Table of Contents

Traffic and Resource Management

This chapter provides an overview of ATM traffic management in general and describes the related configurable features on the ATM switch router.

This chapter includes the following sections:

• "Overview" section
  
  
• "The Traffic and Service Contract" section
  
  
• "Connection Admission Control" section
  
  
• "Hardware Resources" section
  
  

Overview

To meet the demands of multiservice networking, in which traffic of varying bandwidth and delay requirements must be simultaneously serviced, sophisticated traffic and resource management mechanisms are needed. To serve these needs, your ATM switch router uses a shared-memory architecture that offers advanced traffic management and congestion control capabilities.

The traffic management features of the ATM switch router provide the following capabilities:

• Integrated support for different types of ATM services
  
  
• Flexible policies for bandwidth allocation through controlled link-sharing services
  
  
• Effective use of network resources
  
  

The congestion control capabilities of the ATM switch router support the following goals:

• Avoid conditions where congestion can occur
  
  
• Minimize the effects of congestion when it does occur
  
  
• Prevent spreading the congestion state to other parts of the network
  
  

Because ATM networks are designed to carry many different types of traffic, traffic characteristics and QoS requirements of each virtual connection must be described, and delivery of the contract must be guaranteed within the resource allocation policies defined for the network.

These requirements are carried out in three phases:

The first of these two steps can be considered the connection setup phase, while the third step represents the data flow phase. These three phases and their supporting mechanisms are discussed in the following sections.

The Traffic and Service Contract

The traffic and service contract specifies an envelope that describes the intended data flow and uses the following information:

• A connection traffic descriptor, which includes a service category and applicable parameters
  
  
• An optional set of QoS parameters applicable to the service category
  
  

Table 10-1 shows which traffic and QoS parameters used on the ATM switch router for the setup of connections in the ATM Forum service categories.

When establishing the traffic and service contract, target values for QoS parameters can be used as criteria for the connection setup requirements. These values are either metrics (accumulated over multiple hops of a call) or attributes (a gating criterion that is not accumulated, but is checked at each interface). Maximum cell transfer delay (MCTD) and peak-to-peak cell delay variation (ppCDV) are metrics, while cell loss ratio (CLR) is an attribute.

Following are the parameters you can configure to define the service contract:

• The connection traffic table rows
  
  
• The default QoS objective table
  
  
• The default MBS
  
  
• The default CDVT
  
  

Connection Traffic Table

The traffic characteristics used in the traffic and service contract for permanent virtual connections (PVCs) are configured in connection traffic table (CTT) rows. A row in the CTT must exist for each unique combination of service category and traffic parameters. Service requests for virtual path links (VPLs) and virtual channel links (VCLs) then specify a row index in the table per flow (receive and transmit directions). Many VCL/VPL pairs can refer to the same row in the traffic table.

Configured parameters in the CTT are ignored for tag switching virtual connections; see the "CTT Rows" section in the chapter "Tag Switching."

Connection Traffic Table Rows for PVCs and SVCs

Specification of nondefault traffic for a PVC requires configuring a CTT row. Rows used for PVCs are called stable rows

Requested traffic parameters for switched virtual connections (SVCs) are signaled in the setup and do not use the preconfigured values in the CTT. However, the CTT in a SVC setup provides a row identifier for use by the Simple Network Management Protocol (SNMP) or the user interface to read or display traffic parameters for SVCs. Thus, a CTT row index is dynamically created and stored in the connection-leg data structure for each flow of an SVC.

CTT Row Allocations and Defaults

To make CTT management software more efficient, the CTT row-index space is split into rows allocated as a result of signaling (for SVCs) and rows allocated from the CLI and SNMP (for PVCs). Table 10-2 describes the row-index range for both.

The CTT contains a set of well-known, predefined ATM CTT rows, described in Table 10-3. These rows cannot be deleted.

Configuration Overview

Configuring the CTT row requires specifying a row index with parameter values for each of the service categories:

• For VBR-RT and VBR-NRT traffic, you must specify values for PCR and SCR; MBS and CDVT are optional.
  
  
• For CBR traffic, you must specify a value for PCR; CDVT is optional.
  
  
• For ABR and UBR traffic, you must specify a value for PCR; MCR and CDVT are optional.
  
  

Default QoS Objective Table

Since UNI 3.x signaling does not provide information elements (IEs) to signal QoS values, the resource management software on the ATM switch router provides a table of default QoS objective values to apply to SVCs in the guaranteed service categories (CBR and VBR). UNI 4.0 signaling does support signaling of QoS values, but the default QoS objective values configured also apply to connections on UNI 4.0 interfaces.

The ATM switch router uses no default values for these objectives; rather, they are unspecified, as shown in Table 10-4, until defined. If undefined, the objective is not considered in connection setup.

Configuration Overview

Configuring the default QoS objective table requires one or more of the following steps; each objective can have a defined or undefined value.

• Specify a MCTD value in microseconds for CBR, VBR-RT, or for both.
  
  
• Specify a ppCDV value in microseconds for CBR, VBR-RT, or for both.
  
  
• Specify a cell loss ratio exponent for CBR, VBR-RT, VBR-NRT, or for all three.
  
  

  You can specify separate loss ratio exponents for CLP0 and CLP0+1 cells.

The default QoS objective table should be configured with the same values for an entire network.

CDVT and MBS Interface Defaults

If MBS or CDVT values are not explicitly specified in the CTT, the default values for those parameters on the interface are used in the contract. See the "Default CDVT and MBS" section.

Connection Admission Control

Connection Admission Control (CAC) is the set of procedures and actions taken by the ATM network at the connection setup phase or during connection renegotiation phase to determine whether a virtual path connection (VPC) or virtual channel connection (VCC) request can be accepted or not. Each half-leg (connection on an interface) of a connection must pass CAC.

Resource CAC (RCAC) uses the following information provided in the traffic contract for each direction of a requested connection:

RCAC is based on a proprietary Equivalent Bandwidth algorithm in which equivalent bandwidths are used as real constant bandwidths in a statistical time-division multiplexing (STM) CAC environment for fast calculation. The equivalent bandwidth (or effective bandwidth by some) of a source is defined to be the minimum capacity required to serve the traffic source so as to achieve a specified steady-state CLR, MCTD, and ppCDV for CBR/VBR and for the nonzero MCR portion of ABR/UBR+ connections.

Flexibility in the resource management framework is particularly important because it is not easy to fully anticipate the customer and service requirements of emerging applications on the ATM internets. Controlled link sharing makes a key contribution to this flexibility. Discussed in the "Controlled Link Sharing" section, controlled link sharing configuration allows the user to place maximum limits and minimum guarantees on the interface bandwidth dedicated to service categories.

The RCAC algorithm provides a set of administratively configurable parameters (controlled link sharing) that specifies one or both of the following:

• Minimum or maximum bandwidth, or both, in terms of a percentage share of the link bandwidth, allowed for CBR, VBR, ABR, and UBR+
  
  
• Maximum bandwidth for combined guaranteed bandwidth
  
  

Parameter Definitions

Interface parameters used by RCAC are defined as follows:

• SCRM (SCR margin)—extra equivalent bandwidth (over and above SCR) allocated to a VBR connection to account for bursting to PCR.
  
  
• SCRMF (SCRM factor)—configurable safety margin with default = 1%
  
  
• R_CLR_CBR:CLR—estimate for CBR connection transiting an interface
  
  
• R_CLR_VBR:CLR—estimate for VBR connection transiting an interface
  
  
• R_ppCDV_CBR—estimate on maximum transmit ppCDV that might be experienced by a CBR connection transiting an interface
  
  
• R_ppCDV_VBR—estimate on maximum transmit ppCDV that might be experienced by a VBR-RT connection transiting an interface
  
  
• R_MaxCTD_CBR—estimate on maximum transmit maxCTD that may be experienced by a CBR connection transiting an interface
  
  
• R_MaxCTD_VBR—estimate on maximum transmit maxCTD that may be experienced by a VBR-NRT connection transiting an interface
  
  
• MAX_CR—Maximum cell rate in a direction on an interface
  
  
• MAX_BE_CONNS—user-configured maximum number of best-effort (ABR/UBR) connections allowed on an interface
  
  
• MAX_PCR_CBR, MAX_PCR_VBR, MAX_PCR_ABR, MAX_PCR_UBR—user-configured maximum allowed PCR, per service category (default = line rate)
  
  
• MAX_SCR—user-configured maximum allowed SCR (default = line rate)
  
  
• MAX_MCR_ABR, MAX_MCR_ABR— user-configured maximum allowed MCR for ABR, UBR+ (default = line rate)
  
  
• MAX_MBS—user-configured maximum allowed MBS (default = none)
  
  
• MAX_CDVT_CBR, MAX_CDVT_VBR, MAX_CDVT_ABR, MAX_CDVT_UBR:—
  user-configured maximum allowed CDVT, per service category (default = no limit)
  
  

Parameters used in the algorithm are defined as follows:

• CBR_EQ_BW, VBR_EQ_BW, ABR_EQ_BW, UBR+_EQ_BW—the equivalent bandwidth proposed for a CBR, VBR, ABR, or UBR+ connection.
  
  

CAC Algorithm

Here is the basic RCAC algorithm, expressed in a c-like pseudo code. Note that this is performed for each direction (RX/TX) on a half-leg.

Input:
Traffic contract input:   service_category, PCR, SCR, SCRMF, MBS, CDVT, MCR
			   tx_MaxCTD_obj, tx_MaxCTD_acc, tx_ppCDV_obj, tx_ppCDV_acc, CLR 
 
output:CAC_accept - true,  connection is accepted
                     false, connection is rejected 
 
algorithm:
 
Based on service category of the proposed connection:
 
service_category == CBR:
 
   if ((CLR >= R_CLR_CBR) && 
       (tx_MaxCTD_obj >= R_MaxCTD_CBR + tx_MaxCTD_acc) && 
       (tx_ppCDV_obj  >= R_ppCDV_CBR + tx_ppCDV_acc) && 
       (PCR <= MAX_CR) && 
       (PCR <= MAX_PCR_CBR) &&
       (CDVT <= MAX_CDVT_CBR) {
 
          CBR_EQ_BW = PCR
          if (CBR_EQ_BW >  ACR_CBR) 
             CAC_accept = false 
          else 
             CAC_accept = true  
   } else
      CAC_accept = false 
 
service_category == VBR-RT:
 
   if ((CLR >= R_CLR_VBR) && 
       (tx_MaxCTD_obj >= R_MaxCTD_VBR + tx_MaxCTD_acc) &&
       (tx_ppCDV_obj  >= R_ppCDV_VBR  + tx_ppCDV_acc) && 
       (PCR <= MAX_CR) && 
       (PCR <= MAX_PCR_VBR) && 
       (SCR <= MAX_CR) && 
       (SCR <= MAX_SCR) && 
       (MBS <= MAX_MBS) &&
       (CDVT <= MAX_CDVT_VBR) {
 
          SCRM = SCRMF * (PCR - SCR) /* SCRMF = [0,...,1] w. default=0.01 */
          VBR_EQ_BW = SCR + SCRM
          if (VBR_EQ_BW >  ACR_VBR)
             CAC_accept = false
          else 
             CAC_accept = true 
   } else
      CAC_accept = false 
 
service_category == VBR-NRT:
 
   if ((CLR >= R_CLR_VBR) &&
       (PCR <= MAX_CR) &&
       (PCR <= MAX_PCR_VBR) &&
       (SCR <= MAX_CR) &&
       (SCR <= MAX_SCR) &&
       (MBS <= MAX_MBS) &&
       (CDVT <= MAX_CDVT_VBR) {
 
          SCRM = SCRMF * (PCR - SCR) /* SCRMF = [0,...,1] w. default=0.01 */
          VBR_EQ_BW = SCR + SCRM
          if (VBR_EQ_BW >  ACR_VBR)
             CAC_accept = false
          else 
             CAC_accept = true 
   } else
      CAC_accept = false 
 
service_category == ABR:
    
   if ((PCR <= MAX_CR) &&
       (PCR <= MAX_PCR_ABR) &&
       (MCR <= MAX_CR) &&
       (MCR <= MAX_MCR_ABR) &&
       (CDVT <= MAX_CDVT_ABR) &&
       (ABR_count + UBR_count + 1  <= MAX_BE_CONNS)){
 
          ABR_EQ_BW = MCR
          if (ABR_EQ_BW >  ACR_ABR) 
             CAC_accept = false 
          else 
             CAC_accept = true  
   } else
      CAC_accept = false 
 
service_category == UBR:
 
   if ((PCR <= MAX_CR) &&
       (PCR <= MAX_PCR_UBR) &&
       (MCR <= MAX_CR) &&
       (MCR <= MAX_MCR_UBR) &&
       (CDVT <= MAX_CDVT_UBR) &&
       (ABR_count + UBR_count + 1  <= MAX_BE_CONNS)){
 
          UBR+_EQ_BW = MCR
          if (UBR+_EQ_BW >  ACR_UBR+)
             CAC_accept = false
          else
             CAC_accept = true
   } else
      CAC_accept = false
 

Note that the above algorithm does not describe the derivation of available cell rate (ACR) per service category. In the absence of controlled link sharing, this algorithm applies to each direction:

(.95 * MAX_CR - sum of equivalent bw allocated to all connections on interface)
 

If controlled link sharing is configured, it then establishes limits on ACR for all guaranteed bandwidth or a service type (the maximum case) and limits on the encroachment of other service types (the minimum case).

Configurable Parameters

Following are the parameters you can configure that affect the operation of the CAC mechanism in finding an acceptable path for a connection:

• Sustained cell rate margin factor
  
  
• Controlled link sharing
  
  
• Outbound link distance
  
  
• Limits of best-effort connections
  
  
• Interface maximum of individual traffic parameters
  
  
• Service category support per interface
  
  
• Interface overbooking
  
  
• Framing overhead
  
  

The sustained cell rate margin factor is configured globally. The remaining CAC-related features are configured on a per-interface basis.

Sustained Cell Rate Margin Factor

The sustained cell rate margin factor is a measure used by CAC in admitting VBR connections. Expressed as a percent, the sustained cell rate margin factor dictates the aggressiveness of weighting PCR compared to SCR. CAC uses the sustained cell rate margin factor (SCRMF) as follows to define the requested equivalent bandwidth:

bandwidth = (SCRMF * (PCR-SCR))/100 + SCR

Configuration Overview

You can change the default sustained cell rate margin factor for admitting VBR connections using a global configuration command. Configuring this value as 100 causes CAC to treat VBR like CBR.

Controlled Link Sharing

Controlled link sharing is a set of parameters used to specify a variety of minimum and maximum values for guaranteed bandwidth that can be allocated on an interface. These parameters allow fine-tuning of the CAC functions on a per-interface and per-direction (receive and transmit) basis. The relationship among these parameters, when defined, is shown in Table 10-5:

Configuration Overview

Configuring controlled link sharing on an interface requires the following steps:

Step 2   Take one or more of the following steps:

The Outbound Link Distance

The outbound link distance parameter is a measure of the physical link distance for the next ATM hop in the outbound direction on an interface. By altering the outbound link distance, you adjust the propagation delay attribute, which determines the outbound MCTD experienced by connections transiting an interface. This is used by CAC in admitting CBR and VBR-RT connections

Changing the outbound link distance from its default of zero can affect the SVC requests accepted. For example, you might want to discourage use of a transcontinental link by configuring a higher propagation delay.

Configuration Overview

Configuring the outbound link distance requires the following steps:

Step 2   Specify an outbound link distance in kilometers.

Limits of Best-Effort Connections

By limiting best-effort connections, you place a maximum on the number of ABR and UBR connections to admit on an interface. This allows you to control the number of connections that can have less specified and predictable bandwidth requirements.

Configuration Overview

Configuring the limits of best-effort connections requires the following steps:

Step 2   Specify the maximum number of best-effort connections to allow.

Maximum of Individual Traffic Parameters

You can set maximum values for traffic parameters that are allowed by CAC. Connection requests that exceed the configured maximums on the interface are refused. These traffic parameter limits can be configured independently by service category and traffic direction (receive and transmit). You can specify maximum values for PCR, SCR, MCR, CDVT, and MBS.

Configuration Overview

Configuring the maximum traffic parameters on an interface requires the following steps:

Step 2   Do one or more of the following steps for the receive direction, transmit direction, or both, on the interface:

Interface Service Category Supported

This feature allows you to configure which service categories CAC allows on an interface. It allows you explicitly to permit or deny any of the CBR, VBR-RT, VBR-NRT, ABR, and UBR traffic categories.

Interface service category configuration is supported only on physical interfaces and shaped and hierarchical VP tunnel interfaces. The underlying PVP for shaped and hierarchical VP tunnel logical interfaces must use the CBR service category. The default for shaped VP tunnels is to allow only CBR virtual connections to transit the interface. However, interface service category configuration can be used to specify a service category other than CBR for virtual connections within a shaped VP tunnel.

The transit VP—the VP that connects the tunnel across the service provider network—must also have a service category. Table 10-6 shows the service category of the shaped VP tunnel (always CBR), the service categories you can configure for transported virtual connections, and a suggested transit VP service category for the tunnel.

The default for physical interfaces and hierarchical VP tunnels is to allow virtual connections of any service category to transit the interface. However, interface service category configuration can be used explicitly to allow or prevent virtual connections of specified service categories to migrate across the interface.

Configuration Overview

The restrictions that apply to interface service category support are summarized as follows:

• Configuration is allowed on physical interfaces and shaped and hierarchical VP tunnel logical interfaces.
  
  
• On shaped VP tunnel logical interfaces, only one service category is permitted at a time. To replace CBR with another service category on these interfaces, you must first deny the CBR service category, then permit the chosen service category. To deny a service category, you must delete all user VCCs of that service category on the interface.
  
  
• For ABR and UBR, only zero MCR is supported for VCCs on a shaped VP tunnel.
  
  

Configuring interface service category support requires the following steps:

Step 2   Specify which traffic categories to deny on the interface.

Step 3   Specify which traffic categories to permit on the interface.

You can deny or permit any of the CBR, VBR-RT, VBR-NRT, UBR, and ABR traffic categories.

Interface Overbooking

The interface overbooking feature allows the available equivalent bandwidth of an interface to exceed the maximum cell rate (maxCR) or physical line rate on ATM and inverse multiplexing for ATM (IMA) interfaces. The available equivalent bandwidth is by default limited by the maxCR. Increasing the available equivalent bandwidth beyond the maxCR allows the configuration of more connections on an interface than its physical bandwidth would allow. Overbooking allows more flexibility when configuring an interface when the traffic over the interface will be less than the maxCR.

The following restrictions apply to interface overbooking:

• Regular (simple) VP tunnels do not support interface overbooking.
  
  
• You cannot add new hierarchical VP tunnels on a physical interface if the interface's bandwidth guarantees exceed the maxCR regardless of any overbooking configured on that interface.
  
  
• On IMA interfaces, the available equivalent bandwidth for PVCs differs from the available equivalent bandwidth for SVCs. The available equivalent bandwidth for PVCs is based on the number of interfaces configured as part of the IMA group. The available equivalent bandwidth for SVCs on an IMA interface is based on the number of interfaces that are active in the IMA group. Overbooking increases both the available equivalent bandwidth values by the same configured percentage.
  
  
• The MaxCR for transmit and receive flows might differ on output-paced physical interfaces. Configuring overbooking on such interfaces results in different maximum guaranteed services bandwidth values and available cell rates for service categories for transmit and receive flows. Maximum guaranteed services bandwidth is the maximum equivalent bandwidth allocated for guaranteed services on the interface.
  
  
• When an interface is overbooked with traffic, cell flow to the well-known virtual connections might be reduced.
  
  
• Although overbooking increases the available cell rates for various service categories on an interface, various traffic parameters of a connection are still limited by the maxCR.
  
  
• If the overbooking configuration results in a maximum guaranteed services bandwidth that is below the currently allocated bandwidth guarantees on an interface, the configuration is rejected.
  
  

	Caution Overbooking can cause interface traffic to exceed the guaranteed bandwidth that the switch can provide.

Configuration Overview

Configuring interface overbooking requires the following steps:

Step 2   Shut down the interface.

Step 3   Configure interface overbooking for CAC as a percentage of the maximum equivalent bandwidth.

Step 4   Reenable the interface.

Framing Overhead

The interface framing overhead feature determines whether the maxCR of a physical interface conforms to the actual physical line rate, including framing overhead. By default, the unframed rate is used for determining the maxCR.

When framing overhead is considered, maxCR is less than the unframed rate, and some previously configured connections might not be established. The maxCR differs by interface type and framing mode, and whether framing overhead is configured. Refer to the ATM Switch Router Software Configuration Guide for details.

Configuration Overview

To configure framing overhead for CAC you enter a single command to enable the feature. In some circumstances enabling framing overhead might reduce the maximum guaranteed service bandwidth supported on a direction of an interface to below the current allocation. In that event an option is available to force the configuration to take effect.

Hardware Resources

To help ensure that the terms of the traffic and service contract are honored, several mechanisms comes into play. These mechanisms allow for a network policy expressed through hardware and include the following:

• Policing—Usage Parameter Control (UPC)
  
  
• Cell queuing
  
  
• Congestion notification
  
  
• Output scheduling
  
  

Figure 10-1 shows the relationship between these mechanisms, which are discussed in the following sections.

For most applications, when one or more cells are dropped by the network, the corresponding packet becomes corrupted and useless. This results in the need to retransmit the many cells that comprise that packet, and leads to exacerbated congestion. For example, loss of a cell from an IP over ATM packet (RFC 1577) might require resending 192 ATM cells, given an MTU of 9 KB.

To maximize the number of complete delivered packets, the ATM switch router implements a unique tail packet discard and early packet discard (TPD/EPD) scheme that intelligently and selectively discards cells belonging to the same packet. These congestion control mechanisms reduce the effects of fragmentation and make the ATM switch router essentially emulate a packet switch, which discards entire packets.

UPC—Traffic Policing at a Network Boundary

Traffic policing, called UPC by the ATM Forum, monitors and controls the traffic on an ATM connection in terms of cell traffic volume and cell routing validity. The main purpose of UPC is to protect the network resources from abusive connections and to enforce the compliance of a connection to its negotiated traffic contract. UPC is implemented on the ATM switch router in conformance with the ATM Forum Generic Cell Rate Algorithm (GCRA).

UPC on the ATM switch router checks the following parameters:

• PCR and CDVT for CBR, UBR, and ABR connections
  
  
• SCR and MBS for VBR connections
  
  

On systems equipped with hardware to support the dual leaky bucket, there are two policers for VBR connections. One policer uses PCR and CDVT, while the other policer uses SCR and MBS.

Policing Actions and Mechanisms

When a cell is found to be nonconforming, one of the following actions can be triggered:

• Pass—the cell is untouched.
  
  
• Tag—the CLP bit is set in the cell header, tagging the cell for dropping before higher-priority cells.
  
  
• Drop—the cell is dropped.
  
  

CLP

The CLP bit in the ATM cell header can be used to generate different priority cell flows within a virtual connection. When UPC sets CLP = 1, the cell is more likely to experience loss during congestion. This allows a selective cell discarding scheme to be implemented to deal with network congestion.

Per-VCC and per-VPC UPC Behavior

The default UPC behavior is to pass nonconforming cells. You can configure UPC to pass or tag PVCs and SVCs.

Configuration Overview

Configuring the default UPC behavior requires the following steps:

Step 2   For SVCs, select an interface, enter interface configuration mode, and specify tag or drop.

Default CDVT and MBS

You can change the default CDVT and MBS for UPC of cells received on the interface for connections that do not individually request a CDVT or MBS value.

You can specify CDVT or MBS for PVCs through a connection traffic table row. If no CDVT or MBS is specified in the row, then a per-interface, per-service category default is applied for purposes of UPC on the connection.

Configuration Overview

Configuring the default CDVT and MBS on an interface requires the following steps:

Step 2   Specify a CDVT default value for a service category. You can repeat this step for additional service categories you want to configure.

Step 3   Specify an MBS default value for a service category. You can repeat this step for additional service categories you want to configure.

Cell Queuing

The ATM switch router allows for flexible control of queueing through configurable queue limits and thresholds.

The specific features available depend upon the hardware:

• For systems with the FC-PCQ
  
  
  • Oversubscription factor
    
    
  • Service category limit
    
    
  • Maximum queue size per interface
    
    
  • Interface queue thresholds per service category
    
    
• For systems with the FC-PFQ and the Catalyst 8540 MSR, threshold groups can be configured.
  
  

Oversubscription Factor

The switch oversubscription factor is used in determining initial port maximum queue size for VBR-NRT and ABR/UBR queues.

The size of the VBR-NRT queue and ABR/UBR queues is determined by using the oversubscription factor (OSF) in the following formula:

size (vbr-nrt) = .25 * ((osf * 2048) - DefaultSize (cbr) - DefaultSize (vbr-rt))
size (abr-ubr) = .75 * ((osf * 2048) - DefaultSize (cbr) - DefaultSize (vbr-rt))
 

When you configure the oversubscription factor, you are changing the default values. Refer to the ATM Switch Router Command Reference publication for details.

Configuration Overview

Configuring the oversubscription factor requires the following steps:

Step 2   To have the change take effect and resize the queues, save the running configuration to the startup configuration and restart the ATM switch router.

Service Category Limit

The service category limit configuration restricts the number of cells admitted into the switch by output queue type. Limits can be specified for CBR, VBR-RT, VBR-NRT, and ABR/UBR queues.

When you configure the service category limit requirements, you are specifying a new value to use rather than the default. To do so requires just one global configuration command in which you specify the limit in number of cells for a queue type. You repeat this command for each additional queue type for which you want to configure a maximum.

Maximum Queue Size Per Interface

The maximum queue size per interface is used to determine the maximum number of cells in the switch fabric queue. This also implicitly affects MCTD and ppCDV on an output interface. The values set for VBR-RT and ABR/UBR override any set with the oversubscription factor feature.

Configuration Overview

Configuring the maximum queue size for an interface requires the following steps:

Step 2   Specify an output queue maximum size for a service category. Repeat for additional service categories you want to configure.

Interface Queue Thresholds Per Service Category

The queue thresholds can be configured for the service categories on each interface queue.

The following queue thresholds can be configured per interface queue:

• Threshold at which the EFCI bit is set
  
  
• Threshold at which cells are eligible for CLP discard or EPD
  
  
• Threshold for relative rate (RR) marking on ABR connections
  
  

Configuration Overview

Configuring the interface queue threshold per service category requires the following steps:

Step 2   Specify an output percent for the EFCI marking threshold for a service category. You can repeat this step for additional service categories on the interface.

Step 3   Specify an output percent for the discard threshold for a service category. You can repeat this step for additional service categories on the interface.

Step 4   Specify an output threshold percent for RR marking on ABR connections.

Threshold Groups

A threshold group utilizes the memory efficiently among virtual connections of a particular traffic type. By default, each threshold group is programmed with a dynamic memory allocation profile that maps into the needs of the connections of a particular service class, and all the connections in a particular service category map into one threshold group.

Each threshold group has a set of eight regions, and each region has a set of thresholds. When these thresholds are exceeded, cells are dropped to maintain the integrity of the shared memory resource.

Threshold Group Defaults

The initial default configuration of per-VCC queuing on the ATM switch router has all the connections of a service category assigned to one threshold group. However, the assignment of service categories to threshold groups is configurable, with the following restrictions:

• A service category cannot be mapped to more than one threshold group.
  
  
• If you configure a service category to a threshold group more than once, only the last configuration is in effect.
  
  
• The default assigns each service category to a different threshold group. However, you can assign more than one service category to a threshold group.
  
  

The threshold group configuration parameters are as follows:

How It Works

As a threshold group congests (the cumulative number of cells on the queues of virtual connections in the threshold group approaches the configured max-cells value), the maximum number of cells per-queue shrinks from the threshold group max-queue-limit to min-queue-limit.

When congestion is in the range of 0 cells (uncongested) to 1/8th full, the connection queues are limited to max-queue-size. When congestion is in the range of 7/8ths full to completely full, the connection queues are limited to min-queue-size.

Configuration Overview

Configuring the threshold group parameters requires the following steps:

Step 2   Specify the maximum number of cells queued for all connections that are members of the threshold group.

Step 3   Specify the percent at which the per-connection queue is to be considered full for purposes of CLP discard and EPD.

Step 4   Specify the maximum per-connection queue limit (in number of cells) for the threshold group.

Step 5   Specify the minimum per-connection queue limit (in number of cells) for the threshold group.

Step 6   Specify a name to associate with the threshold group (optional).

Step 7   Specify the percent at which the per-connection queue is considered full for EFCI (on all connections) and RR marking (on ABR connections).

You can repeat these steps for any additional service category thresholds you want to configure.

Congestion Notification

The ATM switch router implements two methods to indicate and control congestion:

• EFCI marking (for non-ABR connections)—In this mode the ATM switch router sets the EFCI state in the headers of forward data cells to indicate congestion. When the destination receives an EFCI flag, it marks the congestion indication bit in the resource management cells to indicate congestion and sends the resource management cells back to the destination.
  
  
• Relative rate (RR) marking mode (for ABR connections)—In this mode an ATM switch router that experiences congestion can set the congestion indication bit directly in forward and backward resource management cells. RR marking mode is used only for ABR connections and is not supported on all hardware platforms.
  
  

EFCI and RR marking involve two important functions: detecting incipient congestion and providing selective feedback to the source. As with any feedback mechanism, congestion control schemes operate best when the latency of the feedback path is minimized. Thus RR mode, because of its ability to use backward RM cells to send the congestion indicator rather than relying on the destination end system to reflect it back, can greatly reduce feedback delays and deliver better performance than the EFCI mode.

The two modes can be used independently or in combination to support ABR traffic, and thresholds can be set to indicate when EFCI or RR marking should occur.

ABR Congestion Notification Mode

The ABR congestion notification mode is used to change the type of notification used on ABR connections to alert the end station of congestion. ABR mode configuration determines whether ABR uses EFCI marking, RR marking, or both, for forward and backward RM cells used to control ABR congestion. On systems that support RR mode, the ATM switch router uses that mode by default.

Configuration Overview

When you configure the ABR congestion notification mode you affect all ABR connections. To do so requires just one global configuration command.

Output Scheduling

Output scheduling determines which queued cell is chosen to be transmitted out an interface during a cell time, which is dependent upon the characteristics of the physical interface. The goal of output scheduling is to ensure that bandwidth guarantees are met and extra bandwidth is fairly shared among connections.

An additional benefit of output scheduling is the ability to shape traffic. This capability can be important when connecting across a public UNI to a public network, because many such networks base their tariffs on the maximum aggregate bandwidth. Traffic shaping and traffic policing are complementary functions, as illustrated in Figure 10-2.

Traffic shaping is a feature of shaped and hierarchical VP tunnels. See the "VP Tunnels" section in the chapter "Virtual Connections."

The following configurable features on the ATM switch router are used for output scheduling:

• Interface output pacing (systems with FC-PCQ)
  
  
• Scheduler and service class (systems with FC-PFQ, and the Catalyst 8540 MSR)
  
  

Interface Output Pacing

Output pacing is used to artificially reduce the output speed of an interface in kbps. Output pacing can be changed at any time or enabled or disabled.

Configuration Overview

Interface output pacing is disabled by default on the ATM switch router. Configuring the interface output pacing requires the following steps:

Step 2   Specify the output pacing limit as a a bit rate in Kbps.

This configuration does not take effect if the amount of bandwidth allocated to CBR and VBR connections in the transmit direction on the interface is greater than the configured output pacing value.

Scheduler and Service Class

For purposes of scheduling, up to eight QoS classes (zero to seven) can be allocated to each physical interface port. Each port has an independent logical rate scheduler (RS) and a weighted round robin (WRR) scheduler. The RS guarantees minimum bandwidth and has first priority on supplying an eligible cell for transmission. Second priority is given to the service classes that have been assigned relative weights based on the ratio of the total leftover bandwidth. The service class relative weights are configurable so that you can change the priority of the default values. The virtual connections within a service class also have relative weights. The service classes and virtual connections within a service class are scheduled by the WRR scheduler based on their relative weights.

In scheduling the next cell to be transmitted from a port, RS has first call on supplying an eligible cell. If RS does not have one, then the WRR scheduler chooses a service class with an output virtual connection ready to transmit, and finally a virtual connection within the service class is selected.

On the ATM switch router, the ATM service categories are mapped statically to service classes, as shown in Table 10-8; service class 2 has the highest scheduling priority.

A different set of service classes is used for tag switching virtual connections; see the "Tag Switching CoS" section of the chapter "Tag Switching."

The first scheduling decision is made based on whether any rate-scheduled cell is ready (as decided by the timewheel rate scheduler for an interface). Whether a virtual connection uses the rate scheduler is not user-configurable.

Table 10-9 lists the cell rates that are guaranteed by the rate scheduler for each service category.

If the timewheel RS does not have an output virtual circuit ready to transmit, the WRR scheduler becomes active to pick out a virtual circuit to transmit a cell. The WRR scheduler uses the interface bandwidth left over after guaranteed cell service to transmit cells. Thus, an output virtual circuit of a service category other than CBR can be serviced by both the rate scheduler and the WRR scheduler. A CBR output virtual circuit cannot be serviced by the WRR scheduler because its PCR is already guaranteed by the rate scheduler. (Any additional cell transmission by the WRR scheduler out of that output virtual circuit is likely to arrive too soon at the next switch and might be policed.)

The following service categories can be serviced by the WRR scheduler:

• VBR-RT
  
  
• VBR-NRT
  
  
• ABR
  
  
• UBR
  
  

The combined result of the two schedulers is illustrated in Figure 10-3.

Each service class is assigned a weight. These weights are configurable, in the range of 1 to 15. The default weighting is {15,2,2,2} for classes {2,3,4,5}, respectively. The weighting is not modified dynamically.

Within service classes, individual output virtual circuits are also weighted, again in the range of
1 to 15. A standard weight (2) is assigned to all PVCs in a service class. Optionally, PVCs can be configured with a specific weight per half-leg (applying to the transmit output virtual circuit weight). SVCs take the value 2.

Configuration Overview

Configuring the service class weights on an interface requires the following steps:

Step 2   Specify a service class and a weight value. You can repeat this step for additional service classes.