VSI Functions

VSI is a master-slave protocol through which a master can control the operations of one or many slaves. VSI was invented by Cisco Systems. The current implementation is VSI version 2.4.

The main functions of VSI are as follows:

• Discovery of the switch configuration VSI learns about the controlled switch configuration. It can discover the available logical controlled interfaces, the resources allocated in those interfaces, and their status. The VSI slave informs the master of a set of resources associated with a logical interface.

• Control the switch cross-connects The setup and teardown of cross-connects in the controlled switch allow data forwarding, according to what the network protocol mandates. This includes QoS, different service types, and oversubscription.

• Monitoring of port and channel statistics VSI allows the VSI master to gather statistics or counters about ports and connections from the VSI slaves.

• Resynchronization If the VSI master and the slaves get out of sync as far as the cross-connect databases, the resynchronization capabilities allow them to resynchronize without having to tear down and rebuild every connection from scratch. This is done with checksums.

A controller-controlled switch pair (that comprises the multiservice switch) is shown in Figure 2-1.

Figure 2-1. Multiservice Switch Modules

The different modules and components shown in Figure 2-1 can be classified as follows:

• Controller Controls the operation of the controlled switch.

  - Networking software At this layer are the software modules and APIs that control the cross-connect decisions. In an MPLS application, these are the IP routing protocols, such as Interior Gateway Protocol (IGP) (OSPF, IS-IS, and so on) and Label Distribution Protocol (LDP). In a PNNI application, these are UNI signaling and PNNI signaling and routing.
  
  

  - VSI master The master portion of VSI is implemented in the controller. Using VSI commands, the controller sets up the cross-connects in the switch. One VSI master is responsible for the entire switch. It interfaces to the networking software.
  
  

• Controlled switch The ATM switch under the direction of the controller.

  - VSI slave The implementation of VSI in the switch executes the commands the master sends and updates the master with the switch information. To enable the controllers to act independently, the VSI slave process(es) in the switch must allocate resources to the different control planes.
  
  

  - Platform software The platform software handles general switch management and switch-specific functions.
  
  

  - Controlled interfaces As soon as resources are allocated in the interface for a specific control plane, that interface is made visible from the controller and thus becomes a controlled interface. This is a purely logical concept, as is the virtual switch to which the interface belongs.
  
  

From these architectural modules, an important point discussed in the preceding chapter is made explicit: The VSI slaves present a view of a virtual switch to the VSI master and therefore to the networking software module. A physical interface and a logical interface (for example, a virtual interface that uses only one VPI in the interface, or an Inverse Multiplexing ATM [IMA] interface with multiple T1s) are the same from the VSI master's perspective, and thus from the networking software's perspective. The controller view of controlled interfaces is shown in Figure 2-2.

Figure 2-2. Controller View of Controlled Interfaces

As a consequence, a physical interface with multiple virtual path logical interfaces is seen as multiple virtual interfaces from the controller. On the other end of the spectrum, multiple physical interfaces in an IMA group in the controlled switch are presented to the controller as a single interface. This is summarized in Table 2-1.

Table 2-1. Physical-to-Logical Ratio of Controlled Interfaces

Logical Interface	Physical-to-Logical Ratio
Physical interface	1:1
Virtual interface	1:N
IMA interface	N:1

Another direct consequence of this is the fact that VSI's Application Programming Interface (API) hides the platform hardware-specific parameters, software, and firmware from the networking software, cleanly separating them. VSI provides primitives for generic ATM cross-connect creation, deletion, and modification. Typically, platforms have more parameters (usually hardware-specific) to configure for a connection than the networking protocol (such as PNNI or MPLS). Those parameters are called extended parameters. Figure 2-3 shows this separation concept.

Figure 2-3. VSI Hides Platform Specifics from the Networking Software

In switches that do not implement this modular approach, the mapping from standard protocol parameters to switch-specific extended parameters is typically done at the networking software, making the networking modules difficult to port to another platform with a different set of extended parameters. These parameters might be as important as different QoS queues.

In multiservice switches, the mapping is done at the platform software (eliminating platform-specific code from the networking software). This allows platform-independent networking software and results in each platform's optimally mapping standard parameters to extended platform-specific parameters. VSI achieves this by defining service types, letting the controller specify a category of traffic management when setting up a connection.

NOTE

The switch selects logical interface numbers (LINs) to be used by VSI while updating the controller. Those numbers need to be unique and persistent, but this is done switch-implementation-specific. (LINs are assigned by the slave in a platform-specific way. They need to be unique even if the controlled switch is a multishelf node.) The mapping between physical interface name and LIN is maintained by the VSI slave(s) and is hidden from the VSI master. Moreover, the VSI master does not assume any structure in the number and does not presume the range to be compact (even if sometimes a structured approach is desirable).

The LIN is a 32-bit value that the switch needs to advertise before the controller can use it. Valid LINs range from 0 to 0xFFFFFFFE. The VSI master uses the 0xFFFFFFFF value to indicate to the VSI slave that it wants to start walking the slave's LIN values.