Multipe mac addresses entries for the same port (FE)-Switch 3560

Answered Question
Jul 12th, 2010

Dear All,

I have a problem with a host whitch is connected to 11 port of my cisco 3560. from time to time the, the connection is lost with the host and after some troubleshooting i see two entries of mac-address table for the port 11.

I'm asking if someone has an idee how to explain this issue and how to see if this port is participing to SPT or...

I see also somme error of collision :

===================================

5 minute input rate 1000 bits/sec, 2 packets/sec
  5 minute output rate 7000 bits/sec, 1 packets/sec
     64677029 packets input, 17167881111 bytes, 0 no buffer
     Received 39036768 broadcasts (0 multicasts)
     0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 watchdog, 39036088 multicast, 0 pause input
     0 input packets with dribble condition detected
     54722071 packets output, 8588329003 bytes, 0 underruns
     0 output errors, 992 collisions, 1 interface resets
     0 babbles, 2316 late collision, 0 deferred
     0 lost carrier, 0 no carrier, 0 PAUSE output
     0 output buffer failures, 0 output buffers swapped out

======================================

i have two routers in the same switch: my wan router + un other router used to conneced some separated hosts to internet.

If i use statif addressing for the second subnet (2 hosts + internet router), is there any risk for collision or broadcast domains or errors ?

is the second router distrub my LAN or WAN ?

Manay thanks for your help and support.

Best regards,

I have this problem too.
0 votes
Correct Answer by Nagaraja Thanthry about 6 years 5 months ago

Hello,

You can issue commands like "show interface status" and "show spanning-tree vlan ". In the output of "show interface status" command, you will see if the interface is configured and acting as a trunk. Also, in the output of "show spanning-tree vlan " command, if the port says that it is connected to an edge device, that means most likely you are connected to another switch that participates in the Spanning-tree. You can also issue "show cdp neighbor" to see if the device is another Cisco Switch.

While you can connect two different subnets in the same VLAN, it is not recommended. One of the issues you could see is excessive broadcasts. But, operationally, they will still work.

Hope this helps.

Regards,

NT

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Nagaraja Thanthry Mon, 07/12/2010 - 09:43

Hello,

For the first part of the question, I guess somebody might be connecting a hub to that port. If the hub is not negotiating the speed/duplex with the 3560 switch, then that port will go to half-duplex mode and you will see collisions on the port. That might also explain why you are seeing multiple MAC addresses on that port. Please check the port to see if the hub is connected and remove it. You can use features like port-security to ensure only one MAC address is registered on that port and people are not connecting hubs/dumb switches on that port.

For the second issue, you can certainly use static IP addresses as long as they are not overlapping with other subnets in your network. If they are overlapping, you do need to configure NAT on the router so that they are not affecting rest of the network.

Hope this helps.

Regards,

NT

Ayad Mon, 07/12/2010 - 11:21

Hi Nagaraja,

Thank you for your quick reply. When i make a show spanning-tree root i get the same port FE0/11 witch have two mac-addresses, thus confirme that the port FE0/11 is on forwarding state then connected to an other device.

Is there anyway or command to confirme this or to confirm the layer 2 or 1 connectivity ? like show cdp between cisco devices ?

for the second question, i don't have vlan in my switch just a basic configuration. is there any risk or problem of collision or brodcast when i connect two differents subnets in my switch ?

Thanks again for help and assistane.

Best regards,

AA

Correct Answer
Nagaraja Thanthry Mon, 07/12/2010 - 14:23

Hello,

You can issue commands like "show interface status" and "show spanning-tree vlan ". In the output of "show interface status" command, you will see if the interface is configured and acting as a trunk. Also, in the output of "show spanning-tree vlan " command, if the port says that it is connected to an edge device, that means most likely you are connected to another switch that participates in the Spanning-tree. You can also issue "show cdp neighbor" to see if the device is another Cisco Switch.

While you can connect two different subnets in the same VLAN, it is not recommended. One of the issues you could see is excessive broadcasts. But, operationally, they will still work.

Hope this helps.

Regards,

NT

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