This article applies to PRTG Network Monitor 19 or later

Configuration tips for Cisco routers and PRTG

This article includes several tips and tricks for your Cisco router configuration when using PRTG with SNMP or NetFlow.

SNMP-based monitoring

The following command makes sure that the router does not change the numbering of the interfaces when interfaces are added or removed so you do not have to edit your PRTG sensor setup:

snmp-server ifindex persist

See also The interface numbers on my switch keep changing. What can I do?

Setting up NetFlow version 5, version 9, or IPFIX on Cisco routers for PRTG

Using the NetFlow protocol, you can monitor the bandwidth usage of all packets going through a router. For each flow of data, the router sends a NetFlow packet with connection and bandwidth information to a monitoring system that is running PRTG. In PRTG, you must create a collector that accepts these packets and does the accounting.

The advantage of using NetFlow is that it only requires little CPU load on the router itself (for example, 10,000 active flows create about 7% additional CPU load, and 45,000 active flows account for about 20% additional CPU load) and also on the machine that is running PRTG. This is much less than a Packet Sniffer sensor requires.

You must edit your router's setup and save it as the new startup configuration:

• Log in to the router.
• Run the command enable that gives you admin access.
• Run the command configure.

Enabling NetFlow for each interface

The following command must be repeated for each single interface to enable NetFlow for each one (replace FastEthernet 0/1 with the names of the interfaces on your router):

interface FastEthernet 0/1
ip route-cache flow
exit 

Setting NetFlow options

See the code section below. You may want to copy the script, edit it with your IPs, and paste it into your router telnet session. When you are done and have tested the new configuration, remember to copy the new configuration to the startup configuration.

copy running-config startup-config

Disabling NetFlow

This command will tell the router not to send any NetFlow packets to the specified address:

no ip flow-export destination (address) (port)

To disable NetFlow for a specific interface, use these commands:

interface (interface) (interface number)
no ip route-cache flow
Exit

Sample NetFlow code

!
! Step 1: enabling NetFlow
!
! Note: For PRTG we need NetFlow versions 5 or 9 or IPFIX,
!       no other NetFlow version is supported currently
!
ip flow-export version 5
! or
! ip flow-export version 9
!
! Note: We recommend these values for the timeouts:
!
! timeout active value is in minutes
ip flow-cache timeout active 5
!
! timeout inactive value is in seconds
ip flow-cache timeout inactive 10 
!
! Step 2: setting the target address for NetFlow packets:
!
! Replace 10.0.0.200 with the IP address of your machine running PRTG
! Replace 9991 with another port number of your choice if desired
!
ip flow-export destination 10.0.0.200 9991
!
! Step 3: you can optionally send all NetFlow packets to one other machine, too
!
ip flow-export destination 10.0.0.201 9991
!
! Step 4: create a "NetFlow Sensor" in PRTG (V7 or later) and enter the port number set above
!
! That's it

More

• For more information on the performance and CPU load situation when enabling NetFlow on various Cisco hardware, see the Cisco white paper NetFlow Performance Analysis.
• For more information on configuring NetFlow, see the Cisco website: Configuring NetFlow and NetFlow Data Export Format