“Greasy cloth” in the Internet link

A few weeks ago, I had a small project to replace one ancient Cisco router with one new MikroTik RB 2011 device. To make a long story short, I investigated the old Cisco configuration and then configured MikroTik.

After a few hours, I had the phone call. The local admin called to tell me that their Internet was terrible. There shouldn’t be any problem at all, as they are using the fiber optic link. Therefore, I connected remotely to the MikroTik device and checked its configuration.

 

The log is my best friend

As my configuration was correct, I opened the system’s log. I observed it for a few seconds and then I spotted something weird. There were some blue lines in the log. Something happened that generated warnings.

When I check it closer, I saw that the ether1 interface was generating warnings. Moreover, this is my LAN interface. Is this problem with the network cables? I need to check it thoroughly.

As the first step, I opened the new terminal window and typed the command:

log print file=link.txt where topics~"interface"

This will print all these blue lines in the text file. This is a much easier way for the further analysis. When finished, I downloaded this file link.txt. I opened it and it was full of the following lines:

19:17:50 interface,warning ether1 excessive or late collision, link duplex mismatch?
19:18:20 interface,warning ether1 excessive or late collision, link duplex mismatch?
19:18:50 interface,warning ether1 excessive or late collision, link duplex mismatch?
19:19:20 interface,warning ether1 excessive or late collision, link duplex mismatch?
19:19:50 interface,warning ether1 excessive or late collision, link duplex mismatch?
19:20:20 interface,warning ether1 excessive or late collision, link duplex mismatch?
19:20:50 interface,warning ether1 excessive or late collision, link duplex mismatch?

Link duplex mismatch – this is very strange, indeed. I can’t recall every having this error on any MikroTik router.

I rechecked again the MikroTik’s port settings. I used the Ether1 port on the router and this is the gigabit port. Furthermore, the ports on the MikroTik are in automatic configuration mode. However, it showed now 100 Mbps half-duplex. I know that MikroTik RouterBoard virtually never makes such mistake.

Now, the problem must be either with the network cables or on the core switch.

 

Check that switch port

I connected to the core switch and selected the port 1, assigned to the router:

01 - stara podesenja

These three fields marked in yellow were the root of the problem. Don’t be confused, I made this screenshot after I solved this issue. This is why the current duplex mode is full duplex and auto negotiation is on.

For some reason, this switch and the old router communicated on 100Mbps half-duplex link without auto negotiation. This is a very strange configuration, used in the olden days, when the last dinosaurs walked on the Earth.

02 - novo podesavanje

Anyway, I reconfigured this port to the automatic settings and… voila! I removed the “greasy cloth” and unblocked the Internet link.

The speed on the link raised from 18/9 Mbps to 95/95 Mbps. Additionally, the warning lines disappeared from the MikroTik log.

 

What I have learned?

This should have been a simple operation of equipment exchange. In the end, nothing was simple.

I learned that I should always check both sides, especially when the other side is unknown to me. I shouldn’t think – if it worked, it will continue to works. It can be somehow made to work and then I will break the system when an old device is replaced with a new one.

In this case, I was a bit distracted with other problems that appeared. The documentation was incomplete, too. This is another lecture I learned – don’t trust 100% of everything you read – except here! Try to check everything before you begin and update documentation as necessary.

Stay tuned.

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