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SNMP low priority?

Hi,

I just want to clarify that I've read snmp is a low priority process. So does this means in case of high cpu any other process is going to take cpu utilization and snmp polling is going to stop? If not then what's gonna happen to polling and does polling in high cpu scenario is going to crash the device?

what is normal cpu utilization and above what % it should be considered high cpu?

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Silver

Re: SNMP low priority?

Hi,

Yes, you are correct, SNMP runs with a low priority.  So, during times of high CPU load, SNMP queries may get dropped or time out.

The Network Management station queries routers for their entire route  table to learn about other networks. It uses this information to find  other routers and query them about their knowledge of networks around  them. In this fashion, the management station can learn the topology of  the entire network.

The router stores the route table in a hashed format, more conducive  to quick route searches. However, SNMP responses for the route are  required to be returned in lexicographical order per RFC1213. Therefore,  for each SNMP request the router receives, the hash table must be  sorted lexicographically before a SNMP response PDU can be built. The  larger the route table, the more CPU intensive the sort.

SNMP is a low priority process as far as the CPU scheduler is  concerned, so another process requiring CPU resources takes priority.  Therefore, while CPU spikes occur in this scenario, they should not  affect performance.

If curious about current SNMP usage:

show process cpu

You can also see the OIDs being referenced with

debug snmp

Take care if using  debugs as they may overwhelm your device.

If you are running into %SNMP-3-CPUHOG: messages then there are two probable causes in relation to SNMP:

  1. Large route and/or ARP tables being polled by the NMS station

  2. Polling certain MIBs

For case #1

To avoid performance issues, force the router  to prematurely end queries for the route table from the network  management system server. Configure the router to respond with a complete message as soon as it receives the start of a request for the route table, as follows:

snmp-server view cutdown iso included

!--- ISO is used as a starting point as it is the snmp 'root'.

snmp-server view cutdown 1.3.6.1.2.1.4.21 excluded

snmp-server view cutdown 1.3.6.1.2.1.4.22 excluded

snmp-server view cutdown 1.3.6.1.2.1.3 excluded

snmp-server view cutdown 1.3.6.1.6.3.15 excluded

!--- The additional object snmpUsmMIB is excluded.

snmp-server view cutdown 1.3.6.1.6.3.16 excluded

!--- The additional object snmpVacmMIB is excluded.

snmp-server view cutdown 1.3.6.1.6.3.18 excluded

!--- The additional object snmpCommunityMIB is excluded.

snmp-server community public view cutdown RO

snmp-server community private view cutdown RW

For #2

Network Management Applications frequently  retrieve information from devices using SNMP. One example would be  Resource Manager Essentials (RME), an applications within the CiscoWorks  2000 suite of products. RME can retrieve flash information defined in  the CISCO-FLASH-MIB. If the device has an ATA flash disk, it can cause CPU utilization to jump.

Follow the bug ID link below and see detailed bug information.

CSCdt97325 (registered customers only)

To verify and apply the workaround, follow these steps:

  1. Verify if the device has an ATA Flash Card in one of its slots:

Router#show disk1:

******** ATA Flash Card Geometry/Format Info ********

ATA CARD GEOMETRY

    Number of Heads:        12

    Number of Cylinders     906

    Sectors per Cylinder    63

    Sector Size             512

    Total Sectors           684936

ATA CARD FORMAT

    Number of FAT Sectors  84

    Sectors Per Cluster    32

    Number of Clusters     21372

    Number of Data Sectors 684117

    Base Root Sector       169

    Base FAT Sector        1

    Base Data Sector       201

Router#

  1. If you're not sure or if you do not get output for the show diskX command, run the show tech command and do a search for disk.

  2. Once you know you have an ATA Flash Disk, you can apply the following workaround to prevent the SNMP queries on the FLASH-MIB:

Router(config)#snmp-server view iso included

!--- ISO is used as a starting point as it is the snmp 'root'.

Router(config)#snmp-server view ciscoFlashMIB excluded

!--- The additional object snmpUsmMIB is excluded.

Router(config)#snmp-server view 1.3.6.1.6.3.15 excluded

!--- The additional object snmpVacmMIB is excluded.

Router(config)#snmp-server view 1.3.6.1.6.3.16 excluded

!--- The additional object snmpCommunityMIB is excluded.

Router(config)#snmp-server view 1.3.6.1.6.3.18 excluded

Router(config)#snmp-server community view ro

Router(config)#snmp-server community view rw

Router(config)#exit

Note: Choose the same word on every line for the in the above configuration.

caution Caution: In the above example, snmpUsmMIB is excluded, snmpVacmMIB is excluded, and snmpCommunityMIB is excluded. Together these objects can be used to get information  about configured users and community strings to gain administrative  access to the device. It is recommended that these objects be exlcluded  from view on any device which can be accessed by public users.

Other bugs related to polling MIBs:

Follow the Cisco bug ID links below and see detailed bug information.

You can find most of the above here.

3 REPLIES
Silver

Re: SNMP low priority?

Hi,

Yes, you are correct, SNMP runs with a low priority.  So, during times of high CPU load, SNMP queries may get dropped or time out.

The Network Management station queries routers for their entire route  table to learn about other networks. It uses this information to find  other routers and query them about their knowledge of networks around  them. In this fashion, the management station can learn the topology of  the entire network.

The router stores the route table in a hashed format, more conducive  to quick route searches. However, SNMP responses for the route are  required to be returned in lexicographical order per RFC1213. Therefore,  for each SNMP request the router receives, the hash table must be  sorted lexicographically before a SNMP response PDU can be built. The  larger the route table, the more CPU intensive the sort.

SNMP is a low priority process as far as the CPU scheduler is  concerned, so another process requiring CPU resources takes priority.  Therefore, while CPU spikes occur in this scenario, they should not  affect performance.

If curious about current SNMP usage:

show process cpu

You can also see the OIDs being referenced with

debug snmp

Take care if using  debugs as they may overwhelm your device.

If you are running into %SNMP-3-CPUHOG: messages then there are two probable causes in relation to SNMP:

  1. Large route and/or ARP tables being polled by the NMS station

  2. Polling certain MIBs

For case #1

To avoid performance issues, force the router  to prematurely end queries for the route table from the network  management system server. Configure the router to respond with a complete message as soon as it receives the start of a request for the route table, as follows:

snmp-server view cutdown iso included

!--- ISO is used as a starting point as it is the snmp 'root'.

snmp-server view cutdown 1.3.6.1.2.1.4.21 excluded

snmp-server view cutdown 1.3.6.1.2.1.4.22 excluded

snmp-server view cutdown 1.3.6.1.2.1.3 excluded

snmp-server view cutdown 1.3.6.1.6.3.15 excluded

!--- The additional object snmpUsmMIB is excluded.

snmp-server view cutdown 1.3.6.1.6.3.16 excluded

!--- The additional object snmpVacmMIB is excluded.

snmp-server view cutdown 1.3.6.1.6.3.18 excluded

!--- The additional object snmpCommunityMIB is excluded.

snmp-server community public view cutdown RO

snmp-server community private view cutdown RW

For #2

Network Management Applications frequently  retrieve information from devices using SNMP. One example would be  Resource Manager Essentials (RME), an applications within the CiscoWorks  2000 suite of products. RME can retrieve flash information defined in  the CISCO-FLASH-MIB. If the device has an ATA flash disk, it can cause CPU utilization to jump.

Follow the bug ID link below and see detailed bug information.

CSCdt97325 (registered customers only)

To verify and apply the workaround, follow these steps:

  1. Verify if the device has an ATA Flash Card in one of its slots:

Router#show disk1:

******** ATA Flash Card Geometry/Format Info ********

ATA CARD GEOMETRY

    Number of Heads:        12

    Number of Cylinders     906

    Sectors per Cylinder    63

    Sector Size             512

    Total Sectors           684936

ATA CARD FORMAT

    Number of FAT Sectors  84

    Sectors Per Cluster    32

    Number of Clusters     21372

    Number of Data Sectors 684117

    Base Root Sector       169

    Base FAT Sector        1

    Base Data Sector       201

Router#

  1. If you're not sure or if you do not get output for the show diskX command, run the show tech command and do a search for disk.

  2. Once you know you have an ATA Flash Disk, you can apply the following workaround to prevent the SNMP queries on the FLASH-MIB:

Router(config)#snmp-server view iso included

!--- ISO is used as a starting point as it is the snmp 'root'.

Router(config)#snmp-server view ciscoFlashMIB excluded

!--- The additional object snmpUsmMIB is excluded.

Router(config)#snmp-server view 1.3.6.1.6.3.15 excluded

!--- The additional object snmpVacmMIB is excluded.

Router(config)#snmp-server view 1.3.6.1.6.3.16 excluded

!--- The additional object snmpCommunityMIB is excluded.

Router(config)#snmp-server view 1.3.6.1.6.3.18 excluded

Router(config)#snmp-server community view ro

Router(config)#snmp-server community view rw

Router(config)#exit

Note: Choose the same word on every line for the in the above configuration.

caution Caution: In the above example, snmpUsmMIB is excluded, snmpVacmMIB is excluded, and snmpCommunityMIB is excluded. Together these objects can be used to get information  about configured users and community strings to gain administrative  access to the device. It is recommended that these objects be exlcluded  from view on any device which can be accessed by public users.

Other bugs related to polling MIBs:

Follow the Cisco bug ID links below and see detailed bug information.

You can find most of the above here.

Community Member

Re: SNMP low priority?

Thanks very helpful and detailed answer.

  Also what is considered as normal cpu utilization? and above what % it is critical?

SNMP low priority?

it is not possible to define a normal CPU utilization value generally; This value depends on several factors mainly the platform, device configuration (function in the network design), typical network load and so on;

you can do baselining (monitor the value over a period of time for your devices and define what is typicall) or - what is not accurate but possible (and sometimes enough) - you can define "your" threshold (like 60 %) and see if any of the devices are constantly above or if some are above at cetain times (e.g. when server backups are done). Then you can define individual values for these devices.  

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