We are using NIC bonding under the following configuration: 
 
eth0 = first port of first Intel Quad port Gigabit Ethernet controller. 
eth4 = first port of second Intel Quad port Gigabit Ethernet controller. 
bond0 = bonded master interface for eth0+eth4. 
 
When eth0/eth4 cables were disconnected and reconnected to same ports on a 
running system, we experienced lost of network connectivity to the bond0 
interface. 
 
Upon cable reconnect, ethtool showed valid link status as well as correct speed 
& duplex settings for the slave interfaces. (1000/FDX). 
 
bond0 would not recognize a valid slave interface after reconnect. 
 
"grep 'Active Slave' /proc/net/bonding/bond0" produced the following result: 
Currently Active Slave: None. 
 
running "ifconfig <if> down" and "ifconfig <if> up" for each slave interface 
eth0 and eth4 failed to affect the result. 
 
"ifconfig bond0" showed the correct IP address assigned to the bond0 interface, 
but IP traffic failed to reach the system over bond0. 

A reboot of the servers restored service.