Patent Publication Number: US-2015085860-A1

Title: Distributed connectivity verification protocol redundancy

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/853,006, filed Aug. 9, 2010, the content of which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to computer networks, and, more particularly, to connectivity verification protocols for use with virtual interfaces. 
     BACKGROUND 
     Connectivity verification protocols (CVPs), such as bidirectional forwarding and detection (BFD), may be used in computer networks in order to verify the connectivity between devices within the network. In particular, it is often desirable to execute CVPs over virtual interfaces (VIs), such as tunnels and/or pseudowires, which are not bound to a particular interface or line card of the network devices. That is, the interface and line card on which a particular VI is operating (e.g., as an egress or ingress) depends upon convergence of various routing protocols. Executing CVPs over VIs of a network device, however, presents numerous challenges, particularly with regard to preventing false alarms and managing operability and accountability of the line cards responsible for managing the CVPs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments herein may be better understood by referring to the following description in conjunction with the accompanying drawings in which like reference numerals indicate identically or functionally similar elements, of which: 
         FIG. 1  illustrates an example computer network; 
         FIG. 2  illustrates an example network device; 
         FIG. 3  illustrates an example signaling message/packet; 
         FIGS. 4A-G  illustrate example message/packet handling within a network device; and 
         FIG. 5  illustrates an example procedure for distributed connectivity verification protocol redundancy. 
     
    
    
     DESCRIPTION OF EXAMPLE EMBODIMENTS 
     Overview 
     According to one or more embodiments of the disclosure, a connectivity verification protocol (CVP) session for a particular virtual interface (VI) may be configured to operate on a particular group of two or more line cards (LCs) on a network device. Accordingly, the group of LCs may then transmit CVP session packets, at a reduced rate that is sufficient to maintain the CVP session based on a negotiated CVP full rate (e.g., less than a negotiated CVP full rate), onto the particular VI through ingress path processing on the network device. Ingress path processing, in particular, takes transmitted CVP session packets and egresses them onto an appropriate LC of the network device currently responsible for the VI egress. Also, in response to receiving CVP session packets for the VI on an LC of the network device currently responsible for the VI ingress, the receiving LC may forward the received CVP session packets to the particular corresponding group of LCs, which may then process the received CVP session packets. In this manner, the CVP sessions for a VI may be distributed and redundant. 
     DESCRIPTION 
     A computer network is a geographically distributed collection of nodes interconnected by communication links and segments for transporting data between end nodes, such as personal computers and workstations. Many types of networks are available, with the types ranging from local area networks (LANs) to wide area networks (WANs). LANs typically connect the nodes over dedicated private communications links located in the same general physical location, such as a building or campus. WANs, on the other hand, typically connect geographically dispersed nodes over long-distance communications links, such as common carrier telephone lines, optical lightpaths, synchronous optical networks (SONET), or synchronous digital hierarchy (SDH) links. The Internet is an example of a WAN that connects disparate networks throughout the world, providing global communication between nodes on various networks. The nodes typically communicate over the network by exchanging discrete frames or packets of data according to predefined protocols, such as the Transmission Control Protocol/Internet Protocol (TCP/IP). In this context, a protocol consists of a set of rules defining how the nodes interact with each other. Computer networks may be further interconnected by an intermediate network node, such as a router, to extend the effective “size” of each network. 
     Since management of interconnected computer networks can prove burdensome, smaller groups of computer networks may be maintained as routing domains or autonomous systems. The networks within an autonomous system (AS) are typically coupled together by conventional “intradomain” routers configured to execute intradomain routing protocols, and are generally subject to a common authority. To improve routing scalability, a service provider (e.g., an ISP) may divide an AS into multiple “areas” or “levels.” It may be desirable, however, to increase the number of nodes capable of exchanging data; in this case, interdomain routers executing interdomain routing protocols are used to interconnect nodes of the various ASes. Moreover, it may be desirable to interconnect various ASes that operate under different administrative domains. As used herein, an AS, area, or level is generally referred to as a “domain.” 
       FIG. 1  is a schematic block diagram of an example computer network  100  illustratively comprising nodes/devices, such as routers  200  interconnected by links  110  as shown. Illustratively, on top of the links  110  may be one or more virtual interfaces (VIs)  120 , such as tunnels, pseudowires, etc., as may be appreciated by those skilled in the art. Those skilled in the art will also understand that any number of nodes, devices, links, etc. may be used in the computer network, and that the view shown herein is for simplicity. Data packets  140  (e.g., traffic) may be exchanged among the nodes/devices of the computer network  100  using predefined network communication protocols such as the Transmission Control Protocol/Internet Protocol (TCP/IP), User Datagram Protocol (UDP), Asynchronous Transfer Mode (ATM) protocol, Frame Relay protocol, Internet Packet Exchange (IPX) protocol, Multi-Protocol Label Switching (MPLS), Generic Route Encapsulation (GRE), etc. 
       FIG. 2  is a schematic block diagram of an example node/device  200  that may be used with one or more embodiments described herein, e.g., as a router. The device comprises a plurality of line cards, one or more processors  220 , and a memory  240  interconnected by a system bus  250 . The line cards (LCs)  210  contain the mechanical, electrical, and signaling circuitry for communicating data over physical network interfaces  214  (links) coupled to the network  100 . The network interfaces may be configured to transmit and/or receive data using a variety of different communication protocols, including, inter alia, TCP/IP, UDP, ATM, synchronous optical networks (SONET), wireless protocols, Frame Relay, Ethernet, Fiber Distributed Data Interface (FDDI), etc. Notably, a physical network interface  214  may also be used to implement is one or more virtual network interfaces (VIs), such as tunnels (MPLS, GRE, etc.), pseudowires, etc., known to those skilled in the art. Also, as described herein, LCs  210  may also comprise one or more processes (executed by a processor), such as an illustrative “CVP process”  212 . 
     The memory  240  comprises a plurality of storage locations that are addressable by the processor(s)  220  for storing software programs and data structures associated with the embodiments described herein. The processor  220  may comprise necessary elements or logic adapted to execute the software programs and manipulate the data structures, such as routing tables, etc. An operating system  242  (e.g., the Internetworking Operating System, or IOS®, of Cisco Systems, Inc.), portions of which are typically resident in memory  240  and executed by the processor(s), functionally organizes the node by, inter alia, invoking network operations in support of software processes and/or services executing on the device. These software processes and/or services may comprise routing process/services  244 , a VI process  245 , an ingress path process  246 , and a connectivity verification protocol (CVP) process  248 , as described herein. It will be apparent to those skilled in the art that other types of processors and memory, including various computer-readable media, may be used to store and execute program instructions pertaining to the techniques described herein. Further, the one or more processes as described herein may alternatively be embodied as hardware, software, and/or firmware modules configured with the functionality of the corresponding process, accordingly. 
     Routing services  244  contain computer executable instructions executed by processor  220  to perform functions provided by one or more routing protocols, such as the Interior Gateway Protocol (IGP) (e.g., Open Shortest Path First, “OSPF,” and Intermediate-System-to-Intermediate-System, “IS-IS”), the Border Gateway Protocol (BGP), etc., as will be understood by those skilled in the art. These functions may be configured to manage a forwarding information database (not shown) containing, e.g., data used to make forwarding decisions. In particular, changes in the network topology may be communicated among routers  200  using routing protocols, such as the conventional OSPF and IS-IS link-state protocols (e.g., to “converge” to an identical view of the network topology). Notably, routing services  244  may also perform functions related to virtual routing protocols  245  for virtual interfaces (VIs)  120 , such as maintaining tunneling protocols, such as for Multi-Protocol Label Switching, etc., or pseudowire protocols, each as will be understood by those skilled in the art. In addition, as part of the virtual routing/interface process  245 , an illustrative ingress path process  246  (e.g., a component of process  244  and/or  245 ) may also be illustratively configured to properly forward packets to physical interfaces based on virtual routing convergence, as described herein and as may be appreciated by those skilled in the art. For instance, as shown in  FIG. 1 , a VI labeled VI-A may be routed out a first physical interface and line card, but then for routing protocol purposes, may be moved to another physical interface and/or line card, shown as VI-B. 
     Connectivity verification protocol (CVP) services or process  248  contain computer executable instructions executed by processor  220  to verify connectivity between two systems/devices. Illustratively, a connectivity verification protocol that may be used is the bidirectional forwarding and detection (BFD) protocol. CVP services  248  may illustratively verify connectivity between two systems/devices depending on the transmission of control packets/messages between the two devices. Assume, for example, that a first device (a monitoring node) wishes to verify its connectivity to a second device (a monitored node) using a CVP such as BFD. The first device may transmit a message to the second device, and may verify connectivity based on a response/non-response from the second device, e.g., within a particular time period. If the first device does not receive a response from the second device within the time period, the first device determines that the CVP session has failed or “timed out” (or is “down”), accordingly. These messages may be transmitted back and forth at a negotiated rate, such that an opposing device can be deemed unreachable in the event a message is not received from that device within the expected time frame (negotiated rate plus a given detection multiplier to allow for a certain number of missed messages). 
       FIG. 3  illustrates an example connectivity verification protocol message  300  that may be transmitted by capable devices  200  (e.g., LCs  210 ). Illustratively, the message  300  is a generic message modeled after BFD messages, and those skilled in the art will understand that other messages (e.g., Echo messages) may comprise other fields accordingly. The message  300  includes an encapsulation protocol header  310 , an optional CVP header field  315 , a discriminator value field  320  (e.g., a “My_discriminator”  322  and/or “Your_discriminator  324 ), and a field for other information  325 . As those skilled in the art will understand, CVP messages  300  are sent in an encapsulation appropriate to the environment (e.g., TCP/IP, MPLS, etc.). Thus, the encapsulation protocol header  310  contains information standard for the specific type of encapsulation. 
     The CVP header field  315  may comprise standard CVP (e.g., BFD) header information, such as, e.g., a CVP version number, a message length, certain flags, etc., or other information (more or less) as configured by the sending device. Because a sending device may have more than one CVP session established at a time (e.g., with the same receiving device, or other receiving devices), the discriminator fields  320  contain sufficient information to demultiplex the messages  300  to the correct CVP session once it has been received by the receiving device. An example discriminator may be an opaque value that identifies each CVP session, and which is unique among all CVP sessions at each device. For instance, a “My_discriminator” value  322  is unique at the sending device, while a “Your_discriminator”  324  value is unique at the receiving device. Also, the other information field  325  may be used according to the CVP protocol, such as, e.g., timer interval values, authentication, etc. Notably, CVPs may operate across any number of links and at any protocol layer, e.g., Physical, Data Link, Network, Virtual, etc., as will be understood by those skilled in the art. Conventionally, if a CVP message  300  (e.g., Echo message) is not returned to the first device, the session is declared to be down by the first device. When a failure is detected (of the link or a remote node/device), interested applications, such as routing protocols, etc., may take appropriate action, e.g., removing any reference to the adjacency from routing/forwarding tables, and route traffic around the point of failure. 
     As noted above, it is often desirable to execute CVPs over virtual interfaces (VIs), such as tunnels and/or pseudowires, which are not bound to a particular interface or line card of the network devices. That is, the interface and line card on which a particular VI is operating (e.g., as an egress or ingress) depends upon convergence of various routing protocols. Executing CVPs over VIs of a network device, however, presents numerous challenges, particularly with regard to preventing false alarms and managing operability and accountability of the line cards responsible for managing the CVPs. 
     For example, a single-hop CVP session may be used to detect connectivity on tunnel interfaces or pseudowire (PW) Head-End interfaces. That is, a PW Head-End (PW-HE) interface may be used to terminate PW traffic on a service router (e.g., provider edge or “PE” device) and provide services to the PW traffic that originated from a customer site, and CVP may be used to monitor reachability over that interface. Since VI traffic (e.g., tunnel and PW-HE traffic) may terminate on any line card on the service router, the line card on which the traffic would terminate would depend on the routing state (e.g., IGP reachability) of the tunnel or PW-HE destination. Accordingly, to operate a CVP session for a VI, a distributed CVP may be used which executes on any chosen line card  210 , rather than the current interface  214  itself. As such, the CVP Session packets  300  may be injected into ingress path processing  246  with an internal label/context, such that process  246  would switch the packets and make them egress out the current line card  210  where the tunnel/PW-HE interface is currently egressing. 
     While the above arrangement helps account for the ability of a virtual interface to change line cards (and interfaces), the CVP session is still tied to a single LC, which may fail independently of the VI. For instance, an LC may be “down” in response to LC online insertion and removal (OIR), LC hardware faults, CVP software upgrade on the LC, or CVP process crashes on LC. One option to provide redundancy is an “active/standby” redundancy with 2 LCs maintaining state about the CVP session. The active/standby solution, however, depends on the detection time of the local line card (or at least the CVP process) being down. If the CVP session interval is smaller than the detection time of the above events, false positives (alarms) may occur. In other words, latencies associated with routing protocols or central CVP processes operating on routers however, are not particularly well suited for use with high speed CVPs, such as BFD. For instance, to switch LCs in response to one going down requires detecting that the LC is down, switching to a backup (or altogether new) LC, and then sending CVP packets from that activated LC, thus resulting in many opportunities for extended and detrimental delay. The probability of false positives increases with the number of CVP sessions hosted on the downed line card, as well, since it takes more time to process all of the displaced sessions. 
     Distributed CVP Redundancy 
     According to one or more embodiments of the disclosure, due to the pitfalls of the active/standby solution, an “active/active” redundancy mechanism is described herein for the CVP session. Specifically, each CVP session operates on at least two line cards (LCs), where each LC maintains state about the CVP session, one LC assuming an active, primary role, the other LC(s) assuming a passive, backup role. As described herein, each LC actively sends CVP packets once the session is up, though at a reduced rate (e.g., a half rate) sufficient to maintain the CVP session. Illustratively, the techniques described herein may be performed by hardware, software, and/or firmware, such as in accordance with central CVP process  248  (or “CVP central”), which may contain computer executable instructions executed by the processor  220  to perform functions relating to the novel techniques described herein and in conjunction with LC CVP processes  212 , which also contain computer executable instructions executed by a processor to perform functions relating to the novel techniques described herein. 
     Operationally, a plurality of LCs  210  on a router  200  may be divided into groups of at least two LCs, where in each group one LC is designated as a primary and the other LC as a backup. For example, with reference to  FIG. 4A , which illustrates an example alternative view  400  of a router  200 , showing LCs  1 - 4 , a CVP central process  248 , and an ingress path process  246 . Accordingly, CVP central may determine LC groups, and may assign, to each group, a subset of VIs and corresponding CVP sessions for which each group is responsible. In other words, a CVP session for a particular VI may be configured for a particular group of LCs. (Note that not all LCs need be assigned to a group, and in an alternative embodiment, an LC may be assigned to more than one group.) For instance, CVP central may inform the LCs of each group that they are to manage a CVP single-hop session for any VI having a destination “X” (or with an inLabel=“L” for MPLS). Illustratively, assume that a group responsible for the VI  120  shown comprises LC- 3  as a primary LC and LC- 4  as a backup LC. 
     In addition, hardware of the line cards may be programmed to direct all CVP packets  300  to a particular group of two or more LCs based on the VI ingress of the packet  300 . In particular, when an LC receives a CVP packet with a your_discriminator value  324  corresponding to a certain group ID (e.g., GroupX/ 16 ), then that receiving LC may be programmed to quickly multicast the packets  300  to the two corresponding LCs. Also, as described below, when one LC in the group goes down, CVP central can assign another LC to the group, and re-program the hardware to direct GroupX/ 16  to this new LC along with the old LC. 
     Generally, only a primary LC would be responsible for CVP session establishment and tear down. For example, in a conventional manner, a primary LC (e.g., LC- 3 ) may establish a CVP session over the VI by sending out a CVP packet  300  containing a my_discriminator value of its corresponding group ID (such that replies will be forwarded to its group of LCs), and a your_discriminator value of some initializing value, e.g., “0”. The response CVP packet would then include a my_discriminator (of the opposing device) to replace the initializing value in a subsequent your_discriminator field. 
     Note that in the reverse, any packets received with a your_discriminator value of “0” (or other initializing value) would be multicasted to all LCs of the router hosting CVP sessions, as shown in  FIG. 4B . The primary LC responsible for the CVP session corresponding to the VI on which the CVP packet was received (e.g., based on a VI ID and knowing which VIs the LC is responsible for, as noted above) may then respond to the packet, accordingly. That is, the primary LC may return a packet with an appropriate my_discriminator value to replace the initializing value, such that a receiving LC in the future will know which group is to receive the packet. 
     Once CVP central initiates a CVP session on a primary LC, then the primary and backup LC of the corresponding group would execute at a reduced rate from a highest timer value (either received in a CVP packet or given by the CVP central process). That is, both LCs of a group may transmit CVP session packets  300  onto the particular VI at a reduced rate that is less than a negotiated CVP full rate, e.g., half (since there are two LCs). As shown in  FIG. 4C , the two LCs LC- 3  and LC- 4  each transmit a CVP packet at half rate (single line), which results in a full rate (double line) entering the VI, currently at LC- 1 . 
     For example, assuming the CVP is BFD, a corresponding hold-down timer may be configured as, e.g., 100 ms. Since, according to BFD, the packets  300  may not be sent at a rate higher than the negotiated rate, if there are two LCs in a particular group, then one condition to be met by the reduced rate is that the individual rate of each LC should be sufficient to send at most one packet every 100 ms. That is, the reduced rate of the primary and backup LCs could be one packet every 200 ms. 
     As another example, for other CVPs less restricted by higher rates, that is, the packets  300  may be sent at a rate sufficient to send at least one packet during the hold-down timer. As such, if a normal (“full”) rate of a primary line card is established as one packet per every 50 ms, then a reduced rate may be, e.g., one packet every 75 ms in order to ensure that at least one packet is received within the hold-down period. In addition, the respective rates could also be different between the LCs, such as transmitting a packet by the primary LC every 75 ms and a packet by the secondary LC every 100 ms. 
     Notably, a control mechanism may be implemented to prevent ill-spaced bursts of packets, since the two LCs of a group typically act independently of one another. That is, assuming that a packet is to be sent at a full rate equal to once every 100 ms, half rate would be once every 200 ms. Without controlling the two LCs, both may transmit the packet every 200 ms (or at 20 ms and 180 ms, etc.), resulting in a burst or otherwise offset transmission of packets. Under certain conditions, this may result in false positives (false conclusions of failed CVP sessions) due to the sub-optimal spacing. Accordingly, measures may be implemented such as synchronizing clocks on the LCs and spacing the reduced rate packets appropriately (e.g., randomly or substantially perfectly alternating, i.e., near the negotiated rate). Alternatively or in addition, a CVP detection multiplier may be negotiated for the session that allows for a certain number of missed CVP packets (e.g., 2-4) prior to declaring a session down, thus accounting for the independent reduced rate transmission of CVP. 
     During an established session, a current ingress LC for a VI may receive CVP session packets for the VI, and may forward the received CVP session packets to the particular group of two or more LCs, accordingly. For instance, as described above, this may be based on the group ID within a your_discriminator value of the CVP packet. Both LCs of the group may then receive the packets, and at least one (e.g., the primary) may process the received CVP session packets to determine whether the state of the opposing device (peer) is up or down. Generally, the primary LC is responsible for informing a dataplane and CVP central process  248  of a state of the CVP session.  FIG. 4D  illustrates receipt of the CVP packets at near-full rate, and the forwarding of the packets via the ingress path process  246  to the two corresponding LCs for that VI. 
     Note that as mentioned above, through ingress path processing on the network device, the network device egresses outgoing transmitted CVP session packets onto an appropriate LC that is currently responsible for the VI egress. Since this is based on routing protocols, in the event the VI egress line card changes, such as from LC- 1  to LC- 2  as shown in  FIG. 4E , then ingress path processing simply redirects CVP packets from the responsible group&#39;s LCs to the appropriate egress LC. 
     In response detecting that one of the LCs within a particular CVP session group goes down, e.g., from OIR, failure, etc., the remaining LC of the group may be directed to begin transmitting CVP session packets the CVP full rate (or other assigned rate), as shown in  FIG. 4F . Notably, in the time it takes to direct this remaining CVP to transmit at the full rate, the reduced rate packets should be sufficient to maintain the CVP session. If it was the primary LC that went down, then the backup LC (once informed from CVP central) may be reassigned as the primary to assume control of the state maintenance. A new LC of the network device may later be assigned to the particular group, e.g., LC- 2  as shown in  FIG. 4G , at which time the remaining LC and new LC of the group may be directed to resume CVP packet transmission at the reduced rate. Notably, when another LC is assigned to the group, CVP central can re-program the hardware of the LCs  210  to direct received (ingress) CVP packets for this VI (based on the discriminator values) to this new LC along with the remaining LC. 
     Note also, that in order to preserve the local CVP session&#39;s state upon a primary LC failure, the central CVP process  248  may keep the last known good state received from the primary LC for at least T seconds, T being large enough for the backup LC to take over and confirm the state of the CVP session. Specifically, while the backup LC need not send the state information to CVP central regularly, the backup LC may still queue any BFD state transition for T seconds, such that if it becomes active before the T delay, the queued state may then be sent to CVP central. Generally, state information from a backup LC (not yet a primary) may be discarded as it is the role of the active primary LC to do so, but it may be beneficial to configure the central CVP process to handle overlapping notifications for the same CVP session. 
       FIG. 5  illustrates an example simplified procedure for providing distributed CVP redundancy in accordance with one or more embodiments described herein. The procedure  500  starts at step  505 , and continues to step  510 , where virtual interfaces  120  and any corresponding configured CVP sessions are assigned to groups of two (or more) LCs  210 , one primary LC and one backup LC, as described above. Also, in step  515 , hardware of the LCs may be programmed to direct (e.g., multicast) CVP packets  300  to the corresponding LCs depending on the associated CVP session of the packets (e.g., via a discriminator value  324 ). 
     In step  520 , the group of LCs assigned to a particular CVP session may transmit CVP session packets  300  onto corresponding VIs  120  through ingress path processing  246  at the reduced rate (e.g., half). Optionally, as described above, the transmission timing may proceed with certain control measures, such as synchronized clocks, detect multipliers, etc. In step  525 , any LC currently responsible for a VI ingress may receive a CVP session packet, and in step  530  checks the discriminator (group ID) value  324  to determine where to forward the packet. When there is an ID present, then in step  535  the received CVP session packet is forwarded (e.g., multicasted) to the corresponding group of LCs as directed in step  515 . If, on the other hand, there is no ID present (e.g., a value of “ 0 ”), then in step  540  the packet may be forwarded to all LCs hosting CVP sessions to allow an appropriate LC to respond. Accordingly, in step  545 , at least one LC (e.g., primary) within the corresponding group, based on the group ID or VI ID, may receive and process the CVP session packet. 
     The transmission and receipt of CVP packets may continue in this manner, and at some point in step  550  an LC within a particular group may go down (e.g., intentionally or unintentionally). If so, then in step  555  the remaining LC of the group may be directed to transmit CVP session packets at the full rate, and may, in step  560 , be reassigned as a new primary LC for the group if the downed LC was the primary. Optionally, a new LC may be assigned to the group in step  565 , at which time the remaining LC and newly assigned LC may transmit the CVP session packets at the reduced rate once again. The procedure  500  continues to transmit and receive packets, and react to downed LCs, until the CVP sessions are torn down or otherwise ceased. Note that while a particular order of steps is shown and described, the order is merely one representative example. For instance, step  520  may fall after  525 - 545 , while steps  555 - 565  may occur in any order. 
     The novel techniques described herein provide distributed CVP redundancy, particularly for virtual interfaces, in a computer network. By having two LCs maintaining state for a given CVP session, both transmitting (e.g., at a reduced rate) and receiving all CVP packets for a given session, the novel techniques alleviate the latency associated with transitioning LCs, and reduce the number of false CVP alarms. In particular, the “Active/Active” techniques described above offer improved performance through smaller detection and reaction times (e.g., compared to an Active/Standby model) by having both LCs transmitting packets as soon as the CVP session is UP, thus relaxing the critical period where the other LC has to take over the full-rate transmission and the ownership of the CVP state machine before the session flaps. The solutions herein are also robust, being independent of detection time for LC OIR, hardware/ASIC errors (e.g., LC reload), CVP software upgrade on an LC, CVP process crashing on an LC, etc. Moreover, the dynamic aspects of one or more embodiments described herein (e.g., CVP central assignment of LCs to CVP sessions) alleviate the need for cumbersome and inefficient manual configuration. 
     While there have been shown and described illustrative embodiments that provide distributed CVP redundancy in a computer network, it is to be understood that various other adaptations and modifications may be made within the spirit and scope of the embodiments herein. For example, the embodiments have been shown and described herein using BFD as a primary example CVP. However, the embodiments in their broader sense are not so limited, and may, in fact, be used with other suitable CVPs. Also, while the illustrative example relies on a central CVP process in communication with a plurality of line cards, the embodiments herein may also be applied to other physical arrangements of network devices, such as those without physically removable line cards, or those that simply classify their egresses and ingresses as network interfaces (e.g., rather than one LC controlling multiple interfaces, each interface may act independently of one another). 
     The foregoing description has been directed to specific embodiments. It will be apparent, however, that other variations and modifications may be made to the described embodiments, with the attainment of some or all of their advantages. For instance, it is expressly contemplated that the components and/or elements described herein can be implemented as software being stored on a tangible computer-readable medium (e.g., disks/CDs/etc.) having program instructions executing on a computer, hardware, firmware, or a combination thereof. Accordingly this description is to be taken only by way of example and not to otherwise limit the scope of the embodiments herein. Therefore, it is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the embodiments herein.