I need help understanding how a router learns a MAC address from a switch that doesn't appear to have it?
The router is C9500-40X, and the directly connected switch is WS-C3850-24XU.
From the router:
#sh mac add add 0000.00ff.ef52
Mac Address Table
-------------------------------------------
Vlan Mac Address Type Ports
---- ----------- -------- -----
100 0000.00ff.ef52 DYNAMIC Te1/0/9
108 0000.00ff.ef52 DYNAMIC Te1/0/9
110 0000.00ff.ef52 DYNAMIC Te1/0/9
120 0000.00ff.ef52 DYNAMIC Te1/0/9
180 0000.00ff.ef52 DYNAMIC Te1/0/9
181 0000.00ff.ef52 DYNAMIC Te1/0/9
182 0000.00ff.ef52 DYNAMIC Te1/0/9
183 0000.00ff.ef52 DYNAMIC Te1/0/9
184 0000.00ff.ef52 DYNAMIC Te1/0/9
185 0000.00ff.ef52 DYNAMIC Te1/0/9
186 0000.00ff.ef52 DYNAMIC Te1/0/9
187 0000.00ff.ef52 DYNAMIC Te1/0/9
188 0000.00ff.ef52 DYNAMIC Te1/0/9
200 0000.00ff.ef52 DYNAMIC Te1/0/9
300 0000.00ff.ef52 DYNAMIC Te1/0/9
800 0000.00ff.ef52 DYNAMIC Te1/0/9
801 0000.00ff.ef52 DYNAMIC Te1/0/9
875 0000.00ff.ef52 DYNAMIC Te1/0/9
910 0000.00ff.ef52 DYNAMIC Te1/0/9
Total Mac Addresses for this criterion: 19
#sh cdp nei Te1/0/9
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone,
D - Remote, C - CVTA, M - Two-port Mac Relay
Device ID Local Intrfce Holdtme Capability Platform Port ID
216-501-cis2
Ten 1/0/9 145 S I WS-C3850- Ten 1/1/4
Total cdp entries displayed : 1
When I connect to that switch though, it's not there:
216-501-cis2#sh mac add add 0000.00ff.ef52
Mac Address Table
-------------------------------------------
Vlan Mac Address Type Ports
---- ----------- -------- -----
216-501-cis2#
If I shut down the interface (Ten1/0/9) on the 9500 then the MAC address table entries go away, and when I bring it back up again they come back, so it's not something old and leftover -- it's actively re-learned more or less immediately.
Also interesting is that I see this MAC address across multiple VLANs, but not all existing vlans. I've also verified that this MAC address is not the address of any of the interfaces on these devices.
Any suggestions as to how this MAC address table is being learned?
Thanks!!
-Chris
