# Unexpected ATM Behavior During Unified ISSU

The following aspects of ATM behavior during unified ISSU are different than the behavior during normal router operations.

## ILMI Sessions Not Maintained

The router does not maintain ILMI sessions during a unified in-service software upgrade. The router terminates all ILMI sessions and restarts them during the upgrade. If the ILMI protocol is enabled on any port, you are warned during the initialization phase when unified ISSU is verifying the prerequisites for the upgrade. You can choose to continue the upgrade—and bring down the sessions—or to halt the unified in-service software upgrade.

## OAM CC Effects on VCC

When an ATM VC is configured as an OAM CC source, periodic OAM cells are generated for about 15 seconds. The device configured as the OAM CC sink is likely to declare the VCC down during this time. Unified ISSU generates a warning when it detects an OAM CC source configuration during the initialization phase while unified ISSU is verifying the prerequisites for the upgrade. You can choose to continue or halt the unified in-service software upgrade

When an ATM VC is configured as OAM CC sink, it cannot receive OAM CC cells generated by the source for about 15 seconds. The OAM CC does not time out and the VCC is not declared down. OAM CC cell generation resumes when the unified ISSU operation is completed.

## OAM VC Integrity Verification Cessation

During the unified ISSU operation, verification of OAM VC integrity stops. This verification resumes when the unified ISSU operation is completed.

ATM does not respond to incoming OAM loopback cells during the upgrade for a period of less than 30 seconds. The lack of response might cause ATM peers to declare VCC (VPC) down.

## Port Data Rate Monitoring Cessation

The monitoring of ATM port data rates is halted during a unified in-service software upgrade. Monitoring resumes immediately after the unified ISSU operation is completed. The data rates reported by the show atm interface command are inaccurate for the period of one configured load interval after unified ISSU is completed.

## VC and VP Statistics Monitoring Halts Unified ISSU Progress

A unified in-service software upgrade cannot proceed if VC or VP statistics monitoring is in progress.

#### Related Documentation

- Application Support for Unified ISSU