Abstract:
A network device connecting a plurality of Ethernet links, includes: an Ethernet maintenance and administration section for periodically checking whether a link fault occurs on each Ethernet link; a link manager for updating link status information for each Ethernet link according to a check result of the Ethernet link; and a link switching processor for switching from a fault-detected Ethernet link to another Ethernet link according to link status information of the Ethernet links.

Description:
[0001]    This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2008-157428, filed on Jun. 17, 2008, the disclosure of which is incorporated herein in its entirety by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a communication network and, more particularly, to a network device having Ethernet maintenance and administration functionality, as well as a link switching method used in the same. Note that “Ethernet” is a registered trademark. 
         [0004]    2. Description of the Related Art 
         [0005]    Ethernet was originally created as a local area network (LAN) technology but, in recent years, has become used for wide area networks. However, Ethernet, which is standardized as a technology for LAN, is not provided with OAM (Operations, Administration and Maintenance) functionality that allows monitoring of the state of a remote network device, bypassing of a link fault, and the like. 
         [0006]    TCP/IP-based simple network management protocol (SNMP) is used in many cases to maintain and administer an Ethernet network. In this case, however, when a remote network device has become unable to be managed with SNMP, it is impossible to determine whether the cause resides in the IP (Internet Protocol) layer or in the Ethernet network. Accordingly, for Ethernet, a function is needed that makes it possible to maintain and administer a remote network device, and the standardization of Ethernet OAM functionality has been pursued (see ITU-T recommendation Y.1731 and IEEE 802.1ag). 
         [0007]    As well known, the main functions of Ethernet OAM are limited to those for fault detection and performance measurement such as delay measurement. For the fault detection functions, defined are the continuity check (CC) function, loop back (LB) test function, and link trace (LT) function. For example, a method for detecting a fault using the CC function is disclosed in Japanese Patent Application Unexamined Publication No. 2007-243466. 
         [0008]    However, the functions of Ethernet OAM are confined in the scope of fault detection and performance monitoring, and operations after fault detection are not standardized. Therefore, recovery after fault detection depends on manual operations, which means that it takes much time to recover from a fault. 
         [0009]    Regarding the detection of a network fault and the generation of a path bypassing the fault, for example, Japanese Patent Application Unexamined Publication No. 2002-016617 and others disclose techniques, which use a general wide area network technology in which an OAM cell is transmitted over an ATM network. 
       SUMMARY OF THE INVENTION 
       [0010]    An object of the present invention is to provide a network device and a link switching method by which high-speed link switching can be achieved through fault detection utilizing Ethernet OAM functionality. 
         [0011]    According to the present invention, a network device connecting a plurality of Ethernet links, includes: an Ethernet maintenance and administration section for periodically checking whether a link fault occurs on each Ethernet link; a link manager for updating link status information for each Ethernet link according to a check result of the Ethernet link; and a link switching processor for switching from a fault-detected Ethernet link to another Ethernet link according to link status information of the Ethernet links. 
         [0012]    According to the present invention, a method for switching Ethernet links in a network device connecting a plurality of Ethernet links, includes the steps of: periodically checking whether a link fault occurs on each Ethernet link; updating link status information for each Ethernet link according to a check result of the Ethernet link; and switching from a fault-detected Ethernet link to another Ethernet link according to link status information of the Ethernet links. 
         [0013]    According to the present invention, high-speed link switching can be achieved through fault detection utilizing Ethernet OAM functionality. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a block diagram showing a basic functional configuration of a network device according to an exemplary embodiment of the present invention. 
           [0015]      FIG. 2  is a block diagram showing a basic functional configuration of a router according to a first example of the present invention. 
           [0016]      FIG. 3  is a diagram showing a network structure, to describe link switching operation according to the first example of the present invention. 
           [0017]      FIG. 4  is a flowchart schematically showing the internal operation of each router that executes the link switching operation according to the first example. 
           [0018]      FIG. 5  is a diagram showing a network structure, to describe link switching operation according to a second example of the present invention. 
           [0019]      FIG. 6  is a block diagram showing a basic functional configuration of a router according to a third example of the present invention. 
           [0020]      FIG. 7  is a diagram showing a network structure, to describe link switching operation according to the third example of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     1. EMBODIMENT  
       [0021]      FIG. 1  is a block diagram showing a basic functional configuration of a network device according to an exemplary embodiment of the present invention. In the present exemplary embodiment, a network device having a function of switching between a plurality of Ethernet links will be illustrated as an example. In this disclosure, a network device is defined as a communication device connected to a network. Examples of the network device include user communication equipment, a router on a network, and the like. 
         [0022]    Referring to  FIG. 1 , the network device is provided with a plurality of transmission/reception (T/R) control sections  101 . 1  to  101 .N which are connected to a plurality of Ethernet links  1  to N, respectively, and individually execute processing prescribed by Ethernet. The transmission/reception control sections  101 . 1  to  101 .N are connected to input/output ports of a switching processing section  102 , respectively, and the switching processing section  102  executes link switching in accordance with link status information from a link management section  104 , which will be described later. 
         [0023]    The network device is further provided with an Ethernet OAM processing section  103  which can perform maintenance and administration of the links  1  to N through the respective transmission/reception control sections  101 . 1  to  101 . N. Here, it is assumed that the continuity of each link is checked by utilizing the CC function of Ethernet OAM in such a manner that the network device in question and a network device on the other end of the link transmit a CC message (CCM) to each other at predetermined time intervals. Note that it is also possible to monitor each link by using the LB function or LT function. 
         [0024]    The Ethernet OAM processing section  103  determines that a network fault has occurred between its own network device and a network device on the other end when receiving no CCM from the other-end network device even after a predetermined period of time has passed, and makes a notification to that effect to the link management section  104 . 
         [0025]    The link management section  104  receives fault detection information from the Ethernet OAM processing section  103  as input and performs link management using a link status table  105 . The link status table  105  keeps the respective states (LINK-UP (LINK-U) or LINK-DOWN (LINK-D)) of the links  1  to N, which are updated based on the fault detection information from the Ethernet OAM processing section  103 . Updated link status information is output from the link management section  104  to the switching processing section  102 . 
         [0026]    For example, upon receipt of a notification from the Ethernet OAM processing section  103  to the effect that a network fault has occurred in the link  1 , the link management section  104  changes the state of the link  1  from LINK-U to LINK-D. At this time, if the backup link  2  is in LINK-DOWN state, the link management section  104  changes the state of the link  2  to LINK-UP. The thus updated link status information is output to the switching processing section  102 . The switching processing section  102  switches the currently used link from the link  1  to the link  2  in accordance with the link status information, whereby a continuity check on the link  2  can be executed. 
         [0027]    As mentioned below, a continuity check on the backup link  2  can also be performed independently of the state of the link  1 . The Ethernet OAM processing section  103  can periodically transmit and receive a CCM to/from a network device on the other end of each link, irrespective of the link status in the link status table  105 . Accordingly, it is possible to check the continuity of the backup link  2  in advance. 
         [0028]    Additionally, the functions equivalent to the switching processing section  102 , Ethernet OAM processing section  103 , and link management section  104  can also be implemented with software by executing programs on a program-controlled processor such as a CPU (Central Processing Unit). 
       2. FIRST EXAMPLE  
       [0029]    Hereinafter, a first example of the present invention will be described in more detail by taking a router as an example of the network device shown in  FIG. 1 . 
       2.1) Configuration 
       [0030]      FIG. 2  is a block diagram showing a basic functional configuration of a router according to the first example of the present invention. Note that the blocks having the same functions as those of the network device shown in  FIG. 1  are denoted by the same reference numerals as in  FIG. 1  and a description thereof will be simplified. 
         [0031]    In the router  10  according to the first example, the switching processing section  102  in  FIG. 1  is composed of a routing processing section  201 , a routing table  202 , and a routing information management section  203 . The routing processing section  201  has N input/output ports connected to the transmission/reception control sections  101 . 1  to  101 .N, respectively, and executes routing of a transmission/reception signal in accordance with route information in the routing table  202 . 
         [0032]    The routing information management section  203  updates the routing table  202 , based on the link status information from the link management section  104 . For example, it is assumed that the link  1  is set as a primary route for communication with a network device on the other end. When a fault has occurred in the link  1 , the link management section  104  updates the link status information, whereby the routing table  202  is updated, and thus the route can be switched to the link  2  set as a secondary route. 
         [0033]    Incidentally, the functions equivalent to the Ethernet OAM processing section  103 , link management section  104 , routing processing section  201 , and routing information management section  203  can also be implemented with software by executing programs on a program-controlled processor such as a CPU. 
       2.2) Operation 
       [0034]      FIG. 3  is a diagram showing a network structure, to describe link switching operation according to the first example of the present invention.  FIG. 4  is a flowchart schematically showing the internal operation of each router executing the link switching operation according to the first example. 
         [0035]    To avoid complicating the description, here assumed is a network in which four routers  10 A to  10 D are connected in a ring shape as shown in  FIG. 3 , with a direct connection between the router  10 A and the neighboring router  10 B being a primary route, and a connection via the routers  10 C and  10 D being a secondary route. 
         [0036]    The Ethernet OAM processing section  103  of the router  10 A transmits a CCM at predetermined time intervals from the transmission/reception control section  101 . 1  to the router  10 B, which is the other end of the link  1 , and also receives a CCM from the router  10 B at predetermined time intervals. The primary route using the link  1  operates normally as long as a CCM is normally received at the predetermined time intervals. 
         [0037]    As shown in  FIG. 4 , when the Ethernet OAM processing section  103  of the router  10 A does not receive a CCM from the router  10 B even after a predetermined period of time has passed, a timeout occurs on a timer of the Ethernet OAM processing section  103  of the router  10 A, whereby it is detected that a fault has occurred in the link  1  to the router  10 B (Step  20 ). 
         [0038]    The link management section  104  notified of the occurrence of a fault checks the current link status by referring to the link status table  105  (Step S 21 ). Here, it is assumed that the link  1  is in LINK-UP state and the link  2  is in LINK-DOWN state as shown in  FIG. 4 . Subsequently, the link management section  104  updates the link status table  105 , according to the notification of the occurrence of a fault in the link  1  (Step S 22 ). Here, the state of the link  1  is changed from LINK-UP to LINK-DOWN, and the state of the link  2  is changed from LINK-DOWN to LINK-UP as shown in  FIG. 4 . The thus updated link status information is output to the routing information management section  203 . 
         [0039]    The routing information management section  203  updates the routing table  202  in accordance with the updated link status information (Step S 23 ). Here, since the link  1 , the primary route, is in LINK-DOWN state and the link  2 , the secondary route, is in LINK-UP state, the routing table  202  is updated so that the transmission and reception of a signal to/from the router  10 B will be performed through the secondary route. Similar switching is also made at the router  10 B. Accordingly, at the router  10 A, the currently used link to the router  10 B is switched from the link  1  to the link  2 , and at the router  10 B, the currently used link to the router  10 A is switched from the link  1  to the link  3 . Resultantly, the connection between the routers  10 A and  10 B is switched from the primary route to the secondary route as shown in  FIG. 3 . 
       2.3) Effects 
       [0040]    As described above, according to the first example of the present invention, high-speed link switching can be achieved through fault detection utilizing Ethernet OAM. In other words, it is possible to carry out an instantaneous update of the routing table  202  by performing fault detection on the layer  2 , and it is thus possible to provide a high-speed backup. 
       3. SECOND EXAMPLE  
       [0041]    A router  10  according to a second example of the present invention has a functional configuration similar to the router according to the first example shown in  FIG. 2 . However, the Ethernet OAM processing section  103  according to the second example can perform fault monitoring not only on the primary route but also on the secondary route. Specifically, fault monitoring is performed by periodically transmitting and receiving a CCM to/from a network device on the other end, as in the case of the primary route. 
         [0042]      FIG. 5  is a diagram showing a network structure, to describe link switching operation according to the second example of the present invention. Assuming a network in which four routers  10 A to  10 D are connected in a ring shape as in the first example shown in  FIG. 3 , the router  10 A monitors whether the reception of a CCM is normally performed over the primary route to the neighboring router  10 B, and also concurrently monitors the reception of a CCM over the secondary route via the routers  10 C and  10 D in a similar manner. 
         [0043]    As described above, a continuity check is performed also on the secondary route, whereby, when a network fault in the primary route is detected, it is possible to promptly secure the secondary route into which the communication should be diverted, and it is thus possible to achieve high-speed switching. 
         [0044]    Moreover, even after switching to the secondary route is made, a continuity check on the primary route is continued. When the primary route has recovered, the link management section  104  updates the link status table  105 , whereby it is possible to switch again from the secondary route to the original primary route. 
       4. THIRD EXAMPLE  
       [0045]    According to the present invention, it is also possible to make an Ethernet link between routers redundant. Hereinafter, a router and a network using a redundant system will be described with reference to  FIGS. 6 and 7 . 
       4.1) Configuration 
       [0046]      FIG. 6  is a block diagram showing a basic functional configuration of a router according to a third example of the present invention. Note that the blocks having the same functions as those of the router shown in  FIG. 2  are denoted by the same reference numerals as in  FIG. 2  and a description thereof will be simplified. According to the third example, the transmission/reception control sections  101 . 1  and  101 . 2  connected to the links  1  and  2  respectively are connected to a redundant system switching section  301 , and any one of the transmission/reception control sections  101 . 1  and  101 . 2  selected in accordance with a switching signal from the link management section  104  is connected to a single input/output port of the routing processing section  201 . Here, it is assumed that the link  1  is an active (currently used) link and the link  2  is a standby link. 
         [0047]    The routing information management section  203  updates the routing table  202  in accordance with the link status information from the link management section  104 , as described in the first example. However, apart from the route information, link status about the redundant system is also stored in the routing table  202  according to the third example. Here, it is assumed that the link  1  is set as an active link and the link  2  is set as a standby link. 
         [0048]    When a fault has occurred in the link  1 , which is being used as an active link, the link management section  104  updates the link status information, thereby switching the redundant system switching section  301  from the link  1  to the link  2 . Moreover, the routing table  202  is updated as described above, whereby the active link is switched from the link  1  to the link  2 . 
       4.2) Operation 
       [0049]      FIG. 7  is a diagram showing a network structure, to describe link switching operation according to the third example of the present invention. To avoid complicating the description, it is assumed that the network has a redundant structure in which the routers  10 A and  10 B are connected through two Ethernet links  1  and  2 , with the link  1  set as an active link, and the link  2  set as a standby link, as described above. 
         [0050]    The Ethernet OAM processing section  103  of the router  10 A transmits a CCM at predetermined time intervals from the transmission/reception control section  101 . 1  to the router  10 B on the other end of the link  1 , and also receives a CCM from the router  10 B at predetermined time intervals. The active link  1  operates normally as long as a CCM is normally received at the predetermined time intervals. 
         [0051]    When the Ethernet OAM processing section  103  of the router  10 A does not receive a CCM from the router  10 B even after a predetermined period of time has passed as shown in  FIG. 7 , a timeout occurs on the timer of the Ethernet OAM processing section  103  of the router  10 A, whereby it is detected that a fault has occurred in the link  1  to the router  10 B. 
         [0052]    The link management section  104  notified of the occurrence of a fault checks the current link status by referring to the link status table  105 . Here, it is assumed that the link  1  is in LINK-UP state and the link  2  is in LINK-DOWN state. Subsequently, the link management section  104  updates the link status table  105 , according to the notification of the occurrence of a fault in the link  1 , switches the redundant system switching section  301  from the link  1  to the link  2 , and outputs the updated link status information to the routing information management section  203 . 
         [0053]    The routing information management section  203  updates the routing table  202  in accordance with the updated link status information. Here, since the link  1  is in LINK-DOWN state and the link  2  is in LINK-UP state, the routing table  202  is updated so that the transmission and reception of a signal to/from the router  10 B will be performed through the link  2 . The switching of the redundant system switching section  301  from the link  1  to the link  2  and the update of the routing table  202  are also performed at the router  10 B similarly. Thus, at the router  10 A, the connection to the router  10 B is switched from the link  1  to the link  2 . 
       4.3) Effects 
       [0054]    As described above, according to the third example of the present invention, even in a network where a plurality of Ethernet links are made redundant, high-speed protection can be realized through fault detection utilizing Ethernet OAM. Thus, it is possible to enhance the reliability of communication. 
         [0055]    The present invention, which makes it possible to detect a fault in a link to a remote network device and to recover from the fault, can be applied to the networks of carriers and Internet providers, as well as private networks. Moreover, owing to the characteristics of Ethernet OAM, monitoring can be performed in domain units, with a network divided into a plurality of domains. Therefore, the present invention can also be applied to each of the plurality of divided domains, such as between customer devices, between edge routers, or between core routers. 
         [0056]    The present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The above-described exemplary embodiment and examples are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.