Technical data

Configuring SNMP
13.4 Configuring SNMP
On startup, each subagent first sets up a default session timeout (see the
AGENTX_SESSION_TIMEOUT option). It then registers its MIB regions. The
subagent can register each of its MIB regions with a different timeout. A value of
0 causes the session timeout for the entire subagent to be used.
The master agent listens for SNMP requests. The timeout value is 10 seconds,
unless the SNMP_MASTER_TIMEOUT option has been defined. After a timeout
occurs, the master agent updates counters, checks for requests, then loops to wait
for another SNMP request. When an SNMP request arrives, the master agent
determines which if any registered subagents can handle it. It then resets the
SNMP_MASTER_TIMEOUT timeout to use the maximum of the timeouts for all
MIB regions involved.
When it is not processing an SNMP request, a subagent may send
are_you_there
messages to the master agent at a default interval determined by the subagent.
For the chess example, the default is 30 seconds; for the OS_MIBS and HR_MIB
subagents, the default is 5400 seconds (90 minutes). Both values are derived
from those used in the UNIX implementation of SNMP; the second value was set
high to minimize system overhead.
The following relationships among configuration option values are recommended
but are not enforced. See the descriptions of the specific options for details.
• SNMP_ARE_YOU_THERE_TIME and SNMP_INACT_TIME
The SNMP_ARE_YOU_THERE_TIME option determines the time between
are_you_there messages. If the SNMP_INACT_TERM option is set, and
if the master agent does not receive any SNMP request or are_you_there
mesages from a subagent during the time associated with the SNMP_INACT_
TIME option, the master agent automatically exits. By default, the SNMP_
INACT_TERM option is not set.
If the SNMP_ARE_YOU_THERE_TIME option is not set and no external
SNMP requests are received, the master agent will exit even if subagents are
still active.
• SNMP_INACT_TIME and SNMP_POLL_TIME
The values for these options should be a multiple of the value of the SNMP_
MASTER_TIMEOUT option.
The master agent checks whether these intervals have elapsed after the time
specified by the SNMP_MASTER_TIMEOUT option. Therefore, a value for
these two options that is not a multiple of SNMP_MASTER_TIMEOUT will
have the same effect as one that is the next higher multiple.
• The client should allow a large enough timeout interval to accommodate
the server to avoid query failures or unnecessary retries. Particular care is
required when network load is high and when communicating with heavily
used servers and those in which tracing is turned on. See Table 13–6 for
details on using trace.
Configuring SNMP 13–17