Abstract:
The present invention relates to a method and system for implementing link level protocol redundancy in a router. In particular, the invention relates to providing redundancy of the Open Shortest Path First (OSPF) routing protocol. An active processor provides OSPF operations. In the present invention, a standby processor is coupled to the active processor. During an initial synchronization, all network link protocol information from the active processor is forwarded to the standby processor. The network link information can include OSPF state information, OSPF configuration information, OSPF adjacencies information, OSPF interface information and OSPF global protocol information. Thereafter, any updates of network link protocol information are immediately forwarded to the standby processor. Upon failure of the active processor, the router is switched to the standby processor and all OSPF protocol operations are performed on the standby processor. In the present invention, all states of the link protocol immediately function as if a failure had not occurred.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to network communications and more particularly to redundancy of routing protocols, such as the Open Shortest Path First (“OSPF”) protocol and apparatus for protecting protocol services of a router and neighbor routers from failure. 
   2. Related Art 
   The Internet Protocol (“IP”) is the foundation for many public, such as the Internet, and private, such as a corporate Intranet, data networks. Convergence of voice, data and multimedia networks has also been largely based on IP-based protocols. 
   Data packets progress through the data networks by being sent from one machine to another towards their destination. Routers or other types of switches are used to route the data packets over one or more links between a data source, such as a customer&#39;s computer connected to the data network, and a destination. Routing protocols such as Border Gateway Protocols (“BGP”), Routing Information Protocol (“RIP”), and Open Shortest Path First Protocol (“OSPF”) enable each machine to understand which other machine is the “next hop” that a packet should take towards its destination. Routers use the routing protocols to construct routing tables. Thereafter, when a router receives a data packet and has to make a forwarding decision, the router “looks up” in the routing table the next hop machine. Conventionally, the routers look up the routing table using the destination IP address in the data packet as an index. 
   In the basic OSPF algorithm, a router broadcasts a hello packet including the router&#39;s own ID, neighbors&#39; IDs the router knows and also receives such messages from other routers. If a router receives a Hello packet, which includes its own ID, from another router that the router has been aware of, on the understanding that the two routers have become aware of each other, the two routers exchange network link-state information by sending routing protocol packets. The router creates a routing table based on the network link-state information collected by running the link-state routing algorithm, typically the Dijkstra algorithm. In OSPF, the routing table can specify the least-cost path, based on a cost determined by considering many factors including network link bandwidth, as the packet route. When a network link changes, each router calculates the shortest path for itself to each of the networks and sets its own routing table accordingly to the paths. A route calculation unit is used for creating a routing table. 
   Each router, while it transmits or receives control packets and network link-state information, manages the states of other routers on the network to which this router is connected and also manages the states of the interfaces through which this router is connected to networks. With regard to the states of routers, each router manages the routers&#39; ID&#39;s, and checks if each of those routers is aware of this router, or checks if each of those routers has completed the transmission and reception of network link-state information. With regard to interface state, each router manages the addresses of the interfaces and other routers connected to a network to which an interface is connected. 
   When conventional IP edge routers lose their primary circuitry and operation falls back to a redundant controller, a five to fifteen minute outage ensues while the router releases the routing states and packet forwarding tables. In order to enhance the reliability of the router device, it is important to multiplex the above-mentioned route calculation units. The multiplex router device includes a plurality of route calculation units, and always has one route calculation unit placed in the active mode to make it execute an ordinary process while keeping the remaining route calculation units in a standby mode. When the route calculation unit in the active mode runs into trouble, the multiplex router device brings one of the waiting route calculation units into the active mode (this is referred to as a system switchover of route calculation units), and the one other route calculation unit takes over and continues to execute the process that was previously being executed by the route calculation unit in trouble. 
   U.S. Pat. No. 6,049,524 describes a multiplex router device which reduces the amount of information to be transmitted from a route calculation unit in operation to a route calculation unit in standby mode. The route calculation unit in the active mode is connected by an internal bus to the route calculation unit in the standby mode. The route calculation unit in the active mode stores network link state information showing connections of the router and other routers with networks, neighboring router states showing states of neighboring routers and interface states showing states of network interfaces to connect the multiplex router device to the network. The route calculation unit in the active mode sends to the route calculation unit in the standby mode only the network link state information. In the route calculation unit in the standby mode, a database integration module that received the link-state information stores its contents in a link-state database. When a failure occurs in the route calculation unit in the active mode, the route calculation unit performs the routing protocol process by using the stored link-state database, so it is not necessary to exchange information with other routers to collect the network link state information over again. For awhile after the switchover to active mode the route calculation unit has no information about the neighbor route state and interface state. Hello packets are transmitted from the route calculation unit brought into the active state. The route calculation brought into the active state gradually accumulates information about the neighbor router states and interface states in order to gradually bring a complete list of ID&#39;s of other routers which is included in later Hello packets that the route calculation unit sends out. 
   It is desirable to provide high network availability by providing improved redundancy which can be implemented as a link level protocol running over IP having a backup link level process in total real time synchronization with an active one in order to enable an expeditious switchover when a failure occurs on the active control card. 
   SUMMARY OF THE INVENTION 
   The present invention relates to a method and system for implementing link level protocol redundancy in a router. In particular, the invention relates to providing redundancy of the Open Shortest Path First (OSPF) routing protocol. An active processor provides OSPF operations. In the present invention, a standby processor is coupled to the active processor. During an initial synchronization, all network link protocol information from the active processor is forwarded to the standby processor. The network link information can include OSPF state information, OSPF configuration information, OSPF adjacencies information, OSPF interface information and OSPF global protocol information. Thereafter, any updates of network link protocol information are immediately forwarded to the standby processor in an orderly and controlled manner. Upon failure of the active processor, the router is switched to the standby processor and all OSPF protocol operations are performed on the standby processor. In the present invention, all states of the link protocol immediately function as if a failure had not occurred. Neighbor routers will not notice any difference after switch-over, and no additional information is needed from neighbor routers after the switch-over. Accordingly, the router&#39;s forwarding capability will remain unaffected and a neighbor router will not notice that a system failure has occurred. 
   In an embodiment of the present invention, a hidden OSPF interface is determined at the active processor and the standby processor for each area of the router during the initial synchronization. The hidden interface is considered a point-to-point unnumbered interface which is not exposed to the outside world. A link-state database of the active processor is synchronized with the standby processor using the hidden OSPF interface. Link-protocol information is also forwarded from the active processor to the standby processor over the hidden OSPF interface. Upon synchronization of the standby processor with the active processor, the hidden OSPF interface for each area is removed. 
   In the present invention the active and standby OSPF processors stay in a highly synchronized state, referred to as a hot-standby state. Accordingly, an expeditious switchover to the standby processor occurs when the active processor fails. 
   The invention will be more fully described by reference to the following drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a system for implementing OSPF redundancy. 
       FIG. 2  is a schematic diagram of a redundancy software implementation. 
       FIG. 3  is a schematic diagram of an implementation of a hidden interface for each OSPF area. 
       FIG. 4  is a schematic diagram of states of an OSPF process running on the active OSPF control card. 
       FIG. 5  is a flow diagram of steps for transfer of network link state information from an active process to a standby process. 
   

   DETAILED DESCRIPTION 
   Reference will now be made in greater detail to a preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings. Wherever possible, the same reference numerals will be used throughout the drawings and the description to refer to the same or like parts. 
     FIG. 1  is a schematic diagram of a system for implementing link protocol redundancy in a router  10  in accordance with the teachings of the present invention. Router  11  includes active OSPF control card  12 . Active OSPF control card  12  performs OSPF operations. OSPF operations include mechanisms for building maintaining and verifying one or more adjacencies  14  to one or more neighbor routers  15 , exchanging network information with neighbors and updating best network routes to a local routing table. When a link-state database of two neighboring routers is synchronized, the routers are referred to as adjacent. Adjacencies control distribution of routing-protocol packets which are sent and received only at adjacencies. 
   Standby OSPF control card  18  is removably coupled to router  11 . In the absence of standby OSPF control card  18 , active OSPF control card  12  operates in a non-redundant mode. Active OSPF control card  12  communicates network link protocol information  15  over communication channel  16  to standby OSPF control card  18 . Preferably, communication channel  16  is a fast and reliable communication channel. For example, communication channel  16  can be a duplex Ethernet. Network link protocol information  15  can be forwarded in the form of Inter Process Control (IPC) messages. The same redundancy software for OSPF operations  19  runs on both active OSPF control card  12  and standby OSPF control card  18 . Redundancy software for OSPF operations  19  controls updating of network link protocol information  15  between active OSPF control card  12  and standby OSPF control card  18  and distinguishes between an active mode and a backup mode using system state information, as described in more detail below. 
   One embodiment of the present invention utilizes OSPF protocols running on the Amber Network ASR2000 router (or, alternatively, the ASR2020). The Amber Network ASR2000 and ASR2020 technical manuals are incorporated herein by reference as if fully set out. Active OSPF control card  12  and standby OSPF control card  18  are processors which are coupled to a line card and ASIC driver of router  11 . It will be appreciated that although system  10  is described in terms of the OSPF protocol the teachings of the present invention can be used with other conventional link protocols. 
   After standby OSPF control card  18  is coupled to router  11 , an initial synchronization is performed as a bulk update of network link information  15  from running active OSPF control card  12  to standby OSPF control card  18  using redundancy software for OSPF operations  19 . Network link information  15  can include configuration, state and learned information. 
   After the initial synchronization, ospf active and standby processes become fully redundant, an OSPF process running in the redundancy software for OSPF operations  19  operates in an incremental updating mode. Updates can be posted to active OSPF control card  12 . All updates are forwarded to standby OSPF control card  18 . Standby OSPF control card  18  receives all OSPF messages and updates in order to maintain total real time synchronization between active OSPF control card  12  and standby OSPF control card  18 . Accordingly, standby OSPF control card  18  mirrors active OSPF control card  12  for implementing redundancy. In this state, referred to as hot-standby, active OSPF control card  12  and standby OSPF control card  18  maintain a substantially synchronous state. Thereafter, if a failure of active OSPF control card  12  occurs, standby OSPF control card  18  will become active and be capable of immediately taking over all operations which were previously performed by active OSPF control card  12 . 
     FIG. 2  illustrates a detailed schematic diagram of redundancy software for OSPF operations  19  of active OSPF control card  12  and standby OSPF control card  18 . Redundant card manager (RCM)  20  is a task that maintains a synchronization state machine for each task. All tasks of redundancy software for OSPF operations  19  of active OSPF control card  12  interact with RCM  20  to send network link information  15  to standby OPF control card  18 . OSPF task  21  is a task for determining a status of OSPF processes running on active OSPF control card  12 . Software redundancy manager  22  is a module that interacts with RCM  20  for determining switching over from an active state in which active OSPF control card  12  performs OSPF operation to a standby state in which standby OSPF control card  18  takes over OSPF operations. 
   During an initial synchronization, redundant card manager (RCM)  20  on standby OSPF control card  18  contacts OSPF task  21  on active OSPF control card  12  for retrieving task information. OSPF task  21  on active OSPF control card  12  automatically processes OSPF messages and calculates routes stored in routing table manager (RTM)  34 . Active OSPF control card  12  marks corresponding internal states and transfers link-state database information  23 , OSPF state information  24  and OSPF configuration information  25 , OSPF adjacencies information  26 , OSPF interface information  27  and OSPF global protocol information  28  to backup OSPF control card  18  through RCM  20 . 
   During the initial synchronization, locks can be used with active OSPF processes running on active OSPF control card  12 . For example, on active OSPF control card  12 , a lock can be maintained on creating an OSPF adjacency such that a new OSPF adjacency is not established during the initial synchronization. 
   Hidden OSPF interface  30  is created on both active OSPF control card  12  and standby OSPF control card  18  for each area during initial synchronization. An area refers to a group of contiguous networks and attached hosts. Hidden OSPF interface  30  is a point-to-point unnumbered interface which is used with system  10  and is not exposed to the outside world. Hidden OSPF adjacency  32  is built automatically over hidden OSPF interface  30  due to OSPF neighbor discovery. Database  33  is synchronized through hidden OSPF adjacency  32 . Accordingly, there is one hidden OSPF adjacency  32  between active OSPF control card  12  and standby OSPF control card  18  for each area. Accordingly, hidden OSPF adjacencies  32  can be used to synchronize link state database information  23  stored in database  33 . 
     FIG. 3  illustrates an implementation of hidden OSPF interfaces. Router  11  has two interfaces, interface  14   a  belongs to area  0  connecting to router  15   a , and interface  14   b  belongs to area  2  connecting to Router  15   b . In router  11 , two hidden OSPF interfaces are created for area  0  and area  2 , hidden interface  30   a  is created for area  0 , and hidden interface  30   b  is created for area  2 . Hidden OSPF adjacency  32   a  runs over hidden OSPF interface  30   a , and hidden OSPF adjacency  32   b  runs over hidden OSPF interface  30   b . External link state advertisements (LSAs) are synchronized through hidden interface  30   a  for area  0  only. 
   Referring to  FIG. 2 , active OSPF control card  12  and standby OSPF control card  18  processes OSPF packets and calculates the shortest path first which decides the shortest path from a router to a destination network by considering cost. Active OSPF control card  12  can send OSPF packets to the line card for transmission to neighbor routers. Standby OSPF control card  18  does not send any OSPF packets to the line card for transmission to neighbor routers. Active OSPF control card  12  and standby OSPF control card  18  route updates to routing table manager (RTM)  34 , as shown in  FIG. 2 . RTM  34  of standby OSPF control card  18  can update redistribution routes to active OSPF control card  12 . IP interface manager  35  interfaces system  10  to the Internet Protocol (IP). Command Line Interface (CLI) commands are used to provide the OSPF configuration using datastore  36 . Datastore  36  is a task that is responsible for providing storage in memory  38 . For example, memory  38  can be a compact flash disc. Accordingly, all information obtained by standby OSPF control card  18  is directly obtained from either active OSPF control card  12 , IP interface manager  35  or datastore  36 . 
   An active state is associated with active OSPF control card  12 . A standby state is associated with standby OSPF control card  18 . A switchover from active OSPF control card  12  to standby OSPF control card  18  can clear upon failure of active OSPF control card  12 . When a switchover occurs, standby OSPF control card  18  changes its state to active and takes over all OSPF operations. Standby OSPF control card  19  resumes any suppressed OSPF actions and begins sending OSPF packets to the line card. 
     FIG. 4  is a schematic diagram of states of an active OSPF process  40  running on active OSPF control card  12 . OSPF_FAULT_INIT state  41  is an initial state of active OSPF process  40 . If system  10  is operating with only active OSPF control card  12  operating, system  10  remains in OSPF-FAULT_INIT state  41  awaiting initiation of a standby OSPF control card  18 . 
   Once standby OSPF control card  18  begins operating, OSPF_FAULT_VERIFY state  42  is entered in which standby OSPF control card  18  installs OSPF configuration information  25  received from data store  36  of active OSPF control card  12  which OSPF configuration has been activated on active OSPF control card  12 , as shown in  FIG. 2 . At this time the configuration on active OSPF control card  12  is disabled. OSPF configuration on standby OSPF control card  18  from data store  36  is synchronized and verified with information of active OSPF process  40 . Active OSPF process  40  verifies whether standby OSPF process  44  running on standby OSPF control card  18  has a totally synchronous configuration and system information from data store  36 . For example, active OSPF control card  12  can verify the interface number and parameters. If the verification fails, active OSPF process  40  can retry after a predetermined time interval, such as a few seconds. 
   After verification of the OSPF configuration, active OSPF processes  40  and standby OSPF process  44  enter OSPF_FAULT_SYNC state  45 . In OSPF_FAULT_SYNC state  45  neighbor information is transferred over communication link  16  between active OSPF control card  12  and standby OSPF control card  18 , as shown in block  50  of  FIG. 5 . Neighbor information can be transferred from active OSPF process  40  as an IPC message. A plurality of IPC messages can be used to send a large number of neighbors. Standby OSPF process  44  acknowledges the received IPC message and sends an acknowledged IPC message to active OSPF control card  12 , as shown in block  52 . 
   During forwarding of neighbor information, active OSPF control card  12  will not accept any new neighbors by ignoring Hello packets from unknown persons. Once all neighbor information has been transferred from active OSPF control card  12  to standby OSPF control card  18 , active OSPF control card  12  will forward an end message, as shown in block  53 . 
   Thereafter, standby OSPF process  44  downloads link-state database information from active OSPF control card  12 , in block  54 . Link-state database information can be synchronized with the use of the internal database synchronization mechanism provided by OSPF, as described in RFC  2328  hereby incorporated by reference into this application. The database synchronization uses a “Database Exchange Process” in which each router describes its database by sending a sequence of Database Description packets to its neighbor. The two routers enter a master/slave relationship. Each Database Description Packet describes a set of LSA&#39;s belonging to the router&#39;s database. When a neighbor sees an LSA that is more recent than its own database copy, it makes a note that the newer LSA should be requested. Each Database Description packet has a sequence number. Database Description packets (Polls) sent by the master are acknowledged by the slave by echoing the sequence number. Both Polls and responses contain summaries of link state data. The master is the only one allowed to retransmit Database Description Packets which can be done at fixed intervals. When the Database Description Process has completed, the databases are deemed synchronized and the routers are marked fully adjacent. At this time the adjacency is fully functional and is advertised in the two routers-LSA&#39;s. Hidden OSPF adjacency  32  is determined between active OSPF control card  12  and standby OSPF control card  18  for downloading the link-state database information  23 . Upon receipt of a database requirement message at active OSPF control card  12  from standby OSPF control card  18 , active OSPF control card  12  is aware that standby OSPF control card  18  is starting to download link-state database information  23 . Downloading of link-state database information continues until a synchronous link-state database exists in active OSPF control card  12  and standby OSPF control card  18 . 
   After standby OSPF control card  18  has a synchronous link-state database with active OSPF control card  12 , active OSPF control card  12  and standby OSPF control card  18  enter OSPF_FAULT_FULL state  46 . OSPF_FAULT_FULL state  46  is a hot standby state in which standby OSPF control card  18  can immediately take over all operations of active OSPF control card  12  upon failure. In OSPF_FAULT_FULL state  46 , hidden OSPF interfaces  30  and hidden adjacencies  32  are removed. Active OSPF process  40  incrementally updates any changes to standby OSPF process  44  by immediately sending updated OSPF state information  24 , OSPF configuration information  25 , OSPF adjacencies information  26 , OSPF interface information  27  and OSPF global protocol information  28  to standby OSPF control card  18  through RCM  20  using IPC messages. Any neighbor state or loss of a neighbor adjacency changes to active OSPF control card  12  are immediately transferred to standby OSPF control card  18  over communication link  18 . Any link-state database change is transferred to backup OSPF control card  18  with conventional OSPF synchronization mechanisms over communication link  15 . 
   Configuration changes in the active OSPF control card can be forwarded to backup OSPF control card  18  as an IPC message to trigger standby OSPF control card  18  to read updated information from data store  36 . Alternatively, a configuration command can be forwarded from CLI to backup OSPF control module  18 . 
   If a failure of active OSPF control card  12  occurs when standby OSPF control card  18  is in the OSPF_FAULT_FULL state, the standby OSPF control card  18  immediately takes over all OSPF operations. If a failure of active OSPF control card  12  occurs when standby OSPF control card  18  is in one of the states of OSPF_FAULT_INIT state  41 , OSPF_FAULT_VERIFY state  12  or OSPF_FAULT_SYNC state  45 , it indicates that the standby is not in a full redundant state, and the standby card will be reset. Because the system has not reached a redundant state, a failure of the active card will interrupt the service. 
   It is to be understood that the above-described embodiments are illustrative of only a few of the many possible specific embodiments which can represent applications of the principles of the invention. Numerous and varied other arrangements can be readily devised in accordance with these principles by those skilled in the art without departing from the spirit and scope of the invention.