR network use a topology discovery mechanism (described in Chapter 10 and Annex K of the standard) to automatically keep track of the topology of the ring. Topology messages are broadcast from each node to the other nodes on the ring. Each node constructs a topology map, containing information about the location, capabilities, and “health” of other nodes on the ring. Topology messages are generated periodically and upon the detection of changes in local status. When a node is removed or a fiber span between nodes fails, the nodes adjacent to the failure record the status in their topology maps and send protection messages around the ring. All the nodes update their topology maps to reflect the change in connectivity.  
         [0006]     The RPR standard (Annex E) also defines a mechanism for bridging between 802.1D and 802.1Q LANs via the ring network. Bridging of this sort is carried out by bridge nodes on the ring, which connect the ring to other LANs. When a bridge node receives a packet from another LAN, it adds a RPR header with an appropriate RPR MAC destination address and forwards the packet across the ring. If the particular RPR MAC address for the packet is unknown, the bridge node uses a broadcast MAC address to flood the packet to all the nodes on the ring.  
         [0007]     Busi et al. describe methods for making transparent local area network (LAN) connections over a RPR network in U.S. Patent Application Publications US 2003/0074469 A1 and US 2004/0022268 A1, whose disclosures are incorporated herein by reference. A transparent LAN service (better known as a Virtual Private LAN service—VPLS) provides bridge-like functionality between multiple sites over a large network.  
         [0008]     General methods for creating a VPLS, not specifically related to the RPR context, are described by Kompella et al., in “Virtual Private LAN Service” (IETF draft-ietf-12vpn-vpls-bgp-02.txt, May, 2004) and by Lasserre et al., in “Virtual Private LAN Services over MPLS” (IETF draft-ietf-12vpn-vpls-ldp-03.txt, April, 2004), which are incorporated herein by reference. (These drafts, as well as other Internet drafts cited herein, are available from the Internet Engineering Task Force (IETF) at www.ietf.org/internet-drafts.) Users connect to the VPLS via regular Ethernet interfaces. The VPLS entity itself is formed by virtual connections (referred to as “Pseudo-Wires,” or PWs) between the nodes to which the users are connected.  
         [0009]     Every node in a VPLS acts as a virtual bridge. A virtual bridge node has “virtual ports,” which are the endpoints of PWs that are part of the VPLS. The interfaces to which the users are actually connected are physical ports at the network edges. Both virtual and real interfaces are treated identically from the point of view of frame forwarding and MAC address learning. A single provider node can participate in multiple VPLS instances, each belonging to different users. From the perspective of the end-user, the VPLS network is transparent. The user is provided with the illusion that the provider network is a single LAN domain. User nodes on different physical LANs can thus be joined together through VPLS connections to define a virtual private network (VPN), which appears to the users to be a single Ethernet LAN.  
       SUMMARY OF THE INVENTION  
       [0010]     Although bi-directional ring networks, such as RPR networks, have built-in failure protection mechanisms, these mechanisms do not adequately protect against all failure scenarios that may occur in a VPLS that is provisioned over the ring. For example, if multiple failures occur concurrently, some of the nodes in the ring may be isolated from other nodes, leading to segmentation of the VPLS. As another example, if the VPLS is provisioned across both the ring and another network connected to the ring, a failure in the connection between the ring and the other network may similarly lead to VPLS segmentation. Users in one segment of the VPLS will then find that they are unable to communicate with users in other segments.  
         [0011]     Embodiments of the present invention provide failure protection mechanisms that can respond to and overcome these sorts of VPLS failure scenarios quickly and efficiently. In these embodiments, one or more standby connection termination points (CTPs) are defined as part of the VPLS at one or more of the nodes in the ring network. Each CTP connects the respective node to a network external to the ring network. In the absence of a network failure, these standby CTPs are blocked. When a failure occurs, the nodes in the ring network exchange topology messages and inform one another of the failure. Based on these messages, the nodes may determine that the VPLS has been segmented. In this case, the nodes choose one or more of the standby CTPs to be activated in order to overcome the segmentation.  
         [0012]     This protection mechanism may be implemented individually in each VPLS that is provisioned on the ring network. It takes advantage of the built-in topology discovery mechanism that operates at the physical ring level to provide fast protection at the virtual LAN level. The use of standby CTPs in this manner, with well-defined criteria for determining which nodes should activate their CTPs and when, also avoids formation of loops that could arise in the VPLS topology due to the existence of multiple paths external to the ring network.  
         [0013]     Although the embodiments described herein refer to particular standards (such as RPR) and use particular terminology (particularly VPLS terminology) to refer to virtual private networks, these standards and terminology are used solely for the sake of convenience and clarity. The principles of the present invention may similarly be applied in provisioning and protection of substantially any type of virtual private network over bi-directional packet rings of any suitable type.  
         [0014]     There is therefore provided, in accordance with an embodiment of the present invention, a method for communication over a bi-directional ring network that includes nodes connected by spans of the ring network, the method including:  
         [0015]     provisioning a virtual private local area network service (VPLS) to serve users over the bi-directional ring network, the VPLS including connection termination points provisioned respectively on a plurality of the nodes so as to connect each of the plurality of the nodes to a second network external to the ring network;  
         [0016]     as long as the nodes and spans are fully operational, maintaining one or more of the connection termination points in a deactivated state, so that no more than one of the connection termination points to the second network is active;  
         [0017]     exchanging messages among the nodes indicative of a failure associated with the bi-directional ring network; and  
         [0018]     responsively to the messages, activating at least one of the deactivated connection termination points so as to maintain connectivity among the users of the VPLS without creating a loop in the VPLS via the second network.  
         [0019]     In a disclosed embodiment, the bi-directional ring network includes a resilient packet ring (RPR) network, and wherein exchanging the messages includes transmitting and receiving RPR topology messages.  
         [0020]     Typically, the connection termination points are provisioned as virtual users of the VPLS. In some embodiments, provisioning the VPLS includes provisioning multiple VPLS instances over the bi-directional ring network, each of the VPLS instances including respective connection termination points, and activating the at least one of the deactivated connection termination points includes activating the respective connection termination points in each of the VPLS instances that is affected by the failure.  
         [0021]     In one aspect of the invention, exchanging the messages includes determining that the ring network has become segmented into at least first and second separate segments, and activating the at least one of the deactivated connection termination points includes activating one or more of the deactivated connection termination points so that at least one of the connection termination points is active in each of the first and second segments, whereby both of the first and second segments are connected to the second network. In one embodiment, activating the one or more of the deactivated connection termination points includes activating first and second connection termination points in the first and second segments, respectively, so as to connect the first and second segments via a path through the second network.  
         [0022]     In another aspect of the invention, provisioning the VPLS includes designating a node in the bi-directional ring network to serve as a hub, and activating a connection termination point of the designated node to connect the VPLS on the nodes of the ring network to the second network as long as the nodes and spans are fully operational, and exchanging the messages includes detecting and reporting on a failure of the activated connection termination point of the designated node, and wherein activating the at least one of the deactivated connection termination points includes activating one of the deactivated connection termination points of another node in the ring network so that the other node serves as the hub connecting the VPLS on the nodes of the ring network to the second network.  
         [0023]     In a disclosed embodiment, activating the at least one of the deactivated connection termination points includes assigning respective priorities to the nodes, and choosing which of the connection termination points to activate responsively to the priorities. Alternatively or additionally, activating the at least one of the deactivated connection termination points includes determining, based on the messages, a topology of the ring network subject to the failure, and choosing which of the connection termination points to activate responsively to the topology.  
         [0024]     In some embodiments, the method includes exchanging further messages indicative that the failure has been rectified, and responsively to the further messages, deactivating the at least one of the connection termination points that had activated responsively to the messages that were indicative of the failure.  
         [0025]     There is also provided, in accordance with an embodiment of the present invention, a system for communication, including nodes connected by spans so as to define a bi-directional ring network, over which a virtual private local area network service (VPLS) is provisioned to serve users, the VPLS including connection termination points provisioned respectively on a plurality of the nodes so as to connect each of the plurality of the nodes to a second network external to the ring network, wherein as long as the nodes and spans are fully operational, one or more of the connection termination points are maintained in a deactivated state, so that no more than one of the connection termination points to the second network is active, and  
         [0026]     wherein the nodes are arranged to exchange messages indicative of a failure associated with the bi-directional ring network, and responsively to the messages, to activate at least one of the deactivated connection termination points so as to maintain connectivity among the users of the VPLS without creating a loop in the VPLS via the second network.  
         [0027]     The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]      FIG. 1  is a block diagram that schematically illustrates a communication network supporting a VPLS, in accordance with an embodiment of the present invention;  
         [0029]      FIG. 2  is a block diagram that schematically shows details of a RPR network node, in accordance with an embodiment of the present invention;  
         [0030]      FIG. 3  is a flow chart that schematically illustrates a method for protection against segmentation of a VPLS, in accordance with an embodiment of the present invention;  
         [0031]      FIG. 4  is a block diagram that schematically illustrates a protection configuration of a VPLS, in accordance with an embodiment of the present invention; and  
         [0032]      FIG. 5  is a flow chart that schematically illustrates a method for protection against segmentation of a VPLS, in accordance with another embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0033]      FIG. 1  is a block diagram that schematically illustrates a communication network  20 , in accordance with an embodiment of the present invention. Network  20  comprises a RPR network  22 , which comprises nodes  24  (also referred to as “stations”) connected by an inner ringlet  26  and an outer ringlet  28 . For convenience, the nodes are labeled Nl through N 6 , and the spans of the ring connecting the nodes are labeled S 1  through S 6 , as shown in the figure.  
         [0034]     Two or more of nodes  24  are linked by respective connections  34 ,  36 ,  38  to a network  30  that is external to ring  22 . Network  30  may comprise, for example, another RPR network. Alternatively, network  30  may comprise substantially any other type of network with Layer  2  bridging functionality, such as an Ethernet LAN or a system of label-switched tunnels through an IP network. At the simplest level, network  30  may comprise a single Layer 2 switch, which serves as a MAC bridge between connections  34 ,  36  and  38 . Under normal operating conditions, however, no more than one of these connections (for example, connection  34 ) is active in each VPLS, in order to avoid creation of loops in the VPLS.  
         [0035]     A VPLS is provisioned on ring network  22  between user terminals  39  that are connected to one or more of nodes  24 . (In practical implementations, the user terminals typically connect to nodes  24  via LANs and switches that are external to the ring network, but these elements are omitted from the figure for the sake of simplicity.) The same VPLS may also be provisioned across other portions of network  20 , so as to serve user terminals connected to nodes  32  of network  30 , for example. Although only a single VPLS instance will be considered in the description that follows, multiple VPLS instances may be provisioned on network  22 , each with its own topology and set of users. The methods of failure protection that are described hereinbelow may be applied to each of the VPLS instances individually.  
         [0036]      FIG. 2  is a block diagram that schematically shows details of one of nodes  24  on ring network  22 , in accordance with an embodiment of the present invention. The node shown in  FIG. 2  is assumed to be a bridge node, which is connected to external network  30 . The figure is simplified to show only the elements of the node that are significant in the context of VPLS processing and protection. The blocks shown in the figure and described hereinbelow represent functional elements of node  24  and do not necessarily reflect the actual physical structure of the node circuitry. The functional elements may be implemented in dedicated hardware logic or in software running on a programmable processor or in a combination of hardware and software components, as will be apparent to those skilled in the art.  
         [0037]     Node  24  comprises a RPR MAC processor  40 , which performs filtering and forwarding of RPR packets on the ring. Processor  40  comprises “east” and “west” ring interfaces  42  and  44 , which connect to the neighboring spans of the ring network and perform physical layer and MAC processing functions mandated by the 802.17 standard. Optionally, a VPLS filter  46  handles broadcast and multicast traffic received from ring network  22  in order to enhance the efficiency of MAC processor  40 . The VPLS filter, as well as other elements of node  24 , are described in greater detail in the above-mentioned U.S. patent application Ser. No. 10/993,882.  
         [0038]     When MAC processor  40  determines that a given packet should be processed by this node  24  (and not simply forwarded around the ring), it passes the packet directly to a protocol processor  50 . Processor  50  performs higher-level processing functions with respect to packets transmitted from ring network  22  to other parts of network  20 , and vice versa. When a RPR packet encapsulates a VPLS frame, a VPLS forwarding engine  52  looks past the RPR header and processes the underlying VPLS frame. Processor  50  then passes these frames via a port, referred to herein as a connection termination point (CTP)  54 , to a bridge  56  in network  30 . Bridge  56  may comprise a physical Ethernet switch, or it may be implemented as a virtual bridging function of another network element or set of elements that are configured to emulate a Layer 2 network. CTP  54  may comprise a physical port or a virtual port (such as a VLAN port, as defined in IEEE standard 802.1Q). Engine  52  similarly processes PW frames received from bridge  56  for transmission over ring network  22  and performs other VPLS forwarding functions that are described in the above-mentioned U.S. patent application Ser. No. 10/993,882.  
         [0039]     Reference is now made to  FIGS. 3 and 4 , which schematically illustrate a method for protection of a VPLS in network  20  against segmentation of ring network  22 , in accordance with an embodiment of the present invention.  FIG. 3  is a flow chart that shows the steps in the method, while  FIG. 4  is a block diagram showing elements of networks  22  and  30  that are involved in implementation of the method. As noted earlier, although this method is described hereinbelow with reference to a single VPLS, it may be carried out with respect to each separate VPLS instance that is provisioned on network  20 .  
         [0040]     Initially, as part of the provisioning of the VPLS, a protection CTP (referred to hereinafter as a CTP-P) is defined on each of two or more nodes  24  in ring network  22  that serve the VPLS, at a CTP-P connection step  60 . Each CTP-P can also be viewed as a virtual user port, which connects the respective node  24  to a portion of the VPLS in external network  30 . A CTP-P of this sort could be defined on every node  24  that serves the VPLS. In the example shown in  FIG. 4 , however, nodes N 1 , N 4  and N 6  are each connected by a respective CTP-P to a respective bridge  56  in network  30 . As noted above, each CTP-P may be either a physical port or a virtual port. Each node  24  is informed as to which other nodes include a CTP-P for each VPLS. This information may be distributed to the nodes either as part of the provisioning process or by exchange of messages among the nodes.  
         [0041]     During normal operation, as long as ring network  22  is not segmented, the CTP-Ps are set to the “down” state, at an initial CTP-P setting step  62 . In this state, the CTP-P is blocked, so that no packets are forwarded through it. If the CTP-Ps were not blocked in this manner, a looped path could be formed in the VPLS via network  30 . Although the Spanning Tree Protocol (STP) could be used to prevent this sort of loop, STP is not well accepted in wide area networks, and its use in the context of VPLS is not standardized. The present method provides rapid protection against network segmentation without requiring that an additional loop-prevention protocol, such as STP, be carried out.  
         [0042]     Nodes  24  continually exchange topology messages, as mandated by the 802.17 standard. These messages enable the nodes to detect failures in ring network  22  and to reroute packets as necessary when a failure occurs. The nodes evaluate the messages in order to determine whether the ring network has become segmented, at a segmentation detection step  64 . When only a single span of the ring fails, the nodes can wrap or steer packets around the ring, as appropriate, in order to maintain service on the VPLS. When two or more spans fail, however, the ring becomes segmented, and VPLS users connected to one segment may no longer be able to communicate with those connected to the other segment. This situation is illustrated in  FIG. 4 , in which spans S 4  and S 6  have been broken, thus isolating nodes N 5  and N 6  from the remaining nodes in the ring.  
         [0043]     Upon determining that the ring has been segmented, each node having a CTP-P in a given VPLS checks to determine whether any of the other nodes in its own segment of the ring also has a CTP-P in this VPLS, at a protection checking step  66 . At this step, for example, node N 6  determines that it has the sole CTP-P in its segment of network  22 . Node N 6  then activates its CTP-P, at an activation step  70 , thus activating the connection between its segment of the ring network and bridge B 3  in network  30 . There is no need, however (at this step or at any other step in the methods described herein) for any changes to be made in network  30  when CTPs are activated or deactivated. The protection protocol is carried out entirely by nodes  24  in ring network  22 .  
         [0044]     On the other hand, at step  66 , nodes N 1  and N 4  each determine that in their own segment, there are two nodes that have a CTP-P. If both of nodes N 1  and N 4  were to activate their respective CTP-Ps at this point, a loop would be created in the VPLS through network  30  (via bridges B 1 , B 4  and B 2 , as shown in  FIG. 4 ). To avoid this sort of situation, only one of nodes N 1  and N 4  should activate its CTP-P, while the CTP-P of the other node remains blocked. For this purpose, each node has a predetermined protection priority. The priority can be set by the network service provider, or it may be determined by the nodes automatically, based on which node has the lowest IP address, for example. Each of nodes N 1  and N 4  determines which CTP-P node in the segment has a higher priority, at a priority checking step  68 . In the example shown in  FIG. 4 , node N 1  is assumed to have the higher priority. Therefore, only node N 1  activates its CTP-P at step  70 , and the two segments of ring network  22  are connected via bridges B 1 , B 4  and B 3 .  
         [0045]     Thus, the mechanism of  FIG. 3  can provide protection against an arbitrary number of failures in the ring network, as long as there is at least one node with a CTP-P in each ring segment following the failure. When the ring recovers from the failure, the nodes exchange topology messages to inform one another that the segmentation of the ring has been resolved. The nodes immediately disable their CTP-Ps in order to avoid loop creation.  
         [0046]      FIG. 5  is a flow chart that schematically illustrates a method for protection of a VPLS provisioned across ring network  22  and external network  30  against loss of connection between the nodes in the two networks, in accordance with an embodiment of the present invention. This method is described hereinbelow with reference to the network configuration shown in  FIG. 1 . This method protects against two types of failures that may cut off the VPLS connection between nodes  24  in the ring network and nodes  32  on the external network: 
        Loss of the connection to network  30 , due either to failure of node N 3  or failure of connection  34 .     Segmentation of ring network  22  (as shown in  FIG. 4 , for example), leading to isolation of one or more of nodes  24  from node N 3 .          
         [0049]     The method of  FIG. 5  (like the method of  FIG. 3 ) begins with provisioning of virtual VPLS users, in the form of hub CTPs (CTP-H), which connect nodes  24  in ring network  22  to external network  30 , at a CTP-H connection step  80 . For example, as shown in  FIG. 1 , nodes N 3 , N 1  and N 5  have respective CTP-Hs, which are linked to network  30  by respective connections  34 ,  36  and  38 . The CTP-Hs may be either physical or virtual ports. The activation and deactivation of the CTP-Hs is controlled by the ring network nodes, as described below, in such a manner as to prevent loops in the VPLS. Therefore, connections  34 ,  36  and  38  may be linked to standard ports of any suitable bridges in external network  30 . The use of these inactive, standby hub CTPs, in the manner described herein, consumes minimal bandwidth (as long as the hub CTPs are inactive), in contrast to protection schemes known in the art that are based on duplication of information or squelching according to squelching tables. Furthermore multiple CTP-Hs may be configured to provide a wide variety of N:M protection topologies with added redundancy, and not only 1:1 and 1+1 redundancy as in many systems known in the art.  
         [0050]     Each CTP-H is assigned a respective priority, either by the service provider or by automatic setting. Only the CTP-H with the highest priority is activated initially, at a CTP-H activation step  82 . All of the other CTP-H connections to network  30  are blocked in order to prevent formation of network loops. In the example shown in  FIG. 1 , node N 3  has the highest priority, so that connection  34  is activated, while connections  36  and  38  are blocked.  
         [0051]     Node N 3  monitors the status of the active CTP-H, in order to detect possible failures of connection  34 , at a failure monitoring step  84 . Such a failure may occur, for example, if the physical port used by the CTP-H fails or if there is a corresponding failure in network  30 . (Such a failure could be indicated by an Ethernet or tunneling protocol management message, for example, depending on the characteristics of network  30 .) Upon detecting the failure, node N 3  deactivates its CTP-H and sends a message reporting the failure to the other nodes  24  in the VPLS on ring network  22 .  
         [0052]     When a node that has a deactivated CTP-H receives the message indicating that the active CTP-H has failed, it checks which of the remaining CTP-H nodes has the next-highest priority. The node with the next-highest priority activates its CTP-H, at a protection activation step  86 . Thus, for example, node N 1  might activate connection  36 , while connection  38  remains deactivated. Full communication with network  30  is thus restored while loop creation is avoided.  
         [0053]     When the failure in the highest-priority CTP-H is fixed, node N 3  sends a notification to the other nodes in the VPLS on ring network  22  that it is prepared to reopen connection  34 , at a reversion step  88 . Typically, upon receiving this message, the node (N 1 ) with the currently-active CTP-H immediately disables the CTP-H, thereby deactivating connection  36 . Node N 3  waits for a predetermined period to allow node N 1  to complete the deactivation of its connection, and then enables its own CTP-H to reactivate connection  34 .  
         [0054]     Alternatively, the nodes in ring network  22  may be provisioned for non-revert operation. In this case, after node N 3  has failed-over to node N 1 , connection  36  will remain active indefinitely. Node N 3  will reactivate connection  34  only if connection  36  fails or when the network is reset.  
         [0055]     In addition, nodes  24  may determine that ring network  22  has become segmented, at a segmentation detection step  90 . Such segmentation may occur as the result of failures in two spans of the ring network, as shown in  FIG. 4 . The nodes learn of the segmentation by sending and receiving topology messages over the ring network, as at step  64  in  FIG. 3 . This mechanism also enables the nodes to detect when the node with the currently-active CTP-H (N 3  in the present example) has failed, since that node will cease to transmit topology messages to the other nodes. In other words, the failed node effectively “disappears” from the ring topology, as though it had been segmented out of the ring.  
         [0056]     When a node that is provisioned with a CTP-H discovers that the ring network has been segmented, the node checks the current topology to determine whether there is another CTP-H active in its own segment of the ring, at an activity checking step  92 . If spans S 4  and S 6  were broken, for example, then node N 1  would determine that the CTP-H of node N 3  is still active in its own segment of the ring. Node N 1  would therefore take no further action in this case. Node N 5 , on the other hand, would determine that there is no active CTP-H remaining in its segment of the ring. Node N 5  then determines that there is no other CTP-H with higher priority in its segment, and therefore activates connection  38  at step  86 .  
         [0057]     When the segmentation of ring network  22  is resolved, the resulting topology messages indicate to node N 5  that there is now another CTP-H (at node N 3 ) with higher priority in its segment of the ring. As a result, node N 5  immediately disables its own CTP-H at step  88 .  
         [0058]     Although the embodiments described hereinabove are based on RPR network  22  and nodes  24  that are specifically designed to support VPLS over RPR, the principles of the present invention may similarly be applied in provisioning and protection of other sorts of virtual private networks, operating over bi-directional packet rings of any suitable type. It will thus be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled. in the art upon reading the foregoing description and which are not disclosed in the prior art.