This feature was introduced in 12.4(4)T. The NetFlow Dynamic Top Talkers CLI feature is used to obtain an overview of the highest volume traffic (top talkers) in your network. It provides an overview of the traffic by aggregating the flows in the cache based on the aggregation field that you select when you use the NetFlow Dynamic Top Talkers CLI feature.
The NetFlow Dynamic Top Talkers CLI feature does not require modifications to the configuration of the router. The show ip flow top command is the only command that you need to use for the NetFlow Dynamic Top Talkers CLI feature. You can invoke any of the NetFlow Dynamic Top Talkers CLI options directly from the show ip flow top command whenever you need them.
The information that you want to use the NetFlow Dynamic Top Talkers CLI feature to analyze must be available in the cache. For example, if you want to be able to identify the MAC address in the flows, you must configure the ip flow-capture mac-addresses command in order to capture the values from the MAC address fields in the traffic first.
The NetFlow Dynamic Top Talkers CLI feature aggregates flows and allows them to be sorted so that they can be viewed. The flows can be aggregated on fields in the cache such as source or destination IP address, ICMP type and code values, and so forth. For a full list of the fields that you can aggregate the flows on, refer to the show ip flow topcommand in the Cisco IOS NetFlow command reference documentation.
The aggregated top talker flows can be sorted by any of the following criteria:
The aggregated field in the display data
The number of bytes in the display data
The number of flows in the display data
The by number of packets in the display data
In ascending or descending order (to find the least used Top talker)
In addition to sorting top talkers, you can further organize your output by specifying criteria that the top talkers must match, such as source or destination IP address or port. The match keyword is used to specify this criterion. For a full list of the matching criterion that you can select, refer to the show ip flow topcommand in the Cisco IOS NetFlow command reference documentation.
The NetFlow Dynamic Top Talkers CLI feature can help you quickly identify traffic that is associated with security threats such as DoS attacks because it does not require configuration modifications. You can change the NetFlow Dynamic Top Talkers CLI options for identifying and analyzing network threats in the aggregated flows on-the-fly as you learn more about the traffic that is of interest. For example, after you have identified that there is a lot of ICMP traffic in your network by using the show ip flow top 10 aggregate icmp command you can learn what IP networks the traffic is being sent to by using the show ip flow top 10 aggregate icmp match destination-prefix 126.96.36.199/8 command.
A high volume of ICMP traffic might indicate that an ICMP-based DoS attack is in progress.
The show ip flow top command:
Does not require additional NetFlow configuration commands to display top talkers. Therefore you do not need to supply the configuration mode password to the administrators who use the he show ip flow top command to monitor network traffic. The only prerequisite for using the show ip flow top command is that you have configured NetFlow on at least one interface on the router.
Aggregates flows automatically based on the aggregation method that you select, and independently of any netflow aggregation cache(s).
Allows you to change the parameters of the command, such as the number of flows to display, the display order, and match criterion, on-the-fly every time that you use the command without having to change the router’s configuration.
Allows you to sort the display output in ascending or descending order based on:
The aggregated field
The number of bytes
The number of flows,
The number of packets
show ip flow top and show ip cache verbose flow
Many of the values shown in the display output of the show ip cache verbose flow command are in hexadecimal. If you want to match these values using the show ip flow top command with the match keyword, you must enter the field value that you want to match in hexadecimal. For example, to match on the destination port of 00DC in the following except from the show ip cache verbose flow command, you would use the match destination-port 0x00DC keywords and argument for the show ip flow top command.
SrcIf SrcIPaddress DstIf DstIPaddress Pr TOS Flgs Pkts
Port Msk AS Port Msk AS NextHop B/Pk Active
Et0/0.1 10.10.11.4 Et1/0.1 172.16.10.8 06 00 00 209
00DC /0 0 00DC
/0 0 0.0.0.0 40 281.4
MAC: (VLAN id) aaaa.bbbb.cc03 (005) aaaa.bbbb.cc06 (006)
Min plen: 40 Max plen: 40
Min TTL: 59 Max TTL: 59
IP id: 0
Match Criteria with the show ip flow top command
You can limit the top talkers that are displayed by the show ip flow top command by using the match keyword and arguments. For example, you can display the IP destination address top talkers that have a prefix of 188.8.131.52 using the show ip flow top 10 aggregate destination-address match destination-prefix 184.108.40.206/3 command.
For a full list of the matching criterion that you can select, refer to the show ip flow topcommand in the Cisco IOS NetFlow Command Reference
. If you do not configure match criteria all of the flows are considered as candidates for aggregation as top talkers based on the volume of traffic they represent.
The Order That Aggregation Occurs in
With the exception of the flows keyword, all matches are performed prior to aggregation, and only matching flows are aggregated. For example, the show ip flow top5aggregate destination-address match destination-prefix 172.16.0.0/16 command analyzes all of the available flows looking for any flows that have destination addresses that match the destination-prefix value of 172.16.0.0/16. If it finds any matches it aggregates them, and then displays the number of aggregated destination-address flows that is equal to the number of top talkers that were requested in the command-in this case five.
The flows keyword matches the number of aggregated flows post-aggregation. For example, the show ip flow top 2 aggregate destination-address match 6 command aggregates all of the flows on the values in their destination IP address field, and then displays the top talkers that have 6 aggregated flows.
Number of Flows Matched
If you do not specify match criteria and there is traffic in the flows that includes the field that you used to aggregate the flows on, all of the flows will match. For example, if your router has 20 flows with IP traffic and you enter the show ip flow top 10 aggregate destination-address command the display will indicate that 20 of 20 flows matched, and the 10 top talkers will be displayed.
If you use the match keyword to limit the flows that are aggregated to the flows with a destination prefix of 220.127.116.11/3, and only one flow matches this criterion the output will indicate that one out of six flows matched. For example, if your router has 6 flows with IP traffic, but only one of them has a destination prefix of 18.104.22.168/3, and you enter the show ip flow top 10 aggregate destination-address match destination-prefix 22.214.171.124/3 command, the display will indicate that 1 of 6 flows matched.
If the total number of top talkers is less than the number of top talkers that were requested in the command, the total number of top talkers is displayed. For example, if you enter a value of five for the number of top talkers to display and there are only three top talkers that match the criteria that you used, the display will only include three top talkers.
When a match criterion is included with the show ip flow top command, the display output will indicate "N of M flows matched" where N <= M, N = matched flows, and M = total flows seen. The numbers of flows seen could potentially be more than the total number of flows in the cache if some of the analyzed flows were removed from the cache and new flows were created ahead of the current point, as the top talkers feature sweeps through the cache. Therefore, M is NOT the total number of flows in the cache, but rather, the number of observed flows.
If you attempt to display the top talkers by aggregating them on a field that is not in the cache you will see the "% aggregation-field" is not available for this cache" message. For example, if you use the show ip flow top 5 aggregate s ource-vlan command, and you have not enabled the capture of VLAN IDs from the flows, you will see the "% VLAN id is not available for this cache" message.