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Force switch to learn MAC address

alfista16
Beginner
Beginner

This has been discussed before, but i did not manage to find a solid solution .

Thing is that i just joined a company, and while some interfaces across various switches show as "up/up" (connected) the MAC address table is not learning anything there.

Unfortunately I have no idea what is connected on those ports, so i tried the following:

- Pinging from the switch the broadcast IP of the VLAN in which those ports belong

- DNS request to the broadcast IP of that vlan

- shut / no shut the port, in case this triggers the end device to send any traffic (gratuitous arp, dhcp discover, whatever)

Still unable to learn the MAC somehow.

Any ideas?

Thank you in advance,

George

1 Accepted Solution

Accepted Solutions

As a result and after quite some research, there is not a solid way to "force" a switch learn all mac addresses of its connected hosts. The switch learns the MAC address after the host sends some L2 traffic. So, as an example, we should at least know that host's IP address and ping it, so that the switch learns its MAC through the host's echo reply.

Now, on my case where the show interface showed the port as "up/up" but no MAC address was learnt on that port:

It is the Wake-On-LAN (WoL) feature. WoL feature somewhat keeps the PC's network interface into sleep mode rather than powered off. A Windows PC was connected at that port and despite the fact that it was powered off, the WoL feature kept the physical Layer up (current running through the cable) so the interface showed as up/up at the switch.

Hope this might help for future reference.

Regards,

George

View solution in original post

7 Replies 7

Julio E. Moisa
VIP Mentor VIP Mentor
VIP Mentor

Hi,

It could be a bug, as I imagine the switch is working properly because the frame between hosts are being exchanged but the mac address table looks empty, is that right?.

What is the switch model and have you tried to reboot the switch?




>> Marcar como útil o contestado, si la respuesta resolvió la duda, esto ayuda a futuras consultas de otros miembros de la comunidad. <<

Hi Julio,

I would not say it is a bug yet. I have no idea what hosts are connected to those ports and I cannot tell for sure if they exchange any traffic. Given that the MAC address table is not learning anything, I would expect that they actually do not exchange any traffic at all. Also, I have not tried to reboot the switch yet. it is a 3850.

Hi,

If the computers are working properly it could be a bug, you could try rebooting the switch or upgrading the Image, this link could be useful:

https://quickview.cloudapps.cisco.com/quickview/bug/CSCvd15424




>> Marcar como útil o contestado, si la respuesta resolvió la duda, esto ayuda a futuras consultas de otros miembros de la comunidad. <<

Julio,

That is an excellent finding and i will definitely keep that in mind. Thank you so much.

However, I am far away from reaching to the conclusion that the MACs are not learnt due to a 3850 bug.

So my question remains:

Let's assume that a host is not chatty at all and sends nothing but DHCP traffic every couple of hours.

If - in the meantime - I want to learn their MAC and "force" the host to send some traffic, so that the switch learns its MAC, how can I trigger that?

As a result and after quite some research, there is not a solid way to "force" a switch learn all mac addresses of its connected hosts. The switch learns the MAC address after the host sends some L2 traffic. So, as an example, we should at least know that host's IP address and ping it, so that the switch learns its MAC through the host's echo reply.

Now, on my case where the show interface showed the port as "up/up" but no MAC address was learnt on that port:

It is the Wake-On-LAN (WoL) feature. WoL feature somewhat keeps the PC's network interface into sleep mode rather than powered off. A Windows PC was connected at that port and despite the fact that it was powered off, the WoL feature kept the physical Layer up (current running through the cable) so the interface showed as up/up at the switch.

Hope this might help for future reference.

Regards,

George

NOC_Infonet
Beginner
Beginner

I have this problem too, any one explain me  

We are facing in our cisco switch dynamic mac address can't view. Kindly
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