Odd layer 1 situation. MAC address not being stored in switch MAC table

[JohnW63]

I have a device , a MS Surface tablet with a docking station that could get DHCP info with valid IP and gateway, etc, but would not ping anything on the docking station ethernet port, but would get an IP and surf with it's wifi and with a USB to ethernet adapter.  I found the local switch did not show the MAC in the address-table, but, the next switch upstream did have a record of it. Why the first switch could update the mac table with wifi or USB adapter but not the built in port is the big question. I found I could just move the patch cable so that the Surface went directly to the second switch works fine. The problem switch is a newer 3650 and the working one is an old 3750. The 3750 connects to the 3650 via a copper connection on it's port 48. 

[pieterh]

you describe two situations
- usb to ethernet adapter
- docking station, (which could be pass-through or behave as usb to ethernet
all solutions have a different MAC-address, you may be searching for the wrong one?

other option it may be the upstream switch has configured the MAC-address as "sticky"
if so it is not allowed to move the MAC to another port unless you clear the sticky mac first.

btw I would not call this layer1. L1 is cable/connector/electrical signal,
MAC-address will be L2.

[paul driver]

Hello

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@JohnW63 wrote:

I I found I could just move the patch cable so that the Surface went directly to the second switch works fine. The problem switch is a newer 3650 and the working one is an old 3750. The 3750 connects to the 3650 via a copper connection on it's port 48.

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Did you try just using a differrent port the same switch or even just change the cabling on the old port.

Do you have any specific config applied to the port that wasnt working?

[JohnW63]

Nothing special set on ether switch. Yes, I know wifi, USB adapter, and dock ethernet ports have different MACs. The usb one showed up in the closest switch table, but the dock MAC never did. The doc MAC shows on the upstream switch however. Since the local switch does not have the MAC, the packets get lost.  I can try another port on local switch.  But, the other    addresses work fine on the port.

[pieterh]

"The doc MAC shows on the upstream switch however."
so on what interface is it pointing to? -> follow the MAC back to the connected device, maybe there is a duplicate MAC addresses in your network?

is the surface with dock used to create a cloned image? than the mac may have been copied in the image.

also with display-link ethernet connection you can configure the display-link network port to use

- clone clients ethernet mac address

- use its own usb-ethernet mac address

- use a specified mac-address to use

[JohnW63]

Perhaps a texted based diagram will help. 

This was the original config connection path that only worked with USB Ethernet dongle. The Surface dock MAC did not show on the 3650, but did show on the 3750 as being learned from the 3650. 

Surface -> in on port 19 on 3650 -> out port 48 on 3650 -> to port 48 on 3750 -> out on Gi51 on 3750 out to core switch in MDF

I moved the patch cable in the rack to another port on the 3650 and it worked. 

How can a given port allow a full DHCP conversation and get an IP, but not retain the MAC ? 

[pieterh]

>>> How can a given port allow a full DHCP conversation and get an IP, but not retain the MAC ? <<<

-> it can NOT! 

after all the MAC-address is the destination for the L2 packet
at L3 there first is an ARP query to determine the MAC-address attached to the IP-address, then the packet is sent to the MAC-address.

theoretically there could be a bug in the IOS version used that prevented the MAC address to be shown correct, but I doubt this was the situation.

please try to reproduce the situation and post output including the commands used.
also provide IOS versions

[JohnW63]

The results show it does exactly that.  show mac address-table does not show the MAC in question on the local switch, but the upstream switch from the 3650 does show the MAC and shows it discovered it from the local switch. So, the info was passed along to the network, but not retained in the 3650. 

[pieterh]

and what info gives show mac address-table <interface> ?
also important hom many mac-addresses are shown ? maybe you reach some limit?

[paul driver]

Hello

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@JohnW63 wrote:

I moved the patch cable in the rack to another port on the 3650 and it worked. 

How can a given port allow a full DHCP conversation and get an IP, but not retain the MAC ? 

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Which suggests the orignal port is mis-behaving not the switch -  try resetting the port to default.

conf t

default interface x/x

[JohnW63]

Paul,

There is not much to the config of our ports. What does setting it back to to "default" do ?  Just remove any config settings ? 

[romanroma]

Are truly talking about CAM table entries or what is getting returned in the arp table? Layer 2 frames need to know where the mac is located on the switch interface in order to forward layer 2 frames along. However, if there is a SVI higher up in the switching path, and this SVI is the default gateway, then you might not see the ARP entry lower in the swtiches.

If the CAM table entry is not getting populated, then I would suspect it is a bug, since Layer 2 mechanics needs to know where the MAC was seen and on what interface.

[JohnW63]

As I posted above, I am not talking about the ARP table. I am not seeing the MAC in the mac address-table.