Abstract:
In an Ethernet network element comprising at least one line interface element and a central processing unit (CPU) to control forwarding of data packets at the network element, a method comprising receiving continuity check messages (CCMs) at the at least one line interface element, and processing the CCMs in the at least one line interface element to provide continuity checks for connections to the network element without requiring processing of CCMs by the CPU.

Description:
RELATED APPLICATION 
       [0001]    This application is a continuation application of co-pending U.S. patent application Ser. No. 12/345,815, filed Dec. 30, 2008, titled “Metro Ethernet Connectivity Fault Management Acceleration,” the entirety of which U.S. patent application is incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates generally to connectivity fault management in Ethernet networks. More particularly, the invention relates to systems and methods for generating and processing connectivity check messages. 
       BACKGROUND 
       [0003]    With the advent of connection-oriented forwarding technologies such as Provider Backbone Transport (PBT) and Provider Backbone Bridge (PBB), Ethernet is rapidly becoming a dominant broadband technology, particularly in metro networks and wide-area networks. With PBT, service providers are able to establish point-to-point and point-to-multipoint Ethernet tunnels and to specify paths that service traffic will take through their Ethernet networks. With PBB, service providers are able to separate a communications network into customer domains and service provider domains. The separation is achieved by encapsulating the customer packets within a backbone (i.e., service provider) MAC (Media Access Control) header. Network elements in the service provider domain forward packets based on the service provider MAC header while the customer header remains invisible except at the edge of the service provider domain. 
         [0004]    As Ethernet services proliferate, service providers require a robust set of operation, administration, and maintenance (OAM) tools to manage their Ethernet networks and to adapt the Ethernet technology to a carrier-grade service environment. To this end, the IEEE (Institute of Electrical and Electronics Engineers) organization has formalized a standards document for connection fault management in Ethernet networks, referred to as IEEE 802.1ag (also known as Connectivity Fault Management or CFM). The ITU-T Recommendation Y.1731 also defines OAM functions and mechanisms for Ethernet-based networks much like the 802.1ag standard. In general, such standards specify managed objects, protocols, and procedures for, among other things, detecting and diagnosing connectivity faults in end-to-end Ethernet networks. Defined CFM mechanisms for fault detection include continuity check, linktrace (traceroute), loopback (ping), and alarm indication at different levels or domains (e.g., customer level, service provider level, and operator level). 
         [0005]    The IEEE 802.1ag standard also defines various CFM entities and concepts, including maintenance domains, maintenance associations, and maintenance association end points. According to IEEE 802.1ag, a maintenance domain (MD) is “the network or the part of the network for which faults in connectivity can be managed”, a maintenance association end point (MEP) is “an actively managed CFM entity” that “can generate and receive CFM PDUs” (protocol data units or frames), a maintenance association (MA) is “a set of MEPs, each configured with the same MAID (maintenance association identifier) and MD Level, established to verify the integrity of a single service instance”, and a maintenance entity (ME) is “a point-to-point relationship between two MEPs within a single MA”. Additional details regarding such CFM entities are available in the IEEE 802.1ag/D8.1 draft standard, the entirety of which is incorporated by reference herein. 
         [0006]    In metro Ethernet applications, connectivity across tunnels (also called connections) between MEPs is verified continuously through continuity check (CC) messages. A network element transmits such CC messages periodically at a variable interval, which can occur as often as once every 3 milliseconds. Typically, the generating and processing of such CC messages occurs centrally, that is, by a general-purpose central processing unit on a processor card in the network element. The line cards extract the frames of the CC messages from the data path and send them to the processor card. In effect, this frame extraction and forwarding concentrates the CC messages from all line cards at this central point. 
         [0007]    Because many connections (e.g., PBB/PBT tunnels) can terminate on a given physical interface on the network element, the central processor can become overwhelmed by the real-time processing requirements for generating and checking these CC messages. For example, a network element that supports 640 G of service traffic and has a scaling requirement of 1000 MEs per 10 G lane can thus have 64000 MEs to manage, with the corresponding CC messages converging on the single central processor. With a minimum interval for a CC message being 3.1 ms, the central processor can conceivably need to generate a CC message every 48 ns. Even the fastest of today&#39;s CPUs would not measure up to the task. Consequently, the CPU would eventually lag behind with CC message generation and checking, thus eventually leading to false indicators of lost connectivity. Alternatively, multiple general-purpose CPUs can be used in parallel, but this configuration can be impractical with respect to area, power consumption, and cost. 
       SUMMARY 
       [0008]    In one aspect, the invention features a method in an Ethernet network element comprising at least one line interface element and a central processing unit (CPU). The CPU is configured to control forwarding of data packets at the network element. The method comprises receiving continuity check messages (CCMs) at the at least one line interface element, and processing the CCMs in the at least one line interface element to provide continuity checks for connections to the network element without requiring processing of the CCMs by the CPU. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The above and further advantages of this invention may be better understood by referring to the following description in conjunction with the accompanying drawings, in which like numerals indicate like structural elements and features in various figures. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
           [0010]      FIG. 1  is a schematic representation of an embodiment of a maintenance domain. 
           [0011]      FIG. 2  is a block diagram representation of a network model of a maintenance domain. 
           [0012]      FIG. 3  is a functional block diagram of an embodiment of a network element at an edge of the maintenance domain of  FIG. 1 . 
           [0013]      FIG. 4  is a functional block diagram of an embodiment of line card hardware including a CCM (continuity check message) state machine used to generate outgoing CC messages and to check incoming CC messages. 
           [0014]      FIG. 5  is a functional block diagram of an embodiment of line card hardware used in cooperation with the CCM state machine to generate outgoing CC messages and to check incoming CC messages. 
           [0015]      FIG. 6  is a diagram of an embodiment of a data structure used to maintain a list of connections between MEPs. 
           [0016]      FIG. 7  is a table of example associations among interval codes, CCM interval durations, and timer preload values. 
           [0017]      FIG. 8  is a flow diagram of an embodiment of a process for managing connections between MEPs. 
           [0018]      FIG. 9  is a flow diagram of an embodiment of a process for generating one or more CC messages for a connection. 
           [0019]      FIG. 10  is a diagram of an IEEE 802.1ag frame format for uses in the generating of CC messages. 
           [0020]      FIG. 11  is a flow diagram of an embodiment of a process for examining an incoming CC message. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    Continuity check (CC) messaging is one of several valuable operation, administration, and maintenance (OAM) tools for managing metro Ethernet applications. Traditionally, a single CPU card within a network element centrally handled the CC messaging for each line card in the network element. For network elements constructed as described herein, the handling of the CC messaging falls to the line cards. Advantageously, distributing the CC messaging to the line cards enables network elements so constructed to scale to a greater number of simultaneous connections than are possible for those network elements that process CC messages at a central location. In addition, each line card employs hardware and firmware to generate and check CC messages, thereby achieving messaging rates unattainable using software-based message processing. 
         [0022]    In brief overview, each line card maintains a list of supported connections. A generate timer, a receive timer, and an age counter are associated with each connection in the list. The line card generates a CC message for a given connection when the generate timer expires and detects a loss of continuity for a given connection when its age counter exceeds a threshold. 
         [0023]    A state machine, implemented in hardware of the line card, generates CC messages for each supported connection in accordance with a variable interval associated with that connection. When the moment to generate a CC message arrives for a connection, the state machine produces a template packet. A network processor of the line card modifies the template packet to complete the CC message for subsequent transmission over the network. 
         [0024]    On receipt of a CC message, the network processor performs various checks. Upon validating the CC message, the network processor sends a message to the state machine to signify that a valid CC message has arrived for a given connection. The state machine accesses this connection within the list of connections to modify certain timers used to maintain the aging of the connection. If a CC message for a connection is not received within a defined period, the state machine declares a loss of connectivity and initiates preparation of an exception message for delivery to the central processing card. 
         [0025]      FIG. 1  shows an embodiment of a maintenance domain (MD)  10  having a plurality of network elements  12 - 1 ,  12 - 2 ,  12 - 3 ,  12 - 4 ,  12 - 5 ,  12 - 6 , and  12 - 7  (generally,  12 ). Network elements  12  can be any type of network device, examples of which include bridges, routers, and switches. The maintenance domain  10  may be implemented using various transport technologies according to different protocols related to an end-to-end carrier-grade Ethernet service. Examples of these technologies include, but are not limited to, Ethernet over Synchronous Optical Network/Synchronous Digital Hierarchy (SONET/SDH), Ethernet over Asynchronous Transfer Mode (ATM), Ethernet over Resilient Packet Ring (RPR), Ethernet over Multiprotocol Label Switching (MPLS), and Ethernet over Internet Protocol (IP). 
         [0026]    Each network element  12  includes Ethernet ports  14 . An Ethernet port can implement multiple MEs of different types. For network elements  12 - 1 ,  12 - 5 ,  12 - 6 , and  12 - 7 , one of its ports  14  implements a MEP  18  and another port  14  implements a MIP (maintenance intermediate point)  16 . For network elements  12 - 2 ,  12 - 3 , and  12 - 4 , each port  14  implements a MIP  16 . In general, the MEPs  18  and MIPs  16  are software entities executing at the network element, although aspects of the CC messaging are implemented in hardware, as described in more detail below. 
         [0027]    The MEPs  18  operate at an edge of the maintenance domain  10 , whereas the MIPs  16  are inside the domain  10 . Whereas MEPs  18  are active entities that system operators may use to initiate and monitor CFM activity, MIPs  16  passively receive and respond to CFM flows initiated by MEPs. Each MIP  16  and MEP  18  has a unique identifier, usually the MAC address of the interface with which the MEP or MIP is associated, that uniquely identifies the MIP or MEP in the Layer 2 network. 
         [0028]    A MEG (maintenance entity group) includes a set of MEs that satisfy the following conditions: (1) MEs in an MEG exist in the same administrative domain and have the same ME level; and (2) MEs in an MEG belong to the same service provider VLAN (S-VLAN). MEGs can also be called point-to-point or multipoint Ethernet connections. For a point-to-point Ethernet connection, a MEG contains a single ME. For a multipoint Ethernet connection, a MEG contains n*(n−1)/2 MEs, where n is the number of Ethernet connection end points. For example, in  FIG. 1 , there are four Ethernet connection endpoints and, thus, six MEs (4*3/2). 
         [0029]    In  FIG. 1 , the MEP  18  of the network element  12 - 1  periodically sends a multicast CC message  20  within the MD  10 . The period of this “heartbeat” message can range from 3.1 ms to 10 s (the 802.1ag standard defines a set of discrete intervals: 3.1 ms, 10 ms, 100 ms, 1 s, and 10 s). The CC message  20  passes through the MIPs to the other MEPs in the same VLAN (virtual local area network) as the sending MEP. In  FIG. 1 , these other MEPs are at network elements  12 - 5 ,  12 - 6 , and  12 - 7 . Each MEP receiving this CC message  20  catalogs it and knows from the CC message  20  that the various maintenance associations (MAs) are functional, including all intermediate MIPs. Although not shown, these other MEPs are likewise periodically multicasting CC messages throughout the MD  10 . 
         [0030]    Each MEP  18  also performs various checks on received CC messages. For instance, if the received CC message has a MEG level that is lower than the MEG level of the receiving MEP, the MEP has detected an unexpected MEG level. When, instead, the MEG levels are the same, but the incoming CC message has a MEG ID that is different from MEG ID of the receiving MEP, the MEP has detected a mismerge. When the CC message has a correct MEG level and a correct MEG ID, but an incorrect MEP ID, the MEP has detected an unexpected MEP. When the CC message has a correct MEG level, a correct MEG ID, and a correct MEP ID, but also has a period field value that is different from the CC message transmission period of the receiving MEP, the MEP has detected an unexpected period. In addition, if three consecutive CC messages from a given MEP source are lost, the MEP declares a loss of continuity for the connection to the MEP. 
         [0031]    Metro Ethernet networks often encompass multiple administrative domains belonging to different organizations, network operators, and service providers. The customer subscribes to the services of a provider, and the provider subscribes to the services of two or more operators. Accordingly, a service instance spans the provider network covering one or more operators. The provider has responsibility for the service from end to end, and each operator provides transport for the service across its particular sub-network. 
         [0032]      FIG. 2  shows a multi-domain network model  30 . The network model  30  includes customer equipment  40 - 1 ,  40 - 2  at opposite ends of the network, equipment for operator A, which includes network elements  42 - 1 ,  42 - 2 ,  42 - 3 , and equipment for operator B, which includes network elements  44 - 1  and  44 - 2 . An end-to-end path  46  extends from the customer equipment  40 - 1  to the customer equipment  40 - 2  through the equipment of the operators A and B. 
         [0033]    The service network is partitioned into a hierarchy of levels including a customer maintenance level  48 , a provider maintenance level  50 , an operator maintenance level  52 , and a server/transport level  54 , which consists of underlying packet transport links  56 . These links  56  may be single hop Ethernet links, multi-hop MPLS pseudowire, or SONET/SDH paths. Each different domain corresponds to a particular maintenance level. In general, MEPs  18  are implemented at administrative domain boundaries.  FIG. 2  also shows that for a given Ethernet connection, a port  14  of the network element can implement multiple MEPs and MIPs, depending upon the number of domain levels. 
         [0034]      FIG. 3  shows an embodiment of the network element  12 - 1  of  FIG. 1 , as a representative example of network elements that are at an edge of the maintenance domain  10  and implement a MEP  18 . The network element  12 - 1  includes a central processor (CP) card  60  in communication with a plurality of input/output modules or interface modules, referred to herein as line cards  62 - 1 ,  62 - n  (generally,  62 ) through a midplane (or backplane)  64 . The CP card  60  includes a switch fabric (SF)  66  (e.g., an Ethernet switch). Although shown to be part of the CP card  60 , the switch fabric  66  can alternatively be embodied on the midplane (or backplane)  64 . 
         [0035]    Each line card  62  includes one or more Ethernet ports  68  for sending and receiving Ethernet frames externally of the network element (e.g., to and from a user network, a provider network, operator network). Examples of types of line cards  62  that can be used in the practice of the invention include, but are not limited to, SFP (Small Form-Factor Pluggable)-based, Gigabit Ethernet Services modules, 1000 BaseX for SFP modules, 10 Gigabit Ethernet XFP (Gigabit Ethernet Small Form-Factor Pluggable) module, GBIC (Gigabit Interface Converter)-based Gigabit Ethernet Services Module, POS (Packet over SONET) Baseboard supporting up to 6 OC-3 or 3 OC-12 ports, 1000BASE-T, and fixed Gigabit Ethernet. 
         [0036]    In general, the network element  12 - 1  implements the IEEE 802.1ag protocol in software. Software components of the protocol for generating, transmitting, and receiving 802.1ag packets reside on the CP card  60 . As described below, aspects of generating, transmitting, receiving, and processing CC messages, referred to generally as CC messaging, are implemented in hardware on each line card  62 . 
         [0037]      FIG. 4  shows a simplified embodiment of hardware architecture  100  for a data path of the line card  62 - 1  (as a representative example) in  FIG. 3 . The line card  62 - 1  includes a physical Ethernet interface  102  (i.e., a MAC or Media Access Control device), a frame analyzer  104 , one or more network processors (also known as routing and switching processors or RSPs)  106 , and a switch fabric interface  108 . The Ethernet interface  102  is in communication with an external network (e.g., user network, provider network) for forwarding and receiving Ethernet frames, including 802.1ag packets, to and from the line card  62 - 1 . 
         [0038]    In general, the frame analyzer  104  includes a general-purpose CPU for the line card and is in communication with the Ethernet interface  102  to receive and forward 802.1ag packets therethrough. The frame analyzer  104  includes special-purpose hardware that provides a CCM state machine  110 . The state machine  110  is used for periodically generating new CC messages and for checking incoming CC messages. The special-purpose hardware can be implemented with a FPGA (field programmable gate array) or an ASIC (application-specific integrated circuit). Although shown to be part of the frame analyzer  104 , the CCM state machine  110  can alternatively be implemented in the physical Ethernet interface  102 . 
         [0039]    The frame analyzer  104  is in communication with each network processor  106 —the number of network processors  106  on a given line card depends on the card type and number of ports  68  on the line card. Each network processor  106  corresponds to a lane that supports a number of connections. In general, a network processor  106  performs packet processing of incoming CC messages and participates in the generation of outgoing CC messages. An ASIC device with some programmability features can be used to implement the network processor  106 . The switch fabric interface  106  provides a communication interface with the switch fabric  66  through which the line card  62 - 1  exchanges communications with the CP card  60  and other line cards  62 . 
         [0040]      FIG. 5  shows an embodiment of hardware architecture  120  for operating the CCM state machine  110 . The architecture  120  includes the CCM state machine  110 , decode logic  122 , a central timer (or strobe signal generator)  124 , memory  126 , template memory  128 , and output logic  130 . The decode logic  122  receives and decodes internal messages from the network processor  106  related to a received CC message to produce a ME index signal  132  and a reset signal  134  that the decode logic  122  provides to the state machine  110 . The ME index signal  132  identifies a particular connection associated with the received CC message and the reset signal  134  indicates whether to reset a timer. 
         [0041]    The central timer  124  produces four strobe signals  136 - 1 ,  136 - 2 ,  136 - 3 ,  136 - 4  (generally,  136 ), each strobe signal corresponding to a different strobe rate at which a strobe signal is sent to the CCM state machine: 40 us, 320 us, 2.56 ms, and 20.48 ms, respectively. As illustrative examples, the central timer  124  issues a strobe signal on the 320 us strobe signal line  136 - 2  every 320 us and a strobe signal on the 2.56 ms strobe signal line  136 - 3  every 2.56 ms. 
         [0042]    The memory  126  stores a list of connections  138  for which the line card  62  generates outgoing CC messages and checks incoming CC messages. The state machine  110  reads from and writes to the list of connections  138  stored in memory  126 , as described in more detail below. 
         [0043]    The template memory  128  maintains a blank template used to generate CC messages and, optionally, exception messages. The CP card  60  stores the template in the template memory  128  upon start up of the line card  62 . The output logic  130  is in communication with the template memory  128  to obtain the template and, optionally, to fill certain fields of the template with information acquired from the CCM state machine  110 . 
         [0044]      FIG. 6  shows an embodiment of a data structure  150  (here, as an example, a table) used to maintain the list of connections  138  stored in the memory  126  and accessed by the CCM state machine  110 . The data structure  150  has a plurality of entries  152 - 1 ,  152 - 2 ,  152 - 3 ,  152 -N (generally,  152 ). Each entry  152  corresponds to a different virtual connection (i.e., an ME) between MEPs and includes a connection identifier (or slot ID)  154 , a valid field  156 , an interval field  158 , a generate timer  160  (called gentimer), a receive timer  162  (called rcvtimer), and an age counter  164 . 
         [0045]    The connection identifier  154  holds a value for uniquely identifying the associated virtual connection or ME. The valid field  156  indicates whether the associated connection is valid or invalid. The generate timer  160  holds a decrementing count, which, upon reaching zero, signals generation of a CC message for the associated connection. The receiver timer  162  holds a decrementing count which, upon reaching zero, signals expiration of an interval during which a CC message from the remote MEP of the associated connection was expected, but not detected. The incrementing count within the age counter  164  corresponds to a number of consecutive CC messages that were expected but not received from the remote MEP of the associated connection. The interval field  158  holds a code that maps to values that are initially preloaded and subsequently reloaded into the generate timer and receiver timer fields  160 ,  162  for the associated connection. 
         [0046]    During operation, upon each clock cycle, the state machine  110  accesses one of the virtual connections in the data structure  150 , stepping through the list of connections in round-robin fashion. For example, consider that the data structure has 2000 entries corresponding to 2000 different virtual connections and that the state machine operates at 200 MHz. Accordingly, the state machine  110  takes 10 us (2000 connections/2M cycles/sec) to step through every connection in the data structure  150 ; that is, the state machine  110  accesses each entry  152  in the data structure  150  periodically, once every 10 us. In one embodiment, the data structure  150  has a fixed number of connections  138 , which is determinative of the amount of time that the CCM state machine  110  takes to access every connection in the list once. In another embodiment, the data structure  150  is not fixed in its number of entries, and a variable delay can be added (e.g., at the beginning of the list or at the end) to ensure that each connection is accessed at the desired rate. For instance, if the data structure has 1000 entries and the state machine operates at a 200 MHz clock cycle, the state machine takes 5 us to step through all of the entries. If 10 us is the desired rate, a 5 us delay can be added to the process. 
         [0047]      FIG. 7  shows an example of a table  170  having five entries  172 - 1 ,  172 - 2 ,  172 - 3 ,  173 - 4 , and  172 - 5  that each associates an interval code  174  with a CC message interval  176 , a decrement rate (timer LSB)  178 , and a preload value  180 . One of the five different interval codes  174  is stored in the interval field  158  for each connection in the data structure  150 . The state machine  110  uses the interval code  174  assigned to a given connection to determine which one of the strobe signals  136 , if any, applies to that connection. 
         [0048]    As shown in  FIG. 7 , the table  170  has a different interval code  174  for each different CCM interval of 3.33 ms, 10 ms, 100 ms, 1 s, and 10 s, but excludes interval codes for 1 minute and 10 minutes. Such CC messaging intervals for such CC messages are sufficiently long for software executing at the CP card  60  to process. The accelerated processing achieved by the state machine and cooperating hardware at the line card  62  is not critical for such relatively long messaging intervals. Notwithstanding, interval codes can be established for these and other CCM intervals. 
         [0049]    The interval code  174  assigned to a given connection also determines the preload values written initially, and upon each reset, to the generate timer and receive timer fields  160 ,  162  for the connection. The particular preload values shown in  FIG. 7  are designed to achieve, in conjunction with the strobe signals, the corresponding CCM interval for the above-described embodiment in which the state machine  110  accesses each connection entry in the list once every 10 us. The preload values can differ for embodiments in which the state machine operates at a different clock rate (e.g., 250 MHz), is configured to access each connection entry at a rate other than 10 us (e.g., because there are more or fewer than 2000 connections in the list of connections), or uses different strobe rates from the four aforementioned strobe rates. 
         [0050]    For example, the state machine  110  relates the interval code of 010 (binary) to the 40 us strobe signal. The preload value initially written to the generate timer  160  and to the receive timer  162  for a connection assigned the interval code value of 010 is 0x0FA hex (or 250 in decimal). The values in the generate timer and receive timer  160 ,  162  decrement by one every 40 us. Accordingly, the counts in the generate timer and receive timer fields decrement from their preload values to zero in 10 ms, which corresponds to the 10 ms CCM interval associated with that connection. (It is to be understood that instead of decrementing by one, other embodiments can be configured to increment by one or more, or to decrement by more than one.) 
         [0051]    As another example, the interval code value of 001 (binary) corresponds to a 3.33 ms CCM messaging interval. The preload values of connections assigned the interval code value of 001 is 0x14D hex (i.e., 333 decimal). Each timer field  160 ,  162  is preloaded with the value of 333, and decrements by one every 10 us (3.33 ms/333). No strobe signal is used for this CCM messaging interval because the state machine accesses each connection once every 10 us, and thus a 10 us strobe signal is not needed to control whether the state machine examines and decrements the timer values. The counts in the generate timer and receive timer decrement from their preload values to zero in 3.33 ms. 
         [0052]      FIG. 8  shows an embodiment of a process  200  for generating outgoing CC messages and checking incoming CC messages. The particular order of steps in  FIG. 8  is but one illustration of the process  200 ; some of the actions taken by the CCM state machine  110  can occur concurrently or in a different order from that described. At step  202 , the CP card constructs the list of connections  138  (one list for each line card  62 ), by determining the CC message interval for each connection when that connection is established. For each connection, the CP card  60  provides an interval code  158 , flags the connection as valid, and writes the associated preload values to the generate timer and receive timer fields  160 ,  162 . The list of connections  138  passes to the line card  62  for which it is prepared, where it is locally stored in the memory  126  in a data structure  150 . In one embodiment, the CP card  60  keeps track of each connection in the list for each line card and determines whether to add or invalidate connections in the list. 
         [0053]    During the process  200 , the CCM state machine  110  of a given line card steps through, in round robin fashion, the connections in the list of connections  138 . The CCM state machine  110  accesses ( 204 ) the first connection in the list of connections. To determine whether to examine the generate timer and receiver timer fields of the connection, the state machine  110  checks ( 206 ) if the connection is valid and if the appropriate strobe signal, based on the interval code, is asserted ( 208 ). 
         [0054]    If the connection is either invalid or the associated strobe signal is not asserted, the state machine  110  advances ( 204 ) to the next connection in the list. If the presently accessed connection is the last connection in the list, the state machine  110  returns to the first connection in the list. Otherwise, the state machine  110  decrements ( 210 ) the generate timer  160  and the receive timer  162  for the connection. 
         [0055]    The state machine  110  examines ( 212 ) the present value in the generate timer field  160  and receive timer field  162  for the connection. A non-zero value in the generate timer field  160  indicates that the generate timer has not expired, whereas a zero value indicates expiration. Upon expiration of the generate timer, the state machine  110  initiates generation ( 216 ) of a CC message for this connection. The value in the generate timer field is reset ( 218 ) to the preload value. In addition, the CC message is forwarded ( 220 ) towards its destination MEP through an appropriate Ethernet port  68 . 
         [0056]    Similarly to the generate timer, a non-zero value in the receive timer field  162  indicates that the receive timer has not expired, whereas a zero value indicates expiration. If the receive timer  162  has not expired, the state machine  110  advances ( 204 ) to the next connection in the list. If, instead, the receive timer field  162  has expired, the state machine  110  increments by one ( 224 ) the count in the age counter  164 . If the count in the age counter  164  consequently reaches ( 226 ) a predefined threshold (e.g., 3), then the state machine  110  declares ( 228 ) a loss of continuity for the connection, and initiates a reporting of the continuity loss to the CP card  60  in an exception packet. Otherwise, the state machine  110  advances ( 204 ) to the next connection in the list. 
         [0057]    In the generation of the exception packet, the state machine  110  acquires a template frame (e.g., from the template memory  128 ) and sends the template frame to the network processor  106 . The template frame includes the ME index, thereby identifying the connection for which continuity has been lost. The network processor  106  adds any additional information to the template frame to complete building the exception packet. The network processor also encapsulates the exception packet for transport across the switch fabric to the CP card  60 . The state machine  110  subsequently advances ( 204 ) to the next connection in the list. 
         [0058]    The following pseudo code generally outlines the process  200  described in  FIG. 8 : 
         [0000]    
       
         
               
             
           
               
                   
               
             
             
               
                 For n = 1 to 2048 Do 
               
               
                  Begin 
               
               
                   Read Slot n; 
               
               
                   Case INTERVAL of 
               
               
                    001: decrement GenTimer, RcvTimer; 
               
               
                    010: If 40us_strobe then decrement GenTimer, RcvTimer; 
               
               
                    011: If 320us_strobe then decrement GenTimer, RcvTimer; 
               
               
                    100: If 2.56ms_strobe then decrement GenTimer, RcvTimer; 
               
               
                    101: If 20.48ms_strobe then decrement GenTimer, RcvTimer; 
               
               
                    END Case 
               
               
                    IF GenTimer = 0x000 THEN 
               
               
                     Begin 
               
               
                     Generate_Template_Packet; 
               
               
                     Reload Timer according to INTVL field; 
               
               
                     End 
               
               
                    IF RcvTimer = 0x000 THEN 
               
               
                     Begin 
               
               
                     Increment Age Count; 
               
               
                     If Age Count = 3 THEN 
               
               
                      Generate_Exception_Packet; 
               
               
                     End 
               
               
                 END For Loop 
               
               
                   
               
             
          
         
       
     
         [0059]      FIG. 9  shows an embodiment of a process  216  ( FIG. 8 ) of generating a CC message. In the description of the process  216 , reference is also made to  FIG. 4  and to  FIG. 5 . When a generate timer  160  associated with a valid connection decrements to zero, the CCM state machine  110  generates ( 250 ) a template frame for a CC message for forwarding to the network processor  106 . More specifically, the state machine  110  sends a load command to the logic  130 , and the logic  130  acquires the template frame from the template memory  128 .  FIG. 10  shows an example format for the template frame, which is a standard 802.1ag OAM frame format. 
         [0060]    Returning to  FIG. 9 , the logic  130  adds ( 252 ) an ME index identifying the connection to the template frame (in one of the fields of the OAM frame format) and forwards ( 254 ) the partially filled template frame to the network processor  106 . The network processor  106  uses the ME index passed along in the template frame to access ( 256 ) connection information from a data structure. This data structure maintains a correspondence between ME indices and connection information about each destination MEP in the ME, for example, the destination address (DA), source address (SA), VID (VLAN ID), for the MEP. After acquiring the connection information, the network processor  106  completes ( 258 ) a CC message for each MEP by filling in the remaining fields of the template frame and forwards ( 260 ) each completed CC message to the switch fabric  66  with an appropriate destination Ethernet port. Each completed CC message returns from the switch fabric  66  and passes ( 262 ) to the physical Ethernet interface  102  (through the state machine  110 ) for forwarding to the external network through the destination Ethernet port. 
         [0061]      FIG. 11  shows an embodiment of a process  280  for receiving and checking an incoming CC message. At step  282 , an Ethernet frame (or packet) arrives at the Ethernet interface  102  of one of the Ethernet ports  68 . From the OAM E-type  268  and Opcode  273  fields of the Ethernet frame, the frame analyzer  104  determines ( 284 ) that the Ethernet frame is a CC message. The frame analyzer  104  sends ( 286 ) the Ethernet frame to the network processor  106 , signifying that the frame is to undergo CC message reception processing. 
         [0062]    At step  288 , the network processor  106  performs a hash of the source address, destination address, and VLAN ID to acquire an ME index. The network processor  106  then uses the ME index to access ( 290 ) a database that contains connection information for the corresponding ME. Using this connection information, the network processor  106  examines ( 292 ) the CC message to determine if the MA level, the MA ID, and the MEP ID are correct. If the comparisons fail ( 294 ), the network processor  106  sends ( 296 ) an exception packet to the CP card  60 . 
         [0063]    Alternatively, if the comparisons pass ( 294 ), the network processor sends an internal message ( FIG. 5 ) to the CCM state machine  110 . In general, the internal message operates to cause the CCM state machine  110  to reset ( 298 ) the receive timer field (to the associated preload value) and the age counter (to 0) of the entry  152  corresponding to the connection associated with the CC message. In one embodiment, the internal message maps to a specific memory location. The decode logic  122  ( FIG. 5 ) decodes this memory location as a reset command. Decoding the internal message also provides the ME index so that the CCM state machine  110  can determine the connection for which to reset the receive timer and age counter. 
         [0064]    Program code (or software) of the present invention may be embodied as computer-executable instructions on or in one or more articles of manufacture, or in or on computer-readable medium. A computer, computing system, or computer system, as used herein, is any programmable machine or device that inputs, processes, and outputs instructions, commands, or data. In general, any standard or proprietary, programming or interpretive language can be used to produce the computer-executable instructions. Examples of such languages include C, C++, Pascal, JAVA, BASIC, Visual Basic, and Visual C++. 
         [0065]    Examples of articles of manufacture and computer-readable medium in which the computer-executable instructions may be embodied include, but are not limited to, a floppy disk, a hard-disk drive, a CD-ROM, a DVD-ROM, a flash memory card, a USB flash drive, an non-volatile RAM (NVRAM or NOVRAM), a FLASH PROM, an EEPROM, an EPROM, a PROM, a RAM, a ROM, a magnetic tape, or any combination thereof. The computer-executable instructions may be stored as, e.g., source code, object code, interpretive code, executable code, or combinations thereof. Further, although described predominantly as software, embodiments of the described invention may be implemented in hardware (digital or analog), software, or a combination thereof. 
         [0066]    While the invention has been shown and described with reference to specific preferred embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the following claims.