Troubleshooting Network and Server Probes
How do I change the protocol that Intermapper polls with?
- Select the device you want to change.
- From the Monitor menu, select Info Window.
- From the Probe Type menu, select a new probe type. If parameters are required, a parameters window is displayed for the selected probe type.
- Enter parameters as needed and click OK. The device is polled using the new probe type.
For more information, see Status Windows.
What MIB variables does Intermapper poll?
Anytime Intermapper displays traffic for a link, (using the SNMP Traffic Probe, for example) it polls the following variables:
SNMPv1
When you set the SNMP Version to SNMPv1, the following variables are queried:
Intermapper examines these two variables to decide whether an interface is up or down:
| MIB Variable | OID | SNMP Version |
|---|---|---|
| ifAdminStatus | 1.3.6.1.2.1.2.2.1.7 | SNMPv1 |
| ifOperStatus | 1.3.6.1.2.1.2.2.1.8 | SNMPv1 |
Intermapper examines these variables to detect error conditions:
| MIB Variable | OID | SNMP Version |
|---|---|---|
| ifInDiscards | 1.3.6.1.2.1.2.2.1.13 | SNMPv1 |
| ifInErrors | 1.3.6.1.2.1.2.2.1.14 | SNMPv1 |
| ifOutDiscards | 1.3.6.1.2.1.2.2.1.19 | SNMPv1 |
| ifOutErrors | 1.3.6.1.2.1.2.2.1.20 | SNMPv1 |
SNMPv2c
When you set the SNMP Version to SNMPv2c, the following variables are queried:
This variable set is used on an initial scan of the device.
This variable set is polled to display statistics for the device's operation.
How does Intermapper compute traffic statistics?
Intermapper uses ifInOctets and ifOutOctets to compute the Receive and Transmit bytes/second values, respectively.
The Receive and Transmit packets/second numbers are computed using the sum of the (ifInUcastPkts + ifInNUcastPkts) and (ifOutUcastPkts + ifOutNUcastPkts) respectively.
How does Intermapper compute Utilization for a link?
Intermapper queries a device at specified intervals, and requests a number of SNMP MIB variables. To compute utilization, Intermapper does the following:
- It queries ifInOctets (and ifOutOctets) and the sysUpTime and ifSpeed variables.
- It subtracts the octet counts from successive samples, and divides by the difference in the sysUpTime samples to compute a byte/second rate.
- It divides the result by the ifSpeed variable to compute a percentage of the link's capacity/bandwidth. (If the user has overridden the ifSpeed variable, Intermapper uses the user-entered value.)
- If a network is using a shared baseband link (such
as Ethernet, wireless, etc.) Intermapper compares the sum of the transmitted
and received bytes/second against the link speed to get the utilization.
If it's a full-duplex link (such as a frame relay, T-1 or T3, ATM, etc.) then Intermapper compares the higher of the transmitted or received data rate against the link speed.
How does Intermapper compute errors for a link?
Q:
"I see the Received Discards/Minute and Percent Err values for an ATM AAL5 interface are non-zero and I would like to know which variables were used, and what calculation was used to arrive at these numbers.
"We are also graphing the Percent Err: value. This figure is showing errors and my Cisco support folks wanted to know which MIB variables go into the calculation of this percentage and how they are combined to create this number."
A: The Percent Err values are computed as follows:
The one-way percent errors under the Receive section are computed by totalling { ifInUcastPkts, ifInNUcastPkts, ifInErrors, ifInDiscards } as follows:
PERCENT ERROR = totalErrors / totalPkts;
where:
totalErrors = dErrs + dDis and
totalPkts = dUcast + dNUcast + totalErrors
and:
dUcast = ifCurrStats.inUcastPkts - ifPrevStats.inUcastPkts
dNUcast = ifCurrStats.inNUcastPkts - ifPrevStats.inNUcastPkts
dErrs = ifCurrStats.inErrors - ifPrevStats.inErrors
dDis = ifCurrStats.inDiscards - ifPrevStats.inDiscards
The one-way percent errors for outgoing
traffic are similarly computed from the
{ ifOutUcastPkts, ifOutNUcastPkts, ifOutErrors, ifOutDiscards }
statistics.
The two-way Percent error number (just below Utilization on the Interface information menu) is the probability given both one-way error percentages that a packet will be lost making the round-trip across the link and back. If the probability of successful transmission is T and the probability of successful receipt is R (and assuming the act of transmission and receive are relatively independent), then the probability of a successful round-trip is T * R. The probability of error is (1 - T*R).
T and R are computed from the complement of the one-way percent errors above.
Why can't I get a DHCP probe on OSX to work?
When running Intermapper on macOS, disable DHCP and PPP and assign a manually assigned static address to the computer running Intermapper.
To disable DHCP and PPP for all interfaces:
- Open the Network settings in the System Preferences application.
- From the Show menu, select Network Port Configurations.
- Disable any ports that have been configured to use DHCP or PPP, even if nothing is plugged into them and they aren't currently being used.
- If DHCP or PPP is enabled on any interface of your machine, the process "configd" will open UDP port 68, and prevent Intermapper from using it. You can use the Terminal application to test if configd has port 68 open. Type 'sudo lsof -i | grep bootpc' and press return. If configd is listed, you still have DHCP running.
- If Intermapper still marks the device as down after making these changes, you may need to use a DHCP Message Type of "DHCP-Discover" instead of the default "DHCP-Inform". This setting can be toggled in the DHCP/Bootp probe parameters dialog.
If I look at the traffic on a link, wait five seconds, and look again, the traffic rates are the same. Shouldn't these numbers be updated?
The traffic statistics are samples: the numbers do not change until after Intermapper probes the device again.
How does Intermapper compute byte and packet rates?
SNMP only supplies counts of bytes, packets, or errors, etc. that have passed through or occurred in an interface. These counts increment "forever" (or until the counter rolls over to zero like a car's odometer).
During each poll, Intermapper collects the total traffic and computes the difference with the total traffic from the previous poll. It then divides by the amount of time that has passed to compute the rate (per second or per minute).
Technical note: Even when a counter rolls over (e.g., from 999 to 000), Intermapper will compute the traffic rates accurately. Let's say the two successive samples are 995 and 003. Intermapper subtracts the previous count (995) from the new count (003), assumes that the "003" is actually "1003", and gets the proper difference of 8. Although the counters in the SNMP MIB variable are binary numbers, the same arithmetic principles hold. Thus Intermapper can compute these rates accurately.
How does Intermapper compute time intervals?
To compute the elapsed time accurately, Intermapper uses the sysUpTime variable of the device as a timestamp to calculate the time that has elapsed between subsequent two polls. The time elapsed should roughly correspond to the poll interval; however, it is possible for polls to be delayed occasionally so using the change in sysUpTime to measure the elapsed time is more accurate.