Foundry NetIron M2404C and M2404F Metro Access Switches
Configuring SNMP (Rev. 03)
Simple Network Management Protocol
© 2008 Foundry Networks, Inc.
Page 5 of 48
that receives an Inform request acknowledges the message with an SNMP response PDU. If the
manager does not receive an Inform request, it does not send a response. If the sender does not
receive a response after a particular time interval, the Inform request can be sent again.
Because they are more reliable, informs consume more resources in the device and in the network.
Unlike a trap, which is discarded as soon as it is sent, an Inform request must be held in memory
until a response is received or the request times out. Also, traps are sent only once, while an Inform
may be retried several times. The retries increase traffic and contribute to a higher overhead on the
network. Thus, traps and Inform requests provide a trade-off between reliability and resources. If it
is important that the SNMP Manager receives every notification, use Inform requests. On the other
hand, if the user is concerned about traffic on the network or memory in the router and the user
does not need to receive every notification, use traps.
Figure 2
through
Figure 5
illustrate the differences between traps and Inform requests.
In
Figure 2
, the agent successfully sends a trap to the SNMP Manager. Although the manager
receives the trap, it does not send any acknowledgment to the agent. The agent has no way of
knowing whether the trap reached its destination.
Figure 2: Trap Sent to SNMP Manager Successfully
In
Figure 3
, the agent successfully sends an Inform request to the manager. When the manager
receives the Inform request, it sends a response back to the agent. Thus, the agent knows that the
Inform request successfully reached its destination. Notice that, in this example, twice as much
traffic is generated as in
Figure 2
; however, the agent is sure that the manager received the
notification.
Figure 3: Inform Request Sent to SNMP Manager Successfully
In
Figure 4
, the agent sends a trap to the manager, but the trap does not reach the manager. Since
the agent has no way of knowing whether the trap reached its destination, the trap is not sent again.
The manager never receives the trap.