Patent Publication Number: US-9848040-B2

Title: Name services for virtual cluster switching

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 13/092,752, entitled “Name Services for Virtual Cluster Switching,” by inventors Suresh Vobbilisetty, Phanidhar Koganti, and Jesse B. Willeke, filed 22 Apr. 2011, which claims the benefit of U.S. Provisional Application No. 61/352,264, entitled “Name Services for Virtual Cluster Switching,” by inventors Suresh Vobbilisetty, Phanidhar Koganti, and Jesse B. Willeke, filed 7 Jun. 2010, and U.S. Provisional Application No. 61/380,803, entitled “Name Services for Virtual Cluster Switching,” by inventors Suresh Vobbilisetty, Phanidhar Koganti, and Jesse B. Willeke, filed 8 Sep. 2010, the disclosures of which are incorporated by reference herein. 
     The present disclosure is related to U.S. patent application Ser. No. 12/725,249, entitled “REDUNDANT HOST CONNECTION IN A ROUTED NETWORK,” by inventors Somesh Gupta, Anoop Ghanwani, Phanidhar Koganti, and Shunjia Yu, filed 16 Mar. 2010; and 
     U.S. patent application Ser. No. 13/087,239, entitled “VIRTUAL CLUSTER SWITCHING,” by inventors Suresh Vobbilisetty and Dilip Chatwani, filed 14 Apr. 2011; 
     the disclosures of which are incorporated by reference herein. 
    
    
     BACKGROUND 
     Field 
     The present disclosure relates to network design. More specifically, the present disclosure relates to a method for a constructing a scalable switching system that facilitates automatic configuration. 
     Related Art 
     The relentless growth of the Internet has brought with it an insatiable demand for bandwidth. As a result, equipment vendors race to build larger, faster, and more versatile switches to move traffic. However, the size of a switch cannot grow infinitely. It is limited by physical space, power consumption, and design complexity, to name a few factors. More importantly, because an overly large system often does not provide economy of scale due to its complexity, simply increasing the size and throughput of a switch may prove economically unviable due to the increased per-port cost. 
     One way to increase the throughput of a switch system is to use switch stacking. In switch stacking, multiple smaller-scale, identical switches are interconnected in a special pattern to form a larger logical switch. However, switch stacking requires careful configuration of the ports and inter-switch links. The amount of required manual configuration becomes prohibitively complex and tedious when the stack reaches a certain size, which precludes switch stacking from being a practical option in building a large-scale switching system. Furthermore, a system based on stacked switches often has topology limitations which restrict the scalability of the system due to fabric bandwidth considerations. 
     In addition, the evolution of virtual computing has placed additional requirements on the network. For example, as the locations of virtual servers become more mobile and dynamic, it is often important for the network to update its knowledge of the location of these virtual servers quickly. 
     SUMMARY 
     One embodiment of the present invention provides a switch that facilitates name services in a virtual cluster switch. The switch includes a name service database indicating at least one media access control (MAC) address learned at a second switch. The switch also includes a control mechanism. During operation, the control mechanism distributes information on a locally learned MAC address to the second switch. In addition, the control mechanism receives information on a MAC address learned at the second switch. 
     In a variation on this embodiment, the switch and the second switch are members of a virtual cluster switch comprising one or more physical switches which are allowed to be coupled in an arbitrary topology. Furthermore, the virtual cluster switch appears to be one single switch. 
     In a variation on this embodiment, while distributing information to the second switch, the control mechanism constructs a Fibre Channel registered state change notification (RSCN) encapsulated in a transparent interconnection of lots of links (TRILL) header. 
     In a variation on this embodiment, the distributed information to the second switch includes the MAC address and an identifier of the switch. 
     In a further variation, the distributed information further includes an identifier of a port to which a host corresponding to the MAC address is coupled and a virtual local area network (VLAN) tag associated with the MAC address. 
     In a variation on this embodiment, the control mechanism further sends an update to second switch when a link or port within a multi-chassis trunk fails. 
     In a further variation, the update indicates that an end host previously connected via a multi-chassis trunk is now connected with a physical switch. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1A  illustrates an exemplary virtual cluster switch (VCS) system, in accordance with an embodiment of the present invention. 
         FIG. 1B  illustrates an exemplary VCS system where the member switches are configured in a CLOS network, in accordance with an embodiment of the present invention. 
         FIG. 2  illustrates the protocol stack within a virtual cluster switch, in accordance with an embodiment of the present invention. 
         FIG. 3  illustrates an exemplary configuration of a virtual cluster switch, in accordance with an embodiment of the present invention. 
         FIG. 4  illustrates an exemplary configuration of how a virtual cluster switch can be connected to different edge networks, in accordance with an embodiment of the present invention. 
         FIG. 5A  illustrates how a logical Fibre Channel switch fabric is formed in a virtual cluster switch in conjunction with the example in  FIG. 4 , in accordance with an embodiment of the present invention. 
         FIG. 5B  illustrates an example of how a logical FC switch can be created within a physical Ethernet switch, in accordance with one embodiment of the present invention. 
         FIG. 6  illustrates an exemplary VCS configuration database, in accordance with an embodiment of the present invention. 
         FIG. 7  illustrates an exemplary process of a switch joining a virtual cluster switch, in accordance with an embodiment of the present invention. 
         FIG. 8  presents a flowchart illustrating the process of looking up an ingress frame&#39;s destination MAC address and forwarding the frame in a VCS, in accordance with one embodiment of the present invention. 
         FIG. 9  illustrates how data frames and control frames are transported through a VCS, in accordance with one embodiment of the present invention. 
         FIG. 10  illustrates an example of name service operation in a VCS, in accordance with one embodiment of the present invention. 
         FIG. 11  presents a flowchart illustrating the process of distributing learned MAC information by the Ethernet name service in a VCS, in accordance with one embodiment of the present invention. 
         FIG. 12  presents a flowchart illustrating the process of distributing information of a learned MAC address via an MCT, in accordance with one embodiment of the present invention. 
         FIG. 13  presents a flowchart illustrating the process of updating the link state in an MCT group, in accordance with one embodiment of the present invention. 
         FIG. 14  illustrates an exemplary switch that facilitates formation of a virtual cluster switch with Ethernet and MCT name services, in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not limited to the embodiments shown, but is to be accorded the widest scope consistent with the claims. 
     Overview 
     In embodiments of the present invention, the problem of facilitating fast distribution of the location information of each end host is solved by providing a distributed name service throughout a virtual cluster switch. The virtual cluster switch allows a number of switches to be inter-connected to form a single, scalable logical switch without requiring burdensome manual configuration. As a result, one can form a large-scale logical switch (referred to as a “virtual cluster switch” or VCS herein) using a number of smaller physical switches. The automatic configuration capability provided by the control plane running on each physical switch allows any number of switches to be connected in an arbitrary topology without requiring tedious manual configuration of the ports and links. This feature makes it possible to use many smaller, inexpensive switches to construct a large cluster switch, which can be viewed as a single logical switch externally. 
     In a VCS, each member switch performs source media access control (MAC) address learning. Once a new MAC address is observed and learned from a port, the corresponding MAC address and switch/port identifier information is distributed throughout the VCS. In this way, each VCS member switch can maintain a complete set of knowledge of the location of all the end-hosts (including virtual machines). This knowledge allows a frame destined to any MAC address to be properly routed to the correct switch, even if the MAC address is not directly learned by the local switch. (Note that, in a conventional Ethernet switch, an frame with an unknown destination MAC address is flooded too all the ports.) In this disclosure, the description in conjunction with  FIGS. 1-9  is associated with the general architecture of VCS, and the description in conjunction with  FIG. 10  and onward provide more details on the advanced link tracking mechanism. 
     It should be noted that a virtual cluster switch is not the same as conventional switch stacking. In switch stacking, multiple switches are interconnected at a common location (often within the same rack), based on a particular topology, and manually configured in a particular way. These stacked switches typically share a common address, e.g., IP address, so they can be addressed as a single switch externally. Furthermore, switch stacking requires a significant amount of manual configuration of the ports and inter-switch links. The need for manual configuration prohibits switch stacking from being a viable option in building a large-scale switching system. The topology restriction imposed by switch stacking also limits the number of switches that can be stacked. This is because it is very difficult, if not impossible, to design a stack topology that allows the overall switch bandwidth to scale adequately with the number of switch units. 
     In contrast, a VCS can include an arbitrary number of switches with individual addresses, can be based on an arbitrary topology, and does not require extensive manual configuration. The switches can reside in the same location, or be distributed over different locations. These features overcome the inherent limitations of switch stacking and make it possible to build a large “switch farm” which can be treated as a single, logical switch. Due to the automatic configuration capabilities of the VCS, an individual physical switch can dynamically join or leave the VCS without disrupting services to the rest of the network. 
     Furthermore, the automatic and dynamic configurability of VCS allows a network operator to build its switching system in a distributed and “pay-as-you-grow” fashion without sacrificing scalability. The VCS&#39;s ability to respond to changing network conditions makes it an ideal solution in a virtual computing environment, where network loads often change with time. 
     Although this disclosure is presented using examples based on the Transparent Interconnection of Lots of Links (TRILL) as the transport protocol and the Fibre Channel (FC) fabric protocol as the control-plane protocol, embodiments of the present invention are not limited to TRILL networks, or networks defined in a particular Open System Interconnection Reference Model (OSI reference model) layer. For example, a VCS can also be implemented with switches running multi-protocol label switching (MPLS) protocols for the transport. In addition, the terms “RBridge” and “switch” are used interchangeably in this disclosure. The use of the term “RBridge” does not limit embodiments of the present invention to TRILL networks only. The TRILL protocol is described in IETF draft “RBridges: Base Protocol Specification,” available at http://tools.ietf.org/html/draft-ietf-trill-rbridge-protocol, which is incorporated by reference herein 
     The terms “virtual cluster switch,” “virtual cluster switching,” and “VCS” refer to a group of interconnected physical switches operating as a single logical switch. The control plane for these physical switches provides the ability to automatically configure a given physical switch, so that when it joins the VCS, little or no manual configuration is required. 
     The term “RBridge” refers to routing bridges, which are bridges implementing the TRILL protocol as described in IETF draft “RBridges: Base Protocol Specification.” Embodiments of the present invention are not limited to the application among RBridges. Other types of switches, routers, and forwarders can also be used. 
     The terms “frame” or “packet” refer to a group of bits that can be transported together across a network. “Frame” should not be interpreted as limiting embodiments of the present invention to layer-2 networks. “Packet” should not be interpreted as limiting embodiments of the present invention to layer-3 networks. “Frame” or “packet” can be replaced by other terminologies referring to a group of bits, such as “cell” or “datagram.” 
     VCS Architecture 
       FIG. 1A  illustrates an exemplary virtual cluster switch system, in accordance with an embodiment of the present invention. In this example, a VCS  100  includes physical switches  101 ,  102 ,  103 ,  104 ,  105 ,  106 , and  107 . A given physical switch runs an Ethernet-based transport protocol on its ports (e.g., TRILL on its inter-switch ports, and Ethernet transport on its external ports), while its control plane runs an FC switch fabric protocol stack. The TRILL protocol facilitates transport of Ethernet frames within and across VCS  100  in a routed fashion (since TRILL provides routing functions to Ethernet frames). The FC switch fabric protocol stack facilitates the automatic configuration of individual physical switches, in a way similar to how a conventional FC switch fabric is formed and automatically configured. In one embodiment, VCS  100  can appear externally as an ultra-high-capacity Ethernet switch. More details on FC network architecture, protocols, naming/address conventions, and various standards are available in the documentation available from the NCITS/ANSI T11 committee (www.t11.org) and publicly available literature, such as “Designing Storage Area Networks,” by Tom Clark, 2nd Ed., Addison Wesley, 2003, the disclosures of which are incorporated by reference in their entirety herein. 
     A physical switch may dedicate a number of ports for external use (i.e., to be coupled to end hosts or other switches external to the VCS) and other ports for inter-switch connection. Viewed externally, VCS  100  appears to be one switch to a device from the outside, and any port from any of the physical switches is considered one port on the VCS. For example, port groups  110  and  112  are both VCS external ports and can be treated equally as if they were ports on a common physical switch, although switches  105  and  107  may reside in two different locations. 
     The physical switches can reside at a common location, such as a data center or central office, or be distributed in different locations. Hence, it is possible to construct a large-scale centralized switching system using many smaller, inexpensive switches housed in one or more chassis at the same location. It is also possible to have the physical switches placed at different locations, thus creating a logical switch that can be accessed from multiple locations. The topology used to interconnect the physical switches can also be versatile. VCS  100  is based on a mesh topology. In further embodiments, a VCS can be based on a ring, fat tree, or other types of topologies. 
     In one embodiment, the protocol architecture of a VCS is based on elements from the standard IEEE 802.1Q Ethernet bridge, which is emulated over a transport based on the Fibre Channel Framing and Signaling-2 (FC-FS-2) standard. The resulting switch is capable of transparently switching frames from an ingress Ethernet port from one of the edge switches to an egress Ethernet port on a different edge switch through the VCS. 
     Because of its automatic configuration capability, a VCS can be dynamically expanded as the network demand increases. In addition, one can build a large-scale switch using many smaller physical switches without the burden of manual configuration. For example, it is possible to build a high-throughput fully non-blocking switch using a number of smaller switches. This ability to use small switches to build a large non-blocking switch significantly reduces the cost associated switch complexity.  FIG. 1B  presents an exemplary VCS with its member switches connected in a CLOS network, in accordance with one embodiment of the present invention. In this example, a VCS  120  forms a fully non-blocking 8×8 switch, using eight 4×4 switches and four 2×2 switches connected in a three-stage CLOS network. A large-scale switch with a higher port count can be built in a similar way. 
       FIG. 2  illustrates the protocol stack within a virtual cluster switch, in accordance with an embodiment of the present invention. In this example, two physical switches  202  and  204  are illustrated within a VCS  200 . Switch  202  includes an ingress Ethernet port  206  and an inter-switch port  208 . Switch  204  includes an egress Ethernet port  212  and an inter-switch port  210 . Ingress Ethernet port  206  receives Ethernet frames from an external device. The Ethernet header is processed by a media access control (MAC) layer protocol. On top of the MAC layer is a MAC client layer, which hands off the information extracted from the frame&#39;s Ethernet header to a forwarding database (FDB)  214 . Typically, in a conventional IEEE 802.1Q Ethernet switch, FDB  214  is maintained locally in a switch, which would perform a lookup based on the destination MAC address and the VLAN indicated in the Ethernet frame. The lookup result would provide the corresponding output port. However, since VCS  200  is not one single physical switch, FDB  214  would return the egress switch&#39;s identifier (i.e., switch  204 &#39;s identifier). In one embodiment, FDB  214  is a data structure replicated and distributed among all the physical switches. That is, every physical switch maintains its own copy of FDB  214 . When a given physical switch learns the source MAC address and VLAN of an Ethernet frame (similar to what a conventional IEEE 802.1Q Ethernet switch does) as being reachable via the ingress port, the learned MAC and VLAN information, together with the ingress Ethernet port and switch information, is propagated to all the physical switches so every physical switch&#39;s copy of FDB  214  can remain synchronized. This prevents forwarding based on stale or incorrect information when there are changes to the connectivity of end stations or edge networks to the VCS. 
     The forwarding of the Ethernet frame between ingress switch  202  and egress switch  204  is performed via inter-switch ports  208  and  210 . The frame transported between the two inter-switch ports is encapsulated in an outer MAC header and a TRILL header, in accordance with the TRILL standard. The protocol stack associated with a given inter-switch port includes the following (from bottom up): MAC layer, TRILL layer, FC-FS-2 layer, FC E-Port layer, and FC link services (FC-LS) layer. The FC-LS layer is responsible for maintaining the connectivity information of a physical switch&#39;s neighbor, and populating an FC routing information base (RIB)  222 . This operation is similar to what is done in an FC switch fabric. The FC-LS protocol is also responsible for handling joining and departure of a physical switch in VCS  200 . The operation of the FC-LS layer is specified in the FC-LS standard, which is available at http://www.t11.org/ftp/t11/member/fc/ls/06-393v5.pdf, the disclosure of which is incorporated herein in its entirety. 
     During operation, when FDB  214  returns the egress switch  204  corresponding to the destination MAC address of the ingress Ethernet frame, the destination egress switch&#39;s identifier is passed to a path selector  218 . Path selector  218  performs a fabric shortest-path first (FSPF)-based route lookup in conjunction with RIB  222 , and identifies the next-hop switch within VCS  200 . In other words, the routing is performed by the FC portion of the protocol stack, similar to what is done in an FC switch fabric. 
     Also included in each physical switch are an address manager  216  and a fabric controller  220 . Address manager  216  is responsible for configuring the address of a physical switch when the switch first joins the VCS. For example, when switch  202  first joins VCS  200 , address manager  216  can negotiate a new FC switch domain ID, which is subsequently used to identify the switch within VCS  200 . Fabric controller  220  is responsible for managing and configuring the logical FC switch fabric formed on the control plane of VCS  200 . 
     One way to understand the protocol architecture of VCS is to view the VCS as an FC switch fabric with an Ethernet/TRILL transport. Each physical switch, from an external point of view, appears to be a TRILL RBridge. However, the switch&#39;s control plane implements the FC switch fabric software. In other words, embodiments of the present invention facilitate the construction of an “Ethernet switch fabric” running on FC control software. This unique combination provides the VCS with automatic configuration capability and allows it to provide the ubiquitous Ethernet services in a very scalable fashion. 
       FIG. 3  illustrates an exemplary configuration of a virtual cluster switch, in accordance with an embodiment of the present invention. In this example, a VCS  300  includes four physical switches  302 ,  304 ,  306 , and  308 . VCS  300  constitutes an access layer which is coupled to two aggregation switches  310  and  312 . Note that the physical switches within VCS  300  are connected in a ring topology. Aggregation switch  310  or  312  can connect to any of the physical switches within VCS  300 . For example, aggregation switch  310  is coupled to physical switches  302  and  308 . These two links are viewed as a trunked link to VCS  300 , since the corresponding ports on switches  302  and  308  are considered to be from the same logical switch, VCS  300 . Note that, without VCS, such topology would not have been possible, because the FDB needs to remain synchronized, which is facilitated by the VCS. 
       FIG. 4  illustrates an exemplary configuration of how a virtual cluster switch can be connected to different edge networks, in accordance with an embodiment of the present invention. In this example, a VCS  400  includes a number of TRILL RBridges  402 ,  404 ,  406 ,  408 , and  410 , which are controlled by the FC switch-fabric control plane. Also included in VCS  400  are RBridges  412 ,  414 , and  416 . Each RBridge has a number of edge ports which can be connected to external edge networks. 
     For example, RBridge  412  is coupled with hosts  420  and  422  via 10GE ports. RBridge  414  is coupled to a host  426  via a 10GE port. These RBridges have TRILL-based inter-switch ports for connection with other TRILL RBridges in VCS  400 . Similarly, RBridge  416  is coupled to host  428  and an external Ethernet switch  430 , which is coupled to an external network that includes a host  424 . In addition, network equipment can also be coupled directly to any of the physical switches in VCS  400 . As illustrated here, TRILL RBridge  408  is coupled to a data storage  417 , and TRILL RBridge  410  is coupled to a data storage  418 . 
     Although the physical switches within VCS  400  are labeled as “TRILL RBridges,” they are different from the conventional TRILL RBridge in the sense that they are controlled by the FC switch fabric control plane. In other words, the assignment of switch addresses, link discovery and maintenance, topology convergence, routing, and forwarding can be handled by the corresponding FC protocols. Particularly, each TRILL RBridge&#39;s switch ID or nickname is mapped from the corresponding FC switch domain ID, which can be automatically assigned when a switch joins VCS  400  (which is logically similar to an FC switch fabric). 
     Note that TRILL is only used as a transport between the switches within VCS  400 . This is because TRILL can readily accommodate native Ethernet frames. Also, the TRILL standards provide a ready-to-use forwarding mechanism that can be used in any routed network with arbitrary topology (although the actual routing in VCS is done by the FC switch fabric protocols). Embodiments of the present invention should be not limited to using only TRILL as the transport. Other protocols (such as multi-protocol label switching (MPLS) or Internet Protocol (IP)), either public or proprietary, can also be used for the transport. 
     VCS Formation 
     In one embodiment, a VCS is created by instantiating a logical FC switch in the control plane of each switch. After the logical FC switch is created, a virtual generic port (denoted as G_Port) is created for each Ethernet port on the RBridge. A G_Port assumes the normal G_Port behavior from the FC switch perspective. However, in this case, since the physical links are based on Ethernet, the specific transition from a G_Port to either an FC F_Port or E_Port is determined by the underlying link and physical layer protocols. For example, if the physical Ethernet port is connected to an external device which lacks VCS capabilities, the corresponding G_Port will be turned into an F_Port. On the other hand, if the physical Ethernet port is connected to a switch with VCS capabilities and it is confirmed that the switch on the other side is part of a VCS, then the G_Port will be turned into an E_port. 
       FIG. 5A  illustrates how a logical Fibre Channel switch fabric is formed in a virtual cluster switch in conjunction with the example in  FIG. 4 , in accordance with an embodiment of the present invention. RBridge  412  contains a virtual, logical FC switch  502 . Corresponding to the physical Ethernet ports coupled to hosts  420  and  422 , logical FC switch  502  has two logical F_Ports, which are logically coupled to hosts  420  and  422 . In addition, two logical N_Ports,  506  and  504 , are created for hosts  420  and  422 , respectively. On the VCS side, logical FC switch  502  has three logical E_Ports, which are to be coupled with other logical FC switches in the logical FC switch fabric in the VCS. 
     Similarly, RBridge  416  contains a virtual, logical FC switch  512 . Corresponding to the physical Ethernet ports coupled to host  428  and external switch  430 , logical FC switch  512  has a logical F_Port coupled to host  428 , and a logical FL_Port coupled to switch  430 . In addition, a logical N_Port  510  is created for host  428 , and a logical NL_Port  508  is created for switch  430 . Note that the logical FL_Port is created because that port is coupled to a switch (switch  430 ), instead of a regular host, and therefore logical FC switch  512  assumes an arbitrated loop topology leading to switch  430 . Logical NL_Port  508  is created based on the same reasoning to represent a corresponding NL_Port on switch  430 . On the VCS side, logical FC switch  512  has two logical E_Ports, which to be coupled with other logical FC switches in the logical FC switch fabric in the VCS. 
       FIG. 5B  illustrates an example of how a logical FC switch can be created within a physical Ethernet switch, in accordance with one embodiment of the present invention. The term “fabric port” refers to a port used to couple multiple switches in a VCS. The clustering protocols control the forwarding between fabric ports. The term “edge port” refers to a port that is not currently coupled to another switch unit in the VCS. Standard IEEE 802.1Q and layer-3 protocols control forwarding on edge ports. 
     In the example illustrated in  FIG. 5B , a logical FC switch  521  is created within a physical switch (RBridge)  520 . Logical FC switch  521  participates in the FC switch fabric protocol via logical inter-switch links (ISLs) to other switch units and has an FC switch domain ID assigned to it just as a physical FC switch does. In other words, the domain allocation, principal switch selection, and conflict resolution work just as they would on a physical FC ISL. 
     The physical edge ports  522  and  524  are mapped to logical F_Ports  532  and  534 , respectively. In addition, physical fabric ports  526  and  528  are mapped to logical E_Ports  536  and  538 , respectively. Initially, when logical FC switch  521  is created (for example, during the boot-up sequence), logical FC switch  521  only has four G_Ports which correspond to the four physical ports. These G_Ports are subsequently mapped to F_Ports or E_Ports, depending on the devices coupled to the physical ports. 
     Neighbor discovery is the first step in VCS formation between two VCS-capable switches. It is assumed that the verification of VCS capability can be carried out by a handshake process between two neighbor switches when the link is first brought up. 
     In general, a VCS presents itself as one unified switch composed of multiple member switches. Hence, the creation and configuration of VCS is of critical importance. The VCS configuration is based on a distributed database, which is replicated and distributed over all switches. 
     In one embodiment, a VCS configuration database includes a global configuration table (GT) of the VCS and a list of switch description tables (STs), each of which describes a VCS member switch. In its simplest form, a member switch can have a VCS configuration database that includes a global table and one switch description table, e.g., [&lt;GT&gt;&lt;ST&gt;]. A VCS with multiple switches will have a configuration database that has a single global table and multiple switch description tables, e.g., [&lt;GT&gt;&lt;ST0&gt;&lt;ST1&gt; . . . &lt;STn−1&gt;]. The number n corresponds to the number of member switches in the VCS. In one embodiment, the GT can include at least the following information: the VCS ID, number of nodes in the VCS, a list of VLANs supported by the VCS, a list of all the switches (e.g., list of FC switch domain IDs for all active switches) in the VCS, and the FC switch domain ID of the principal switch (as in a logical FC switch fabric). A switch description table can include at least the following information: the IN_VCS flag, indication whether the switch is a principal switch in the logical FC switch fabric, the FC switch domain ID for the switch, the FC world-wide name (WWN) for the corresponding logical FC switch; the mapped ID of the switch, and optionally the IP address of the switch. 
     In addition, each switch&#39;s global configuration database is associated with a transaction ID. The transaction ID specifies the latest transaction (e.g., update or change) incurred to the global configuration database. The transaction IDs of the global configuration databases in two switches can be compared to determine which database has the most current information (i.e., the database with the more current transaction ID is more up-to-date). In one embodiment, the transaction ID is the switch&#39;s serial number plus a sequential transaction number. This configuration can unambiguously resolve which switch has the latest configuration. 
     As illustrated in  FIG. 6 , a VCS member switch typically maintains two configuration tables that describe its instance: a VCS configuration database  600 , and a default switch configuration table  604 . VCS configuration database  600  describes the VCS configuration when the switch is part of a VCS. Default switch configuration table  604  describes the switch&#39;s default configuration. VCS configuration database  600  includes a GT  602 , which includes a VCS identifier (denoted as VCS_ID) and a VLAN list within the VCS. Also included in VCS configuration database  600  are a number of STs, such as ST0, ST1, and STn. Each ST includes the corresponding member switch&#39;s MAC address and FC switch domain ID, as well as the switch&#39;s interface details. Note that each switch also has a VCS-mapped ID which is a switch index within the VCS. 
     In one embodiment, each switch also has a VCS-mapped ID (denoted as “mappedID”), which is a switch index within the VCS. This mapped ID is unique and persistent within the VCS. That is, when a switch joins the VCS for the first time, the VCS assigns a mapped ID to the switch. This mapped ID persists with the switch, even if the switch leaves the VCS. When the switch joins the VCS again at a later time, the same mapped ID is used by the VCS to retrieve previous configuration information for the switch. This feature can reduce the amount of configuration overhead in VCS. Also, the persistent mapped ID allows the VCS to “recognize” a previously configured member switch when it re-joins the VCS, since a dynamically assigned FC fabric domain ID would change each time the member switch joins and is configured by the VCS. 
     Default switch configuration table  604  has an entry for the mappedID that points to the corresponding ST in VCS configuration database  600 . Note that only VCS configuration database  600  is replicated and distributed to all switches in the VCS. Default switch configuration table  604  is local to a particular member switch. 
     The “IN_VCS” value in default switch configuration table  604  indicates whether the member switch is part of a VCS. A switch is considered to be “in a VCS” when it is assigned one of the FC switch domains by the FC switch fabric with two or more switch domains. If a switch is part of an FC switch fabric that has only one switch domain, i.e., its own switch domain, then the switch is considered to be “not in a VCS.” 
     When a switch is first connected to a VCS, the logical FC switch fabric formation process allocates a new switch domain ID to the joining switch. In one embodiment, only the switches directly connected to the new switch participate in the VCS join operation. 
     Note that in the case where the global configuration database of a joining switch is current and in sync with the global configuration database of the VCS based on a comparison of the transaction IDs of the two databases (e.g., when a member switch is temporarily disconnected from the VCS and re-connected shortly afterward), a trivial merge is performed. That is, the joining switch can be connected to the VCS, and no change or update to the global VCS configuration database is required. 
       FIG. 7  illustrates an exemplary process of a switch joining a virtual cluster switch, in accordance with an embodiment of the present invention. In this example, it is assumed that a switch  702  is within an existing VCS, and a switch  704  is joining the VCS. During operation, both switches  702  and  704  trigger an FC State Change Notification (SCN) process. Subsequently, both switches  702  and  704  perform a PRE-INVITE operation. The pre-invite operation involves the following process. 
     When a switch joins the VCS via a link, both neighbors on each end of the link present to the other switch a VCS four-tuple of &lt;Prior VCS_ID, SWITCH_MAC, mappedID, IN_VCS&gt; from a prior incarnation, if any. Otherwise, the switch presents to the counterpart a default tuple. If the VCS_ID value was not set from a prior join operation, a VCS_ID value of −1 is used. In addition, if a switch&#39;s IN_VCS flag is set to 0, it sends out its interface configuration to the neighboring switch. In the example in  FIG. 7 , both switches  702  and  704  send the above information to the other switch. 
     After the above PRE-INVITE operation, a driver switch for the join process is selected. By default, if a switch&#39;s IN_VCS value is 1 and the other switch&#39;s IN_VCS value is 0, the switch with IN_VCS=1 is selected as the driver switch. If both switches have their IN_VCS values as 1, then nothing happens, i.e., the PRE-INVITE operation would not lead to an INVITE operation. If both switches have their IN_VCS values as 0, then one of the switches is elected to be the driving switch (for example, the switch with a lower FC switch domain ID value). The driving switch&#39;s IN_VCS value is then set to 1 and drives the join process. 
     After switch  702  is selected as the driver switch, switch  702  then attempts to reserve a slot in the VCS configuration database corresponding to the mappedID value in switch  704 &#39;s PRE-INVITE information. Next, switch  702  searches the VCS configuration database for switch  704 &#39;s MAC address in any mappedID slot. If such a slot is found, switch  702  copies all information from the identified slot into the reserved slot. Otherwise, switch  702  copies the information received during the PRE-INVITE from switch  704  into the VCS configuration database. The updated VCS configuration database is then propagated to all the switches in the VCS as a prepare operation in the database (note that the update is not committed to the database yet). 
     Subsequently, the prepare operation may or may not result in configuration conflicts, which may be flagged as warnings or fatal errors. Such conflicts can include inconsistencies between the joining switch&#39;s local configuration or policy setting and the VCS configuration. For example, a conflict arises when the joining switch is manually configured to allow packets with a particular VLAN value to pass through, whereas the VCS does not allow this VLAN value to enter the switch fabric from this particular RBridge (for example, when this VLAN value is reserved for other purposes). In one embodiment, the prepare operation is handled locally and/or remotely in concert with other VCS member switches. If there is an un-resolvable conflict, switch  702  sends out a PRE-INVITE-FAILED message to switch  704 . Otherwise, switch  702  generates an INVITE message with the VCS&#39;s merged view of the switch (i.e., the updated VCS configuration database). 
     Upon receiving the INVITE message, switch  704  either accepts or rejects the INVITE. The INVITE can be rejected if the configuration in the INVITE is in conflict with what switch  704  can accept. If the INVITE is acceptable, switch  704  sends back an INVITE-ACCEPT message in response. The INVITE-ACCEPT message then triggers a final database commit throughout all member switches in the VCS. In other words, the updated VCS configuration database is updated, replicated, and distributed to all the switches in the VCS. 
     Layer-2 Services in VCS 
     In one embodiment, each VCS switch unit performs source MAC address learning, similar to what an Ethernet bridge does. Each {MAC address, VLAN} tuple learned on a physical port on a VCS switch unit is registered into the local Fibre Channel Name Server (FC-NS) via a logical Nx_Port interface corresponding to that physical port. This registration binds the address learned to the specific interface identified by the Nx_Port. Each FC-NS instance on each VCS switch unit coordinates and distributes all locally learned {MAC address, VLAN} tuples with every other FC-NS instance in the fabric. This feature allows the dissemination of locally learned {MAC addresses, VLAN} information to every switch in the VCS. In one embodiment, the learned MAC addresses are aged locally by individual switches. 
       FIG. 8  presents a flowchart illustrating the process of looking up an ingress frame&#39;s destination MAC address and forwarding the frame in a VCS, in accordance with one embodiment of the present invention. During operation, a VCS switch receives an Ethernet frame at one of its Ethernet ports (operation  802 ). The switch then extracts the frame&#39;s destination MAC address and queries the local FC Name Server (operation  804 ). Next, the switch determines whether the FC-NS returns an N_Port or an NL_Port identifier that corresponds to an egress Ethernet port (operation  806 ). 
     If the FC-NS returns a valid result, the switch forwards the frame to the identified N_Port or NL_Port (operation  808 ). Otherwise, the switch floods the frame on the TRILL multicast tree as well as on all the N_Ports and NL_Ports that participate in that VLAN (operation  810 ). This flood/broadcast operation is similar to the broadcast process in a conventional TRILL RBridge, wherein all the physical switches in the VCS will receive and process this frame, and learn the source address corresponding to the ingress RBridge. In addition, each receiving switch floods the frame to its local ports that participate in the frame&#39;s VLAN (operation  812 ). Note that the above operations are based on the presumption that there is a one-to-one mapping between a switch&#39;s TRILL identifier (or nickname) and its FC switch domain ID. There is also a one-to-one mapping between a physical Ethernet port on a switch and the corresponding logical FC port. 
     End-to-End Frame Delivery and Exemplary VCS Member Switch 
       FIG. 9  illustrates how data frames and control frames are transported in a VCS, in accordance with an embodiment of the present invention. In this example, a VCS  930  includes member switches  934 ,  936 ,  938 ,  944 ,  946 , and  948 . An end host  932  is communicating with an end host  940 . Switch  934  is the ingress VCS member switch corresponding to host  932 , and switch  938  is the egress VCS member switch corresponding to host  938 . During operation, host  932  sends an Ethernet frame  933  to host  940 . Ethernet frame  933  is first encountered by ingress switch  934 . Upon receiving frame  933 , switch  934  first extracts frame  933 &#39;s destination MAC address. Switch  934  then performs a MAC address lookup using the Ethernet name service, which provides the egress switch identifier (i.e., the RBridge identifier of egress switch  938 ). Based on the egress switch identifier, the logical FC switch in switch  934  performs a routing table lookup to determine the next-hop switch, which is switch  936 , and the corresponding output port for forwarding frame  933 . The egress switch identifier is then used to generate a TRILL header (which specifies the destination switch&#39;s RBridge identifier), and the next-hop switch information is used to generate an outer Ethernet header. Subsequently, switch  934  encapsulates frame  933  with the proper TRILL header and outer Ethernet header, and sends the encapsulated frame  935  to switch  936 . Based on the destination RBridge identifier in the TRILL header of frame  935 , switch  936  performs a routing table lookup and determines the next hop. Based on the next-hop information, switch  936  updates frame  935 &#39;s outer Ethernet header and forwards frame  935  to egress switch  938 . 
     Upon receiving frame  935 , switch  938  determines that it is the destination RBridge based on frame  935 &#39;s TRILL header. Correspondingly, switch  938  strips frame  935  of its outer Ethernet header and TRILL header, and inspects the destination MAC address of its inner Ethernet header. Switch  938  then performs a MAC address lookup and determines the correct output port leading to host  940 . Subsequently, the original Ethernet frame  933  is transmitted to host  940 . 
     As described above, the logical FC switches within the physical VCS member switches may send control frames to one another (for example, to update the VCS global configuration database or to notify other switches of the learned MAC addresses). In one embodiment, such control frames can be FC control frames encapsulated in a TRILL header and an outer Ethernet header. For example, if the logical FC switch in switch  944  is in communication with the logical FC switch in switch  938 , switch  944  can sends a TRILL-encapsulated FC control frame  942  to switch  946 . Switch  946  can forward frame  942  just like a regular data frame, since switch  946  is not concerned with the payload in frame  942 . 
     VCS Name Services 
     VCS allows an interconnected fabric of RBridges to function as a single logical switch. The VCS name services facilitate fast distribution of run-time network state changes, including newly learned MAC addresses (which is referred to as “Ethernet name service” or “Ethernet NS” in this disclosure) and multi-chassis trunk (MCT) port state updates (which is referred to as “MCT name service” or “MCT NS” in this disclosure). More details on MCT are provided in U.S. patent application Ser. No. 12/725,249, entitled “REDUNDANT HOST CONNECTION IN A ROUTED NETWORK,” by inventors Somesh Gupta, Anoop Ghanwani, Phanidhar Koganti, and Shunjia Yu, filed 16 Mar. 2010, the disclosure of which is incorporated by reference herein. 
     The Ethernet NS provides the ability to distribute various information across the VCS. The MAC information learned at one member switch is distributed to all other member switches, which facilitates fast MAC moves (for example, during migration of virtual machines) and global MAC learning. In some embodiments, layer-2 multicast information, which can be a multicast MAC address with corresponding switch/port identifiers and VLAN tag, can be distributed to facilitate efficient VCS-wide multicast. Optionally, Ethernet NS provides a distribution mechanism and does not maintain a central storage of the MAC-related knowledge base. In other words, the Ethernet NS knowledge database is replicated and stored distributively among all the VCS member switches. 
     Each member switch maintains a database of all the MAC addresses learned throughout the VCS. This database can be used to minimize the amount of flooding (a default behavior of Ethernet switch when a frame&#39;s destination MAC address is not recognized). Ethernet NS also provides VCS-wide distribution of multicast MAC-to-RBridge/Port mapping information which can be obtained by Internet Group Management Protocol (IGMP) snooping. (Details about IGMP and IGMP snooping can be found at IETF RFC 3376 available at http://tools.ietf.org/html/rfc3376 and IETF RFC 4541 available at http://tools.ietf.org/html/rfc4541.) Ethernet NS distributes this information to all RBridges, thereby allowing the VCS to behave as a single switch. By tracking and forwarding IGMP join and leave information, the Ethernet NS can efficiently track the multicast MAC information and maintain an accurate layer-2 multicast group. 
     One of the requirements of presenting a VCS as a single switch is to support connection of trunked links from external hosts to different RBridges within the VCS fabric. Such trunking which involves connection to different RBridges is referred to as multi-chassis trunking (MCT). Conceptually, support within the VCS fabric for routing to a MCT destination is achieved by presenting each MCT group (i.e., each trunk) as a virtual RBridge. In some embodiments, the virtual RBridge is not assigned a domain ID and thus does not utilize FSPF for routing setup. Instead, the a primary RBridge hosting the MCT distributes the virtual RBridge ID and the corresponding link state updates to the VCS fabric. The primary RBridge is responsible for learning a new MAC via an MCT and distributing the new MAC information to the VCS. 
     When an RBridge joins the VCS it will request a dump of the local NS database from the remote RBridge. It will not respond to individual updates from the remote RBridge until the DB dump has been received. After the database is in sync between two RBridges, individual changes are detected locally and pushed remotely. If a local database receives domain unreachable it is responsible for removing all records for that remote domain and doing any local notification that this removal implies. 
       FIG. 10  illustrates an example of name service operation in a VCS, in accordance with one embodiment of the present invention. In this example, a VCS  1000  includes four member switches (Rbridges),  1002 ,  1004 ,  1006 , and  1008 . Assume that an end host  1014  is coupled to switch  1002  during operation. When end host  1014  sends its first Ethernet frame, switch  1002  would not recognize the source MAC address of this ingress frame. Upon receiving this ingress frame, switch  1002  then determines the port (or interface) on which the frame arrives and the frame&#39;s VLAG tag. Subsequently, switch  1002  assembles an Ethernet NS update frame which indicates the learned MAC address (which corresponds to end host  1014 ), its switch identifier (which in one embodiment is the RBridge ID of switch  1002 ), the port identifier, and the VLAG tag for the frame. In one embodiment, this frame is an FC registered state change notification (RSCN) encapsulated in a TRILL header. Note that switch  1002  can obtain the information of all other member switches in the VCS by looking up the global configuration database. Subsequently, switch  1002  can send the Ethernet NS update frame to switches  1004 ,  1008 , and  1006 , respectively. Upon receiving the Ethernet NS update frame, each member switch updates its own MAC database accordingly. In this way, when one of the member switches receives an Ethernet frame destined to end-host  1014 , it can forward that frame to switch  1002  (instead of flooding the frame to all of its ports). 
     Also shown in the example in  FIG. 10  is an MCT group  1016 . MCT group  1016  is formed by an end host  1012  which is dual-homed with switches  1006  and  1008 . Assume that switch  1006  is the primary RBridge in MCT group  1016 . When end host  1012  and MCT group  1010  is first configured, switch  1006  assigns a virtual RBridge  1010  to MCT group  1010 . In addition, switch  1006  notifies the rest of VCS  1000  about the MAC address of end host  1012 . Note that the NS update associated the MAC address of end host  1012  indicates the identifier of virtual RBridge  1010  (instead of the identifier of either switch  1006  or switch  1008 ). In this way, the rest of VCS  1000  can associate end host  1012  with virtual RBridge  1010 . When forwarding a frame destined to end host  1012 , a member switch in VCS  1000  would forward the frame toward virtual RBridge  1010  (i.e., by setting RBridge  1010  as the destination RBridge in the TRILL header). Note that switch  1006  is also responsible for distributing the link state information with respect to the virtual connectivity between virtual RBridge  1010  and switches  1006  and  1008  (indicated by the dotted lines). 
     In case when one of the links (i.e., either the link between switch  1006  and end host  1012 , or the link between switch  1008  and end host  1012 ) fails, as part of the MCT NS, in one embodiment, primary RBridge  1006  is responsible for updating the rest of the VCS  1000  that host  1012 &#39;s MAC address is no longer associated with virtual RBidge  1010 . Instead, the MAC address of host  1012  is now associated with the switch to which host  1012  remains connected. In a further embodiment, it can be the responsibility of the switch that remains connected to host  1012  to distribute the updated MAC address association to the rest of VCS  1000 . 
       FIG. 11  presents a flowchart illustrating the process of distributing learned MAC information by the Ethernet name service in a VCS, in accordance with one embodiment of the present invention. During operation, a VCS member switch detects an ingress frame with a new source MAC address (operation  1102 ). The switch then identifies the port on which the ingress frame is received (operation  1104 ). Subsequently, the switch assembles an Ethernet NS update frame with the learned MAC address, the switch identifier, port identifier, and VLAN tag (operation  1106 ). The switch then distributes the Ethernet NS update frames to all member switches in the VCS (operation  1108 ). 
       FIG. 12  presents a flowchart illustrating the process of distributing information of a learned MAC address via an MCT, in accordance with one embodiment of the present invention. During operation, assume that one of the switches in a MCT group detects an ingress frame with a new source MAC address (operation  1202 ). The switch then determines whether the end host which generates the frame is dual-homed with the MCT group (operation  1204 ). In one embodiment, the switch can make this determination by communicating with the other switch of the MCT group. In a further embodiment, the switch can inspect the link aggregation group (LAG) ID of the ingress frame to determine whether the end host is transmitting using a LAG. If the frame is an MCT frame, the switch then assembles an Ethernet NS update frame with the MAC address, the virtual RBridge identifier corresponding to the MCT, a port identifier, and the VLAG tag of the frame (operation  1206 ). 
     If the frame is determined to be from a regular end host (i.e., not a dual-homed host), the switch assembles an Ethernet NS updated frame with the MAC address, the local physical switch identifier (as opposed to the virtual RBridge ID), the identifier of the port on which the frame is received, and the frame&#39;s VLAN tag (operation  1207 ). The switch then distributes the Ethernet NS update frames to all the member switches in the VCS (operation  1208 ). 
       FIG. 13  presents a flowchart illustrating the process of updating the link state in an MCT group, in accordance with one embodiment of the present invention. During operation, assume one of the MCT partner switches detects a link or port failure which is part of the MCT group (operation  1302 ). Note that this failure can be detected locally (which means a port on the local switch or a link coupled to a local port has failed), or be detected remotely (which means that the failure occurs on the partner switch and the local switch is notified of the failure by the partner switch). The switch then determines whether the MCT end host is still connected to the local switch (operation  1304 ). If the end host is no longer connected to the local switch, the local switch optionally notifies the other partner switch in the MCT of the failure (operation  1310 ) and takes no further actions, assuming that the partner switch will assume responsibility of updating the link state (using, for example, the same procedure illustrated in  FIG. 13 ). 
     If the MCT end host is still connected to the local switch, the switch then assembles an NS update frame with the end host&#39;s MAC address, the local switch&#39;s identifier (e.g., the physical RBridge ID of the local switch), the identifier of the port thought which the end host is connected, and the proper VLAN tag (operation  1306 ). The switch then distributes the NS update frames to all member switches in the VCS (operation  1308 ). 
     Exemplary VCS Member Switch 
       FIG. 14  illustrates an exemplary switch that facilitates formation of a virtual cluster switch with Ethernet and MCT name services, in accordance with an embodiment of the present invention. The VCS member switch is a TRILL RBridge  1400  running special VCS software. RBridge  1400  includes a number of Ethernet communication ports  1401 , which can transmit and receive Ethernet frames and/or TRILL encapsulated frames. Also included in RBridge  1400  is a packet processor  1402 , a virtual FC switch management module  1404 , a logical FC switch  1405 , a VCS configuration database  1406 , a name services management module  1407 , and a TRILL header generation module  1408 . 
     During operation, packet processor  1402  extracts the source and destination MAC addresses of incoming frames, and attaches proper Ethernet or TRILL headers to outgoing frames. Virtual FC switch management module  1404  maintains the state of logical FC switch  1405 , which is used to join other VCS switches using the FC switch fabric protocols. VCS configuration database  1406  maintains the configuration state of every switch within the VCS. TRILL header generation module  1408  is responsible for generating property TRILL headers for frames that are to be transmitted to other VCS member switches. Based on the extracted MAC addresses of incoming frames, NS management module  1407  distributes the NS update frames to the rest of the VCS. NS management module  1407  also maintains a copy of NS database  1409 . NS database  1409  stores all the learned MAC address information from every member switch in the VCS. 
     The methods and processes described herein can be embodied as code and/or data, which can be stored in a computer-readable non-transitory storage medium. When a computer system reads and executes the code and/or data stored on the computer-readable non-transitory storage medium, the computer system performs the methods and processes embodied as data structures and code and stored within the medium. 
     The methods and processes described herein can be executed by and/or included in hardware modules or apparatus. These modules or apparatus may include, but are not limited to, an application-specific integrated circuit (ASIC) chip, a field-programmable gate array (FPGA), a dedicated or shared processor that executes a particular software module or a piece of code at a particular time, and/or other programmable-logic devices now known or later developed. When the hardware modules or apparatus are activated, they perform the methods and processes included within them. 
     The foregoing descriptions of embodiments of the present invention have been presented only for purposes of illustration and description. They are not intended to be exhaustive or to limit this disclosure. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. The scope of the present invention is defined by the appended claims.