Network technologies and trends

Jun 28 2008   12:25PM GMT

How to configure ASA/PIX firewall to collect Net flow data from an external router to the netflow collector located in Inside Network.

Yasir Irfan Yasir Irfan Profile: Yasir Irfan

This article provides an example of Net flow configurations in a Cisco Router, ASA/PIX firewall to collect the Net flow data in the internal network.
Components Uses
The information in this document is based on following hardware and software versions
• Cisco Router 3745 – IOS version 12.3(17b. (Network
• PIX 525 7.0.3 ( ASA can also be used) (Internal
• Manage Engine Net flow Analyzer 6 ( Any net flow collector can be used)(

In this example let’s start by configuring Net flow in a Cisco Router

Cisco Router Configuration
Here the IP address for the interface is

Enabling Net flow in an Interface
Enter global configuration mode on the router and issue the following commands for each interface on which you want to enable Net Flow:

interface {interface} {interface_number}
ip route-cache flow

After applying the commands the example will be as follows
router3745#configure terminal
router-3745(config)#interface FastEthernet 0/1
router-3745(config)#ip address
router-3745(config-if)#ip route-cache flow
router-3745(config-if)Bandwidth 1000

Exporting NetFlow Data

Issue the following commands to export Net Flow data to the server on which NetFlow Analyzer is running:

ip flow-export destination {hostname|ip_address} 9996 ( Exports the NetFlow cache entries to the specified IP address. Use the IP address of the NetFlow Analyzer server and the configured NetFlow listener port. The default port is 9996. )

ip flow-export source {interface} {interface_number} (Sets the source IP address of the NetFlow exports sent by the device to the specified IP address. NetFlow Analyzer will make SNMP requests of the device on this address.)

ip flow-export version 5 [peer-as | origin-as] (Sets the NetFlow export version to version 5. Version 5,7 & 9 are available)

ip flow-cache timeout active 1 (Breaks up long-lived flows into 1-minute fragments. You can choose any number of minutes between 1 and 60. If you leave it at the default of 30 minutes your traffic reports will have spikes.It is important to set this value to 1 minute in order to generate alerts and view troubleshooting data.)

ip flow-cache timeout inactive 15 (Ensures that flows that have finished are periodically exported. The default value is 15 seconds. You can choose any number of seconds between 10 and 600. )

snmp-server ifindex persist (Enables ifIndex persistence (interface names) globally. This ensures that the ifIndex values are persisted during device reboots.)

The following example shows the above mentioned commands

router-3745(config)#ip flow-export destination 9996
router-3745(config)#ip flow-export source FastEthernet 0/1
router-3745(config)#ip flow-export version 5
router-3745(config)#ip flow-cache timeout active 1
router-3745(config)#ip flow-cache timeout inactive 15
router-3745(config)#snmp-server ifindex persist

Issue the following commands in normal (not configuration) mode to verify whether NetFlow export has been configured correctly:

show ip flow export (Shows the current NetFlow configuration)
show ip cache flow (These commands summarize the active flows and give an indication of how much NetFlow data the device is exporting

router-3745#show ip flow export
router-3745#show ip cache flow

The next step is make a Natting in ASA/PIX

pix-525# configure t
pix-525# (config)# static (inside,outside) netmask dns

In order to export to the netflow statistics to the netflow analyzer located in the internal network we have configure the following access-list and apply it to outside interface to allow the Netflow traffic

pix-525# configure t
pix-525# (config)#access-list NETFLOW extended permit udp any host eq 9996
pix-525# (config)#access-list NETFLOW extended permit tcp any any

Apply the created access-list to the outside interface
pix-525# (config)#access-group NETFLOW in interface outside

Now install the Netflow Analyzer software and configure it to recieve the netflow statists from the external router.

Troubleshooting tips

Verify Netflow is working in Cisco Router

router-3745#sho ip cache flow
IP packet size distribution (78841980 total packets):
1-32 64 96 128 160 192 224 256 288 320 352 384 416 448 480
.003 .453 .023 .012 .008 .010 .004 .003 .003 .003 .004 .003 .003 .003 .004

512 544 576 1024 1536 2048 2560 3072 3584 4096 4608
.003 .005 .022 .021 .401 .000 .000 .000 .000 .000 .000

IP Flow Switching Cache, 278544 bytes
548 active, 3548 inactive, 4045717 added
84147818 ager polls, 0 flow alloc failures
Active flows timeout in 1 minutes
Inactive flows timeout in 15 seconds
IP Sub Flow Cache, 33416 bytes
548 active, 1500 inactive, 4045717 added, 4045717 added to flow
0 alloc failures, 0 force free
2 chunks, 14 chunks added
last clearing of statistics never
Protocol Total Flows Packets Bytes Packets Active(Sec) Idle(Sec)
——– Flows /Sec /Flow /Pkt /Sec /Flow /Flow
TCP-Telnet 143 0.0 2 52 0.0 0.4 12.7
TCP-FTP 255 0.0 6 100 0.0 9.0 7.2
TCP-FTPD 15010 0.0 1 63 0.0 0.6 15.4
TCP-WWW 1100665 2.5 14 607 37.7 8.2 6.9
TCP-SMTP 171448 0.3 69 633 27.3 35.8 6.2
TCP-X 723 0.0 2 245 0.0 0.4 13.0
TCP-other 1966270 4.5 21 656 95.4 11.7 6.6
UDP-DNS 56825 0.1 12 66 1.5 20.5 11.6
UDP-NTP 8 0.0 1 76 0.0 0.0 15.5
UDP-Frag 1 0.0 1 1476 0.0 0.0 15.0
UDP-other 684203 1.5 11 319 17.9 4.8 14.9
ICMP 48198 0.1 1 78 0.2 1.6 15.4
GRE 1358 0.0 183 182 0.5 50.0 4.2
IP-other 62 0.0 83 108 0.0 53.4 3.2
Total: 4045169 9.2 19 601 180.9 10.6 8.3

SrcIf SrcIPaddress DstIf DstIPaddress Pr SrcP DstP Pkts
Fa0/1 Tu0 11 1705 0D96 8
Fa0/1 Tu0 06 0019 10EC 32
Fa0/1 Tu0 06 0019 714B 29

Check Nating is working in the Firewall

pix-525# show xlate
2in use, 417 most used
Global Local

Check access -list is forwading the netflow traffic

pix-525# sho access-list NETFLOW
access-list NETFLOW; 2 elements
access-list NETFLOW line 1 extended permit udp any host eq 9996 (hitcnt=7)
access-list NETFLOW line 2 extended permit ip any any (hitcnt=140861)

To know more about Netflow Analyzer and its configuration click this link Netflow.

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5  Comments on this Post

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  • Capnjack
    What's the reasoning behind the "access-list NETFLOW extended permit tcp any any" in the ACL? Kind of begs the question, "Why have a firewall?" right? Is there anything necessary other than the one UDP port sourcing from the external router? Thanks, Chaz
    0 pointsBadges:
  • Yasir Irfan
    it can be modified with an the source IP with the specific port number with following accesslist access-list NETFLOW extended permit udp host host eq 9996
    6,800 pointsBadges:
  • Yasir Irfan
    Also we can remove the following access-list access-list NETFLOW extended permit tcp any any no access-list NETFLOW extended permit tcp any any
    6,800 pointsBadges:
  • networkimran
    this consumes CPU utilization well!!
    10 pointsBadges:
  • Yasir Irfan
    Hi @networkimran, it depends upon the Appliance you have, generaly ASA 5540 , 5580 are fine and they works well
    6,800 pointsBadges:

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