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chd Beginner
Beginner

Re: Router learns MAC address from switch that doesn't have it?

The configuration is pretty standard, but I don't think it's something "lingering" anywhere; I think I mentioned in the original post that if I shut down the interface that connects the 3850, these MAC address entries disappear from the router's forwarding table. (And they disappear from the tables in all the other devices in the LAN too.)

 

Note that with this link shut down, the switch IS still reachable through an alternate path, but this MAC address is not re-learned. [EDIT: Not True -- sorry. It does get re-learned through the alternate path, it just took a little time. Sorry for the misleading assumption.]

 

When I bring the router interface back up, the MAC address table entries return, though the connected switch never learns them (all the other switches in the LAN do).

Beginner

Re: Router learns MAC address from switch that doesn't have it?

ok, so the primary link is direct connection, to the switch and the backup, is that also direct?

just to be 100% clear, you have no ACTIVE ports on the switch besides the trunk? no hosts and no other switches, etc?

Beginner

Re: Router learns MAC address from switch that doesn't have it?

Either way, the only thing I can think of left is:

- now that you have the backup link going, check arp now on the router and see if its different

- if you have dhcp configured on the router or switch, check for dhcp binding, maybe the device got an IP

- try using “switch port-mapping” tool, maybe that will detect it

- turn on logging on the switch, level 7, turn on your logging on switch ports, ( just remove “no logging event link-status”) maybe you’ll be able to  get some traces/logs etcc. From there

- I see you have dhcp snooping enabled, do you have that propagated across all vlans?

Highlighted
Beginner

Re: Router learns MAC address from switch that doesn't have it?

I faced with this strange situation also.
I have dozen WS-C3850-48P with IOS Version 16.3.7 connected to central WS-C4507R+E.
I have a lot of MACs like 0000.00ff.* on the central switch and nowhere else.
I guess it may be connected with RSPAN, configured in our environment.

I have not saw MACs 0000.00ff.* from test switch until i have configured RSPAN on it.

 

C3850:
monitor session 1 source vlan 311
monitor session 1 destination remote vlan 901

 

And then it appeared on the central switch:

C4507#sh mac ad
vlan mac address type protocols port
311 0000.00ff.f02e dynamic ip,ipx,assigned,other Port-channel63

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