Cisco Routers and Switches with L3 routing functions are seen to have problems with High CPU usage when SNMP is enabled. This can range anything from 15% to 40%. According to Cisco, these are low priority processes and other priority processes requiring CPU cycles are given priority over these processes and this level of CPU utilisation can be is normal. However, it is always better to be safe than sorry and get the CPU utilisation caused by SNMP to bare minimum so as to ensure the Routers function smoothly.

The reason behind the High CPU usage can be caused by the Network Management Server (SNMP Server) like HP Openview querying for the Routing Tables and ARP tables to learn about other networks  or querying for certain MIBs which can be resource intensive.

We can use the SNMP View configurations to include/exclude SNMP MIBs in Cisco IOS. So modify your SNMP configuration as follows:

Include the SNMP Root (ISO)

ciscorouter(config)# snmp-server view cutdown iso included

Exlcude queries for Routing Table & ARP Table

We exclude the ipRouteTable & ipNetToMediaTable MIBs to avoid polliong for Routing tables and ARP tables.

ciscorouter(config)# snmp-server view cutdown 1.3.6.1.2.1.4.21 excluded
ciscorouter(config)# snmp-server view cutdown 1.3.6.1.2.1.4.22 excluded
ciscorouter(config)# snmp-server view cutdown 1.3.6.1.2.1.3 excluded

Exclude CPU Intensive snmpUsmMIB, snmpVacmMIB,snmpCommunityMIB

These are good to disable for Security reasons as well.

ciscorouter(config)# snmp-server view cutdown 1.3.6.1.6.3.15 excluded
ciscorouter(config)# snmp-server view cutdown 1.3.6.1.6.3.16 excluded
ciscorouter(config)# snmp-server view cutdown 1.3.6.1.6.3.18 excluded

Configure SNMP Community

Configure the SNMP community with the cutdown view as follows:

ciscorouter(config)# snmp-server community public view cutdown RO
ciscorouter(config)# snmp-server community myprivate view cutdown RW

This should help. For simple SNMP configurations, click here

For official Cisco Documentation click here

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