Observer Analyzer : Observer Analyzer : Troubleshooting common issues : Using Discover Network Names on a Layer 3 switch that uses VLANS
Using Discover Network Names on a Layer 3 switch that uses VLANS
Symptoms: While running Discover Network Names against a Layer 3 Switch that uses VLANs, you see only a limited number of MAC addresses, which typically have multiple IP Addresses associated with them.
Causes: Layer 3 Switches that have been configured to perform routing replace the originating station's MAC Address with the MAC Address of the switch port. For example, suppose CADStation1 has a MAC Address of 00:00:03:AB:CD:00 and an IP Address of It is connected to switch port 1 through a hub. Port 1 of this switch has a MAC Address of 00:11:22:33:44:55.
When a probe is connected to a SPAN or mirror port of that switch, it shows CADStation1 with an IP of and MAC address of 00:11:22:33:44:55 rather than 00:00:03:AB:CD:00 because of this substitution.
Now, suppose there is another station (CADStation2) with MAC address of 00:00:03:AB:EF:01 and has an IP address of that is also connected to port 1 of the switch through a hub. Because Discover Network Names stores station information by MAC address (i.e., the MAC address is the unique station identifier), it changes the IP address of switch port 1's MAC address.
Because a switch configured as such hides originating station MAC addresses from Observer, MAC-based station statistics (such as Top Talkers-MAC, Pair Statistics matrix, etc.) can only be calculated by port. To make the Observer displays more useful, follow this solution.
Solutions: By examining the switch configuration you can obtain a list of MAC addresses that are associated with each port of your switch. Then, use Discover Network Names to edit the alias entry for 00:11:22:33:44:55, labeling it “SwitchPort1.“
The IP based statistical modes (Internet Observer, Top Talkers – IP (by IP Address) still show you statistics calculated from individual stations by their IP address. But MAC-based statistical modes (Pairs Statistics Matrix, Protocol Distribution, Size Distribution Statistics, Top Talkers –MAC (by hardware Address) will now show data by Port.