Smarts Discovery is not layering sub-interfaces below the ATM interfaces

Products

• VMware Smart Assurance

Issue/Introduction

Symptoms:


How is ATM interface layering determined in Smarts?
Why are ATM interfaces isolated in Smarts when they should have been layered?


Smarts Discovery is not layering sub-interfaces below the ATM interfaces
Smarts generates multiple alarms when the ATM interface is brought down

Environment

VMware Smart Assurance - SMARTS

Cause

Smarts generates multiple alarms when the ATM interface is brought down because Smarts is discovering a router's ATM and sub-interfaces, but not layering them to each other. When there is no layering between the ATM and its sub-interface, multiple alarms are generated.

Resolution

When the SNMP Agent is not reporting the layering information in the IfStackStatus MIB (.1.3.6.1.2.1.31.1.2.1.3), you need to check the configuration of the device.

Smarts interface layering example
Interface 69 (IF69) is an AAL5 interface. These types of interfaces are typically layered over an ATM sub-interface which in turn is layered over the ATM interface. A good example of this in a normal topology would be IF66 which is layered over IF64 which is layered over IF63. which is layered over IF62. If IF62 were down, we would alarm on IF62, and not on everything else overlaying IF62. 

                                                                                                                     ------------------ IP
                                                                                                                     "      
                                                                                         ------------ AAL5(66)
                                                                                         "
                                                   ------------- ATM SubIf(64)
                                                   "
                    --------------- ATM(63)
                    "
                IF(62)

If you look at the ifStackStatus in the MIB, you will see:

.1.3.6.1.2.1.31.1.2.1.3.66.64: 1
.1.3.6.1.2.1.31.1.2.1.3.64.63: 1
.1.3.6.1.2.1.31.1.2.1.3.63.62: 1

In a topology where layering is missing however, IF69 is layered over IF68 which has nothing underlying it. Then there's IF67 on its own:

 
                                                                                                                           ----------------- IP
                                                                                                                           "     
                                                                                            -------------- AAL5(69)
                                                                                            "
                                                  \ -------------- ATM SubIf(68)
                                                   \"
                                                    \
                                                    "\
                     ------------- ATM(67)  \

In the MIB, you will see:

.1.3.6.1.2.1.31.1.2.1.3.69.68: 1
.1.3.6.1.2.1.31.1.2.1.3.68.0: 1
.1.3.6.1.2.1.31.1.2.1.3.67.0: 1
 

In this case, we will alarm on IF67 and IF68 as the layering data is missing from the MIB. We will not alarm on IF69 because we know that IF69 is layered over IF68.