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New Member

MAC Addresses Disappearing from Table Even When Aging Set to 0

I work in a hotel with hundreds of IPTVs which connect to Cisco 3750s. The TVs are on VLAN 840 and receive a multicast stream depending on what TV channel they are set to. The TVs are functioning fine. I have a list of MAC addresses of all the TVs, and I'm trying to find out which port each TV is connected to.

I initially did a show mac address-table, but the TVs' MAC addresses did not show up. I assumed that this was an aging issue so I set aging time to 0 for that VLAN and gave it some time. Still nothing in the MAC table for those devices. Finally I thought that because the communication is done via multicast then maybe the MAC addresses are never even being put in the table. I started pinging the devices, and finally the MAC addresses showed up in the table. Unfortunately the TVs will only respond to pings when they are fully powered on, so I have to send pings all day long and hope that a hotel guest has their TV on at the same time that I send the ping. However, I'm finding that in the meantime addresses are still disappearing from the table even though aging is still set to 0.

So my questions are as follows:

1. Is it expected that MAC addresses will not be recorded when multicast is used?

2. Why are entries disappearing from the table even when aging is set to 0 for that VLAN?


Thanks for your help!


Hi, 1. - MAC addresses of



1. - MAC addresses of destination devices are not learned during the multicast process, this is because the switch learns which ports to forward traffic out of based on igmp snooping.

This means that just because a client is receiving traffic - there is no MAC address entry in the switch for that receiver.

2. - Have you entered the command 'show mac address-tabel aging-time' to verify what the switch thinks the aging time is?


Very best wishes



New Member

Thank you for clarifying the

Thank you for clarifying the multicast issue for me.

I have verified that the aging is set to zero for that VLAN:


#show mac address-table aging-time

Global Aging Time:    300

Vlan        Aging Time

840              0

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