ether link and dhcp strange behaviour

RouterOS Beginner Basics
1
  1. When I start my desktop, the following lines appears in router’s log:
14:37:29 interface,info ether2 link up (speed 100M, full duplex) 
14:37:59 interface,info ether2 link down 
14:38:01 interface,info ether2 link up (speed 100M, full duplex)

Why ethernet link down for a second (2nd line)?

  1. When I start dhcp client (dhcpcd 5.6.8, Linux 3.9.4) on my desktop:
14:38:06 dhcp,info dhcp1 assigned 192.168.0.252 to 5A:12:4E:55:2E:F1
14:38:46 dhcp,info dhcp1 deassigned 192.168.0.252 from 5A:12:4E:55:2E:F1
14:38:46 dhcp,info dhcp1 assigned 192.168.0.252 to 5A:12:4E:55:2E:F1

Why dhcp deassigned for a second (2nd line)?

2

The interface up/down/up is usually caused by the client computer restarting the ethernet interface connected to the router. If that is the only device connected to that router interface, you will get that message.

The dhcp behavior may be client device related. I had problems like that with only a few specific devices.
http://forum.mikrotik.com/t/dhcp-server-renew-challenge/66256/1
http://forum.mikrotik.com/t/security-help-as-well-as-why-router-deassign-and-assign-ip/65654/1

The MikroTik crew had a go at my router, and think they have corrected my problem starting with V6.1. Maybe that is your problem too.