Private Network-Network Interface (PNNI)

Document ID: 22886

Contents

Introduction
Prerequisites
      Requirements
      Components Used
      Conventions
What is AvCR?
How AvCR Works
      Example
Problem
Solution
Related Information

Introduction

The Private Network-Node Interface (PNNI) protocol is designed to distribute topology information between ATM switches and to establish dynamic connections across ATM networks. The Available Cell Rate (AvCR) attribute is one of the most important PNNI topology state parameters.

This document explains how to check, calculate, and troubleshoot AvCR when switched virtual circuits (SVCs) or Soft permanent virtual circuits (Soft-PVCs) are not created due to unavailable cell rate.

Prerequisites

Requirements

There are no specific requirements for this document.

Components Used

This document is not restricted to specific software and hardware versions.

Conventions

For more information on document conventions, refer to the Cisco Technical Tips Conventions.

What is AvCR?

AvCR is a topology state attribute that measures an available link capacity. This attribute is used to determine whether a given link or node is acceptable and/or desirable to carry a connection. Every ATM switch calculates this value and exchanges it between other ATM switches.

How AvCR Works

ATM switches do not update AvCR attributes immediately after a change has occurred in the switch's resources. Every virtual circuit (VC) creation/deletion changes a local available bit rate value. However, the switch waits until the changes in AvCR become significant before it updates this value. As AvCR approaches a value of zero, changes in AvCR are more important. For example, transitioning AvCR from a non-zero value to zero implies that no calls are processed where some were processed before. This is an important change. Likewise, transitioning from zero to non-zero values implies that some number of calls are now processed where none were processed before. This is an equally important change.

Changes in AvCR are measured in terms of a proportional difference from the last value advertised. An AvCR proportional multiplier (AvCR_PM) parameter, expressed as a percentage, provides flexible control over the definition of "significant change" for AvCR. There is also an AvCR minimum threshold (AvCR_MT) parameter, expressed as a percentage of maximum cell rate (maxCR). This ensures that the range of insignificance is non-zero. Given a previous value for AvCR, the algorithm establishes an upper and lower bound for AvCR values. These define a range of insignificance. Any new value for AvCR computed that is within these bounds is not considered a significant change from the previous value. Any new value for AvCR that is outside the bounds is considered a significant change.

Example

This example presupposes that there are no VCs traveling through the ATM OC3 interface. A certain number of CBR connections with PCR = 10 Mb must be created.

These are the initial values for all variables required to calculate how many VCs are created before AvCR is updated:

With the help of the values from the table and in the algorithm given in the flowchart above, you obtain this:

delta = Prev_AvCR*(AvCR_PM/100) = 147743 * ( 50 / 100 ) = 73871
min_delta = maxCR*(AvCR_MT/100) = 155519 * ( 3 / 100 ) = 4665


Since 73871 > 4665, then, delta > min_delta.
Then Upper AvCR = Prev_AvCR+delta = 221614

Since MaxCR = 155519 and Upper AvCR = 221614, then Upper AvCR > MaxCR 
Therefore Upper AvCR = 155519

Now, since delta = 73871 and Prev_AvCR = 14774, then delta < Prev_AvCR
Therefore 

Lower AvCR = Prev_AvCR - delta = 147743 - 73871 = 73872

Based on the formulas above, AvCR is updated if a new value of AvCR is lower or equal to 73872. In addition, at least eight CBR VC should be created to fulfill this condition.

These are seven CBR VCs that are created:

Casimir#show atm vc conn-type soft-vc 
Interface         VPI  VCI   Type   X-Interface      X-VPI X-VCI Encap  Status 
ATM0/0/2          0    251   SoftVC ATM0/1/3         0     62           UP
ATM0/0/2          0    252   SoftVC ATM0/1/3         0     63           UP
ATM0/0/2          0    253   SoftVC ATM0/1/3         0     64           UP
ATM0/0/2          0    254   SoftVC ATM0/1/3         0     65           UP
ATM0/0/2          0    255   SoftVC ATM0/1/3         0     66           UP
ATM0/0/2          0    256   SoftVC ATM0/1/3         0     67           UP
ATM0/0/2          0    257   SoftVC ATM0/1/3         0     68           UP

Casimir#show atm pnni resource-info a0/1/3
PNNI:56.1 Insignificant change parameters
acr pm 50,  acr mt 3, cdv pm 25, ctd pm 50, resource poll interval 5 sec
Interface insignificant change bounds shown in square brackets
Interface 80103000 (ATM0/1/3)
  CBR    : MCR 155519, ACR 147743 [73871,155519], CTD 50 [25,75],

[ snip ]

Casimir#show atm interface resource a0/1/3

[ snip ]

Resource Management state:
    Available bit rates (in Kbps):
        77743 cbr RX, 77743 cbr TX, 77743 vbr RX, 77743 vbr TX, 
        77743 abr RX, 77743 abr TX, 77743 ubr RX, 77743 ubr TX
    Allocated bit rates:
        70000 cbr RX, 70000 cbr TX, 0 vbr RX, 0 vbr TX, 
        0 abr RX, 0 abr TX, 0 ubr RX, 0 ubr TX
    Best effort connections: 0 pvcs,  0 svcs

These are eight CBR VCs that are created:

Casimir#show atm vc conn-type soft-vc       
Interface         VPI  VCI   Type   X-Interface      X-VPI X-VCI Encap  Status 
ATM0/0/2          0    251   SoftVC ATM0/1/3         0     62           UP
ATM0/0/2          0    252   SoftVC ATM0/1/3         0     63           UP
ATM0/0/2          0    253   SoftVC ATM0/1/3         0     64           UP
ATM0/0/2          0    254   SoftVC ATM0/1/3         0     65           UP
ATM0/0/2          0    255   SoftVC ATM0/1/3         0     66           UP
ATM0/0/2          0    256   SoftVC ATM0/1/3         0     67           UP
ATM0/0/2          0    257   SoftVC ATM0/1/3         0     68           UP
ATM0/0/2          0    258   SoftVC ATM0/1/3         0     69           UP

Casimir#show atm pnni resource-info a0/1/3
PNNI:56.1 Insignificant change parameters
acr pm 50,  acr mt 3, cdv pm 25, ctd pm 50, resource poll interval 5 sec
Interface insignificant change bounds shown in square brackets
Interface 80103000 (ATM0/1/3)
  CBR    : MCR 155519, ACR 67743 [33871,101614], CTD 50 [25,75],

[ snip ]

Casimir#show atm interface resource a0/1/3

[ snip ]

Resource Management state:
    Available bit rates (in Kbps):
        67743 cbr RX, 67743 cbr TX, 67743 vbr RX, 67743 vbr TX, 
        67743 abr RX, 67743 abr TX, 67743 ubr RX, 67743 ubr TX
    Allocated bit rates:
        80000 cbr RX, 80000 cbr TX, 0 vbr RX, 0 vbr TX, 
        0 abr RX, 0 abr TX, 0 ubr RX, 0 ubr TX
    Best effort connections: 0 pvcs,  0 svcs

Problem

An AvCR rate parameter is not updated if changes made are insignificant. A problem also arises if SVC/Soft-PVC is not created due to the unavailability of a requested cell rate, even though there are enough resources to process the call setup.

In order to illustrate and troubleshoot this problem, this setup is used:

Solution

To solve the problem of the AvCR rate parameter not updating if changes made are insignificant, perform these :

1. Shut/no shut an interface where the AvCR attribute information is inconsistent with the PNNI topology information. AvCR is then recalculated from scratch. In the example , reset interfaces a0/1/3 at Casimir or a4/1/4 at LaaLaa.

2. Tune the AvCR_PM and AvCR_MT parameters so that an insignificant change in AvCR is declared as significant.

   • In an ATM environment with a high number of VCs created and removed, these changes overflow the network through a high volume of PNNI updates and resource recalculations.

   • The changes should be done on all switches. Otherwise different AvCR values are received in different directions.

   Example:

   LaaLaa
   ! node 1 level 56 lowest ptse significant-change acr-pm 10 !--- AvCR_PM is changed to 10%. !

The output of the show atm pnni resource-info command displays insignificant change parameters:

Casimir#show atm pnni resource-info a0/1/3
PNNI:56.1 Insignificant change parameters
acr pm 10,  acr mt 3, cdv pm 25, ctd pm 50, resource poll interval 5 sec

[ snip ]

Related Information

• ATM Support Resources
• ATM Forum PNNI 1.0 Specification
• Configuring ATM Routing and PNNI
• Technical Support - Cisco Systems