MAC Address duplicates

[Ranil Herath]

Hi All,

We have a 5-member Cisco 3750x switch stack. Our phone system management recently installed three third-party switches for their equipment and uplinked them to the stack.

Phone switch1 port g25 – Cisco Gi5/0/23

Phone switch2 port g28 – Cisco Gi3/0/48

Phone switch3 port g26 – Cisco Gi4/0/48

Gi5/0/23, Gi3/0/48 and Gi4/0/48 are access ports. The Phone switches are pretty basic. 

Phones on all three "Phone" switches have been randomly going offline. Phone switches don't show any interface errors or PoE errors. We started checking if there were loops. What we found is that MAC addresses of phones on "phone switch3" are also learnt by "phone switch1" on port g25 which is one of the uplinks to the Cisco. Also mac addresses of phones on "phone switch1" are also learnt by "phone switch3" on port g26 which is also and uplink to stack. 

We have " no ip proxy-arp" on all VLAN interfaces on the stack. Phone management has tracked cables and cannot find any loops.

Wonder if anyone could shed some light on this. Very much appreciate your help.

Thanks in advance.

Ranil

[Mark Malone · Accepted solution]

Hi

Ok so are these new switches connected to your stack single homed or dual homed to your stack ?

Are they cisco switches run CDP to make sure there definitely not looped back somehow tracing cables can easily go wrong

when you say the switches are learning each others mac there layer 2 that would be expected to see the macs in each switches cam table as there same voice vlan but not in the actual ports , 2 diff macs ?

Are you getting duplicate mac alerts in the logs on the 5 set cisco stack ? it should be if there duplicates ..Found duplicate .....

the voice ports are using access voice vlan syntax yes and not trunked

do you have any logs or can you show exactly hat your seeing

[Ranil Herath]

Hi Mark,

Thanks for the quick reply! We've just found the loop! Thank you again for the reply. Appreciate it.

Many thanks

Ranil

[Mark Malone]

ah good stuff :)