ERROR: Alarm signal (Nagios time-out)


We have our Nagios box configured for monitoring 600 Odd servers for around 180 countries across the globe. But all of a sudden last week around 400 servers we started getting 2 types of errors as mentioned below .

**ERROR: Alarm signal (Nagios time-out) **

ERROR: Description table : No response from remote host

These 400 servers are domino servers & these checks work through SNMP on UDP ports 161 & 162.

Remaining 200 servers which are windows servers & SNMP query through TCP is working fine.

*Can anybody please help me to figure out What could be the possible reasons for these errors ? *