Spurious mac addresses

[normbeef]

Hello..

I have a 3550 switch (not routing) that is connected directly into a Cisco 2811 router..  there is a single vlan on the switch.

The connection from the switch to the router is  via   fa0/1 so i expect to only see a single MAC address from the router interface. BUT below is the output of the mac address table

I do not know what any of these MAC addresses are, or who they belong to or why they appear to be learned on this interface.

We have been to the site and verified that there is a single cable connecting the switch to the router.

MYSWITCH>sh mac-address-table int fa0/1
          Mac Address Table
------------------------------------------

Vlan    Mac Address       Type       Ports
----    -----------       ----       -----
  20    0013.0ff5.c870    DYNAMIC    Fa0/1
  20    0013.1ef4.c870    DYNAMIC    Fa0/1
  20    0013.57f4.c870    DYNAMIC    Fa0/1
  20    0013.5f35.c870    DYNAMIC    Fa0/1
  20    0013.65fc.c870    DYNAMIC    Fa0/1
  20    0013.6f55.c870    DYNAMIC    Fa0/1
  20    0013.7ef4.c870    DYNAMIC    Fa0/1
  20    0013.7f45.c870    DYNAMIC    Fa0/1
  20    0013.7f54.4570    DYNAMIC    Fa0/1
  20    0013.7f55.c670    DYNAMIC    Fa0/1
  20    0013.7f55.cc70    DYNAMIC    Fa0/1
  20    0013.7f56.c870    DYNAMIC    Fa0/1
  20    0013.7ff4.0875    DYNAMIC    Fa0/1
  20    0013.7ff4.5575    DYNAMIC    Fa0/1
  20    0013.7ff4.8570    DYNAMIC    Fa0/1
  20    0013.7ff4.9875    DYNAMIC    Fa0/1
  20    0013.7ff4.9877    DYNAMIC    Fa0/1
  20    0013.7ff4.c855    DYNAMIC    Fa0/1
  20    0013.7ff4.c870    DYNAMIC    Fa0/1
  20    0013.7ff4.d8b5    DYNAMIC    Fa0/1
  20    0013.7ff4.d8be    DYNAMIC    Fa0/1
  20    0013.7ff4.d978    DYNAMIC    Fa0/1
  20    0040.f63f.d524    DYNAMIC    Fa0/1
  20    004b.7ff4.c870    DYNAMIC    Fa0/1
  20    0053.5ef4.c870    DYNAMIC    Fa0/1
  20    005f.7ff4.c870    DYNAMIC    Fa0/1
  20    0095.7ff4.c870    DYNAMIC    Fa0/1
  20    0813.7ff4.c870    DYNAMIC    Fa0/1
  20    0c30.f147.8f0c    DYNAMIC    Fa0/1
  20    42b6.1c93.0c30    DYNAMIC    Fa0/1
  20    50fa.7ff4.c870    DYNAMIC    Fa0/1
  20    5613.7ff4.c870    DYNAMIC    Fa0/1
  20    6213.7ff4.c870    DYNAMIC    Fa0/1
  20    8071.800c.0000    DYNAMIC    Fa0/1
  20    8071.800c.0010    DYNAMIC    Fa0/1
  20    8079.800c.0000    DYNAMIC    Fa0/1
  20    8079.800c.0050    DYNAMIC    Fa0/1
  20    8079.800c.0060    DYNAMIC    Fa0/1
  20    8079.800c.0080    DYNAMIC    Fa0/1
  20    aa13.7ff4.c870    DYNAMIC    Fa0/1
  20    b61c.930c.30f1    DYNAMIC    Fa0/1
  20    cc13.7ff4.c870    DYNAMIC    Fa0/1
  20    e01a.7ff4.c870    DYNAMIC    Fa0/1
  20    e613.7ff4.c870    DYNAMIC    Fa0/1
Total Mac Addresses for this criterion: 44
SWITCH>

Can anyone explain this ?

[smartd_03]

Could you please place the output of

" sh run int fa0/1 "     of your switch here. will like to know if you have multicast or  ipv6 anycast configured. That should explain this issue.

[normbeef]

As you can see there is nothing really configured here

Building configuration...

Current configuration : 149 bytes
!
interface FastEthernet0/1
description UPLINK

switchport access vlan 20
switchport mode access
no ip address
duplex full
speed 100
end

[cadet alain]

Hi,

Sniff the interface f0/1 either by placing a hub between switch and router or mirror the rx traffic of f0/1 to another port attached to a machine with a sniffer by using the SPAN feature.

monitor session 1 source interface f0/1 rx

monitor session 1 destination interface f0/x

Regards.

Alain.