show mac address-table returns different MAC which is connected from

nwang · ‎08-22-2019

I have two cisco swiches and connected with one cable, i.e. G7/13 on SwitchA is connected to G1/47 on SwitchB. Today, I found if I execute "show mac address-table int g7/13" on SwitchA, I couldn't find the mac address which is connected from the SwitchB. Instead, I saw a different mac address.

SwitchB#sh int g1/47   
GigabitEthernet1/47 is up, line protocol is up (connected) 
  Hardware is Gigabit Ethernet Port, address is c89c.1de7.f1ce (bia c89c.1de7.f1ce)

SwitchA#sh mac address-table interface g7/13
Legend: * - primary entry
        age - seconds since last seen
        n/a - not available

  vlan   mac address     type    learn     age              ports
------+----------------+--------+-----+----------+--------------------------
Active Supervisor:
*  86  0051.5884.d641   dynamic  Yes          0   Gi7/13
*  86  c89c.1de7.f1bf   dynamic  Yes         50   Gi7/13
Standby Supervisor:
*  86  0051.5884.d641   dynamic  Yes          0   Gi7/13
*  86  c89c.1de7.f1bf   dynamic  Yes         45   Gi7/13

An interesting part is, if I run show cdp, they can discover their neighbour correctly.

SwitchA#sh cdp nei g7/13
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
SwitchB
                 Gig 7/13          120             R S I  WS-C4948- Gig 1/47

SwitchB#sh cdp neighbors g1/47
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
SwitchA
                 Gig 1/47          161             R S I  WS-C6509  Gig 7/13

Why the mac address doesn't match?

luis_cordova · ‎08-22-2019

Hi @nwang ,

Correct me if I'm wrong:

The MAC of the g1/47 interface (blue) it is being learned by the g7/13 interface (green).

Regards

nwang · ‎08-22-2019

Sorry luis, I altered the real mac before posted this thread and I accidently pasted the same mac :( My bad.

I have changed the quotes and now they are showing the mac addresses are mismatched.

Giuseppe Larosa · ‎08-23-2019

Hello nwang,

verify if switchB has defined an SVI interface vlan 86

the seen MAC address

c89c.1de7.f1bf

might be associated to the SVI vlan 86 interface. This is because the two MAC addresses look like belonging to the same chassis:

c89c.1de7.f1ce

Hope to help

Giuseppe

nwang · ‎08-25-2019

Hi Giuseppe,

I find a funny thing. Both of the interfaces are not assigned to any vlans when checking the output of "show vlan". Even not in the default vlan

Giuseppe Larosa · ‎08-26-2019

Hello Nwang ,

show vlan lists only access ports

check if the ports are acting as trunk using

show interface trunk

or show interface gix/y switchport

here look at Admin state and operational state

Hope to help

Giuseppe

nwang · ‎08-26-2019

Hi Giuseppe,

Thanks for assisting me. I check the ports status, they're running under the trunk mode. Will that cause the mac address been changed?

SwitchA#show interfaces g7/13 sw
Name: Gi7/13
Switchport: Enabled
Administrative Mode: trunk
Operational Mode: trunk
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: dot1q
Operational Dot1q Ethertype:  0x8100
Negotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
Administrative Native VLAN tagging: enabled
Operational Native VLAN tagging: disabled
Voice VLAN: none
Administrative private-vlan host-association: none 
Administrative private-vlan mapping: none 
Operational private-vlan: none
Trunking VLANs Enabled: 35,86
Pruning VLANs Enabled: 2-1001
Capture Mode Disabled
Capture VLANs Allowed: ALL

Unknown unicast blocked: disabled
Unknown multicast blocked: disabled

SwitchB#sh int g1/47 sw
Name: Gi1/47
Switchport: Enabled
Administrative Mode: trunk
Operational Mode: trunk
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: dot1q
Negotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
Administrative Native VLAN tagging: enabled
Voice VLAN: none
Administrative private-vlan host-association: none 
Administrative private-vlan mapping: none 
Administrative private-vlan trunk native VLAN: none
Administrative private-vlan trunk Native VLAN tagging: enabled
Administrative private-vlan trunk encapsulation: dot1q
Administrative private-vlan trunk normal VLANs: none
Administrative private-vlan trunk associations: none
Administrative private-vlan trunk mappings: none
Operational private-vlan: none
Trunking VLANs Enabled: 10,86
Pruning VLANs Enabled: 2-1001
Capture Mode Disabled
Capture VLANs Allowed: ALL

Unknown unicast blocked: disabled
Unknown multicast blocked: disabled
Appliance trust: none

Giuseppe Larosa · ‎08-27-2019

Hello nwang,

>> Thanks for assisting me. I check the ports status, they're running under the trunk mode. Will that cause the mac address been changed?

The fact of being trunks explains why they do not appear in show vlan output, but it is not the reason for the log message.

Edit:

modified last sentence, trunks do not cause MAC address mismatch as you have seen.

Hope to help

Giuseppe

nwang · ‎08-27-2019

Yes, but still can't explain why the mac address mismatch.

Giuseppe Larosa · ‎08-27-2019

Hello nwang,

you have a mismatch on the list of permitted Vlans on the two switches:

SwitchA:

>>

Trunking VLANs Enabled: 35,86

Switch B

>>

Trunking VLANs Enabled: 10,86

But the issue happens on Vlan 86 that is permitted by both switches.

What happens if you issue

show mac address-table address c89c.1de7.f1bf

on switchB ?

The difference in the last byte (less significant rightmost octet) is exactly 15. This should be the MAC address of port gi1/32 on SwitchB.

Hope to help

Giuseppe