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Table of Contents

Working with Virtual Connections

Working with Virtual Connections

This chapter provides an overview of virtual connections, their characteristics and applications, and a functional explanation of each type of virtual connection. These explanations are accompanied by steps to provide a high-level overview of configuration.

Note The information in this chapter is applicable to the Catalyst 8540 MSR, Catalyst 8510 MSR, and LightStream 1010 ATM switch router. For detailed configuration information, refer to the ATM Switch Router Software Configuration Guide and the ATM Switch Router Command Reference publication.

This chapter includes the following sections:

Understanding ATM Virtual Connections

A virtual connection is established as a bidirectional facility to transfer ATM traffic between two ATM layer users. The following sections provide basic information about the types of ATM virtual connections, their applications, and their auto-negotiated parameters.

Types of Virtual Connections

ATM provides two kinds of virtual connection services, permanent and switched. Permanent virtual connections, or circuits, are manually set up and remain up until manually torn down. Following are the two main types of permanent virtual connections:

Both PVPs and PVCs can support point-to-point and point-to-multipoint connections.

Switched virtual connections are set up through signaling and remain up only as long as they are in use. Following are the two main types of switched virtual connections:

Both SVCs and SVPs also support point-to-point and point-to-multipoint connections.

Soft PVPs and soft PVPs, a hybrid between switched and permanent connections, are specified by a VPI/VCI and the destination ATM address. They are then set up through signaling but, unlike switched connections, remain up until manually torn down.

Transit and Terminating Connections

From the standpoint of the ATM switch router, virtual connections can be further characterized as transit or terminating connections. Transit connections are switched from the ingress to the egress of the connection, while terminating connections terminate at the ATM switch router. Terminating connections usually end on the CPU interface and are used for management and signaling purposes, though the endpoint of a normal data connection can also be considered as terminating.

Connection Components

Figure 3-1 shows an example virtual connection between ATM user A and user D. The end-to-end VCC has two parts:

The common endpoint between an internal connection and a link occurs at the switch interface. The endpoint of the internal connection is sometimes referred to as a connection leg or half-leg. A cross-connect connects two legs together.


Figure 3-1: Virtual Channel Connection Example


Notice that the value of the VPIs and VCIs can change as the traffic is relayed through the ATM network. These values must be configured, either manually or through signaling, at each point along the connection path.

Table 3-1 lists the types of virtual connections supported on the ATM switch router. The third and fourth rows of the table, PVC and PVP, refer to concatenated links and internal connections that comprise an entire virtual connection, such as from user A to user D in Figure 3-1.


Table 3-1: Supported Virtual Connection Types
Connection Point-to-
Point 		Point-to-
Multipoint 		Transit Terminate

Permanent virtual channel link (PVCL)

X

X

---

---

Permanent virtual path link (PVPL)

X

X

---

---

Permanent virtual channel connection (PVCC)

X

X

X

X

Permanent virtual path connection (PVPC)

X

X

X

---

Soft permanent virtual channel connection (soft PVCC)

X

---

X

---

Soft permanent virtual path connection (soft PVPC)

X

---

X

---

Switched virtual channel connection (SVCC)

X

X

X

X

Switched virtual path connection (SVPC)

X

X

X

---

Autoconfigured Parameters of Virtual Connections

When your ATM switch router initially starts up, with no previous configuration, the Integrated Local Management Interface (ILMI) protocol negotiates certain values across the UNI that serve as parameters for virtual connections. Devices on either end of the UNI connection learn and dynamically configure themselves based on the parameters received from their peers. The virtual connection-related parameters that are negotiated via ILMI are as follows:

If there are previously configured PVPs or PVCs, ILMI determines these as well.

Applications for Virtual Connections

The application of various virtual connection types is summarized as follows:

The difference between a PVC and a soft PVC is that a soft PVC is automatically rerouted if a switch or link in the path fails. From that perspective a soft PVC is considered more robust than a simple PVC.

The difference between an SVC and a soft PVC is that a SVC is established on an "as needed" basis through user signalling. With a soft PVC the called party cannot drop the connection.

PVC Connections

The following sections provide a PVC configuration overview and examples of PVC connections.

PVC Configuration Overview

Configuring a PVC, such as the one shown in Figure 3-1, requires the following steps:

Step 1 Configure the connection traffic table rows (optional).

The connection traffic table specifies traffic management parameters for a connection. See the "Connection Traffic Table" section of the chapter "Traffic and Resource Management."

Step 2 Configure the VPI/VCI values for each cross-connect.

Using the example in Figure 3-1, you connect VPI/VCI 0/50 on interface 3/0/1 to VPI/VCI 2/100 on interface 3/0/2, and VPI/VCI 2/100 on interface 0/0/0 to VPI/VCI 50/255 on interface 0/0/1. Note that the VPI/VCI values change on the cross-connect segment, but are the same at each end of a link between two systems.

Terminating PVC Connections

Terminating connections provide internal connections to the ATM switch router's route processor for LAN Emulation (LANE), IP over ATM, and control channels for Integrated Local Management Interface (ILMI), signalling, and Private Network-to-Network Interface (PNNI) plus network management.

In Figure 3-2, the upper diagram shows a point-to-point connection terminating on the CPU of the switch. The lower diagram shows a point-to-multipoint connection with one leaf of the connection terminating on the CPU and the other two leaves transiting the switch into the ATM network cloud.


Figure 3-2: Terminating Virtual Connection Types


The procedure for configuring a terminating PVC is the same as for a transit PVC. The CPU interface, on which the PVC terminates, is always atm0.

Point-to-Multipoint PVC Connections

Figure 3-3 shows a point-to-multipoint PVC in which cells entering the ATM switch router at the root point (0/0/0, VPI = 50, VCI = 100) are duplicated and switched to the leaf points (output interfaces) that connect into the ATM network cloud.


Figure 3-3: Point-to-Multipoint PVC Example


The procedure for configuring a point-to-multipoint PVC is the same as for a point-to-point PVC, except that the VPI/VCI at the root interface must be separately mapped to the VPI/VCI on each of the leaf interfaces.

PVP Connections

Figure 3-4 shows an example of PVPs through the ATM switch routers used to connect user A and user D. Because these are PVPs, not PVCs, they are identified by only VPIs.


Figure 3-4: Virtual Path Connection Example


Configuration Overview

Configuring a PVP, such as the one shown in Figure 3-4, requires the following steps:

Step 1 Configure the connection traffic table rows (optional).

The connection traffic table is used to specify traffic management parameters for a connection. See the "Connection Traffic Table" section of the chapter "Traffic and Resource Management."

Step 2 Configure the VPI value for each cross-connect.

Using the example in Figure 3-4, you would connect VPI 1 on interface 3/0/1 to VPI 2 on interface 3/0/2, and VPI 2 on interface 0/0/0 to VPI 50 on interface 0/0/1. The VPI values change on the cross-connect segment, but not on the link.

Tips
The VPI and VCI values at both ends of a link segment (for example, interface 3/0/2 on switch B and interface 0/0/0 on switch C) must match.

Point-to-Multipoint PVP Connections

Figure 3-5 shows a point-to-multipoint PVP in which cells entering the ATM switch router at the root point (VPI = 50), are duplicated and switched to the leaf points (output interfaces).


Figure 3-5: Point-to-Multipoint Permanent Virtual Path Example


The procedure for configuring a point-to-multipoint PVP is the same as for a point-to-point PVP, except that the VPI at the root interface must be separately mapped to the VPI on each of the leaf interfaces.

Soft PVC Connections

Figure 3-6 illustrates a soft PVC used to connect user A and user D through the ATM network cloud. Unlike hard PVCs, the interface and VPI/VCI identifiers are needed only for the endpoints of the connection; the values for the intermediate switching points are not needed for the configuration, as these are determined by signaling.


Figure 3-6: Soft Permanent Virtual Channel Connection Example


Configuration Overview

Configuring a soft PVC, such as the one shown in Figure 3-6, requires the following steps:

Step 1 Configure the connection traffic table rows (optional).

The connection traffic table specifies traffic management parameters for a connection. See the "Connection Traffic Table" section of the chapter "Traffic and Resource Management."

Step 2 Decide which of the two connection endpoints you want to designate as the destination (or passive) side of the soft PVC.

This decision is arbitrary---it makes no difference which port you define as the destination end of the circuit.

Step 3 Retrieve the ATM address of the destination end of the soft PVC.

Step 4 Retrieve the currently used VPI/VCI values at both ends.

You must select unused VPI/VCI values for the connection.

Step 5 At the source end interface, specify a soft PVC with unused VPI/VCI values to the ATM address and VPI/VCI values of the destination interface.

Using the example in Figure 3-6, you would connect VPI/VCI 0 200 on interface 0/0/0 to the destination address 47.0091.8100.00.0000.1111.1111.1111.1111.1111.1111.00 with VPI/VCI 0 100.

Soft PVP Connections

Figure 3-7 illustrates a soft PVP that connects user A and user D through the ATM network cloud. The information needed to configure the soft PVP is similar to that for a soft PVC, except that only VPI values are used as connection identifiers.


Figure 3-7: Soft Permanent Virtual Path Connection Example


The procedure for configuring a soft PVP is identical to that used for a soft PVC, except that no VCI values are used.

Route Optimization for Soft PVCs

Permanent virtual connections typically have a much longer life than switched virtual connections. This means that the route chosen, for example, during connection setup remains the same even though the topology of the network can change over time, making the original route less than optimal. With route optimization for soft PVCCs and soft PVPCs, the route can be recomputed periodically based on the following parameters:

At the specified time, all of the soft permanent virtual connections on the interface are checked to see if a better route exists. They are only rerouted if there is an available route that passes any QoS requirements and has a cumulative administrative weight that is better than the existing route by a percentage determined by the configured route-optimization percentage-threshold value (default 30 percent). Administrative weight is similar to hop count. For a description of administrative weight, see the "Administrative WeightGlobal Mode and Per-Interface Values" section of the chapter "ATM Routing with IISP and PNNI."

The route optimization feature applies to soft PVCCs and soft PVPCs on both ATM and Frame Relay interfaces.

Configuration Overview

Configuring the route optimization feature for soft permanent virtual connections requires the following steps:

Step 1 From global configuration mode, enable route optimization and specify a threshold value.

Step 2 Select the interface to configure and enter interface configuration mode. You configure route optimization on the source end only of a soft PVC.

Step 3 Specify during what time of day and how often routes should be recomputed on this interface.

You can also manually trigger route optimization on a specific soft PVC.

Note Route optimization for soft permanent virtual connections should not be configured with constant bit rate (CBR) connections.

Soft PVCs with Explicit Paths

PNNI performs dynamic routing of calls using soft PVCs and soft PVPs that are automatically set up over paths that meet the traffic parameter objectives. However, manually configured paths can be used in cases where a fully or partially specified explicit path is preferred. This feature is further described in the "Manually Configured Explicit Paths" section of the chapter "ATM Routing with IISP and PNNI."

The explicit paths are assigned using precedence numbers 1 through 3. The precedence 1 path is tried first; if it fails the soft connection is routed using the precedence 2 path, and so forth. If all of the explicit paths fail, standard on-demand PNNI routing is tried unless routing has been configured to only use explicit paths.

An explicit path is defined using a series of entries. If the soft connection destination address is reachable at one of the included entries in an explicit path, any subsequent entries in that path are automatically disregarded. This allows longer paths to be reused for closer destinations. It is also possible to specify a point in the entries beyond which further path entries should be disregarded.

You can add, modify, or remove explicit paths without tearing down existing soft connections. When you redo a soft connection, you specify the VPI and VCI values; all applicable explicit path options are replaced by the respecified explicit path options.

The soft connection is not immediately rerouted using the new explicit path. However, reroutes using the new explicit path can happen for the following four reasons:

    1. A failure occurs along the current path.

    2. Route optimization has been enabled for the soft connection.

    3. Route optimization has been enabled on the interface and the retry time interval has expired.

    4. The soft PVC is disabled and then reenabled.

Nondefault Well-Known PVCs

ATM needs to set up and maintain well-known virtual connections for purposes such as signaling and management. Normally the default well-known virtual connections are automatically created with the default VCIs defined by the standards. In unusual circumstances, however, you can configure nondefault well-known VCI values on a per-interface basis. Two possible instances in which you might configure nondefault well-known VCI values are:

Table 3-2 lists the well-known VCs and their default VPI/VCI values.


Table 3-2: Well-Known Virtual Channels
Channel Type Virtual Path Identifier Virtual Channel Identifier

Connection control signaling (QSAAL)

0

5

ILMI

0

16

PNNI

0

18

Tag switching

0

32
Caution
Do not swap virtual channel values between two types of well-known VCs.

Nondefault Well-Known PVC Configuration Overview

Following is an overview of the steps needed to configure nondefault well-known virtual connections:

Step 1 Display the currently configured well-known virtual connections on the interface.

Step 2 Delete any existing automatically created well-known virtual connections on the interface.

Step 3 Configure the new VPI/VCI values on the interface and specify the encapsulation type (QSAAL, ILMI, PNNI, tag).

Step 4 Save these changes to your startup configuration file so that they are not lost if the switch reboots.

VPI/VCI Ranges for SVPs and SVCs

VPI/VCI conflicts can inadvertently occur when setting up SVC and SVP connections. For example, suppose you specify a soft PVC with VPI 0 and VCI 50 on the destination interface. An SVC on that interface might have already taken VPI 0 and VPI 50 just before the soft PVC setup message arrives at the destination interface. In this case, the soft PVC is rejected because VPI 0 and VPI 50 are already taken.

Specifying the VPI/VCI range for SVP and SVC connections allows you to avoid such connection setup rejections. ILMI 4.0 uses this range when negotiating the VPI/VCI values for switched connections. Even if you specify a range, you can still configure PVCs and PVPs of any supported value, including any VPI/VCI range you configured for SVC and SVP connections.

The default maximum VPI for an SVP or SVC connection is 255. For interfaces configured with a 12-bit VPI space (NNI only) the default maximum is 4095. See Table 3-3.


Table 3-3: Maximum SVP VPI Range
VPI Bit Type Maximum Value Range

8-Bit VPI

0-255

12-Bit VPI

0-4095
Note The maximum value specified applies to all interfaces except logical interfaces, which have a fixed value of 0.

You can change the maximum VPI value. For example, in Figure 3-8 the maximum SVP VPI is configured as 100. Therefore, VPIs 1 to 100 are reserved for SVP connections. You can use VPIs 101 to 255 for PVPs; however, you are not restricted to that range.


Figure 3-8: Example SVP VPI Range


Note In Figure 3-8 the maximum available VPI value would be 4095 instead of 255 for ports configured with a 12-bit VPI.

The default maximum VCI for an SVC connection is 255, and the default minimum VCI for an SVC connection is equal to 35. However, you can also change the minimum SVCC VCI. In the example shown in Figure 3-9, the maximum SVCC VPI is 100 and the minimum SVCC VCI is 60. Therefore, VPIs 0 through 100 and VCIs 60 through 16,383 are reserved for SVCCs.


Figure 3-9: Example SVC VPI/VCI Range


Note In Figure 3-9 the maximum available VPI value would be 4095 instead of 255 for ports configured with a 12-bit VPI.

Every interface negotiates the local values for the maximum SVPC VPI, maximum SVCC VPI, and minimum SVCC VCI with the peer's local value during ILMI initialization. The negotiated values determine the ranges for SVPs and SVCs. If the peer interface does not support these objects or autoconfiguration is turned off on the local interface, the local values determine the range.

Configuration Overview

Configuring VPI/VCI ranges for SVP and SVCs requires the following steps at each interface where you need to specify a range:

Step 1 Configure the maximum VPI value for SVP connections.

Step 2 Configure the maximum VPI value for SVC connections.

If you want to configure a maximum VPI greater than 255, then you must enable 12-bit VPIs on the interface. This option is platform dependent and is available on NNI interfaces only; see the configuration overview in the "NNI Interfaces" section in the chapter "ATM Network Interfaces."

Step 3 Configure the minimum VCI value for SVC connections.


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Posted: Mon Aug 16 14:09:01 PDT 1999
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