Catalyst doesn't discover a MAC address

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Oct 7th, 2007
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I am using a Cisco Catalyst 2960 on an industrial network. The MAC address of a particular type of device is not discovered by this router. I have done some tests with a simple switch and was able to discover the MAC and assign an ip to this device.

I'm hoping someone can suggest something along the lines of a configuration change that will allow me to see this device.

Thanks in advance,


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Overall Rating: 4 (2 ratings)
lgijssel Sun, 10/07/2007 - 21:42
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Can you specify the mac adress with wich you have this problem?


CaptainFanatic Sun, 10/07/2007 - 21:48
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It is a siemens device that acts as a gateway between ethernet and profibus (a serial bus for addressable I/O)

lgijssel Sun, 10/07/2007 - 22:06
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And the problem is, when connected to the 2960, you do not get an ip adress?

Have you configured "spanning-tree portfast" on the interface? If not, you will see the link-led remaning amber for some 30s after connecting or booting the device. Likely DHCP is timed out before the link starts forwarding.



CaptainFanatic Sun, 10/07/2007 - 23:17
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This bit of gear is a bit strange, it doesn't use DHCP, but does get an ip assigned by a "parent" on the network. The parent uses the MAC address of the device to communicate with it, but cannot find the MAC address through the cisco router as it is not in the mac address tables.

It seems the simpler the switch we use for this the better it works!

lgijssel Mon, 10/08/2007 - 02:53
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Even in that scenario the reasoning above is still valid. The parent does not learn the mac by reading it from the switch' mac table.

Instead, the "child" sends traffic that is received by the parent, mostly via broad- or multicast.

Your statement that the mac is not in the mac address table on the switch (NOT a router)is what can be expected when there is no communication. A mac adress is only learned when a device starts sending traffic so my assumption is that the device may be silent after it's inital connection attempt. Configuring spanning-tree portfast as directed may very well be the solution.

CaptainFanatic Mon, 10/08/2007 - 17:13
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I've tried setting portfast and still no joy.

The software running on the parent node has a "browse for MAC" function that should enable me to find the child node. This is not working through the cisco switch but does work through a simple non-configurable switch. From this I would say that there is a mac discovery broadcast from the parent node that is not getting through the switch.

The default vlan 1 including all ports is all that is configured on the switch, to my knowledge this is what determines the broadcast domain.

Anyone got any tips on broadcasts?

CaptainFanatic Mon, 10/08/2007 - 19:29
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I have solved this issue:

The discovery protocol used by profinet must be similar to that used by ip phones. When I set "switchport voice vlan 1" on the interface used by the child node, the MAC discovery from the Parent node starts working.


lgijssel Mon, 10/08/2007 - 23:00
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That for reporting back.

I consider this as one to remember because it's not the first thing that one thinks of in this kind of situation.



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