dear friends,
I am using Red Hat Linux Enterprise Edition3 update 2. My built in
lan card is: Marvell Yukon 88E8001/8003/8010 PCI Gigabit Ethernet Controller(Built in ASUS P5PE-VM motherbord). My network service is up through "ntsysv" command. But
"ifconfig eth0 up" says---> eth0:unknown interface:no such device.
But during the bootup process it showed "successfully up eth0 interface".


what am I to do?

Could it be a udev rule? I had sort of a similar problem yesterday when I copied a root drive to a new hard disk to just plug into a machine with a different motherboard. It turned out that udev had named two ethernet cards as eth0. It had also renamed my ath0 as ath1. It's something to check if you're desperate. On my machine (Debian) the file was:

/etc/udev/rules.d/z25_persistent-net.rules

Does YOUR Linux Box have an PCI Ethernet Card? Does YOUR Linux Box have ONLY the on-board Marvell Lan? If more than one, try removing the on-board Lan in the BIOS by disabling the Ethernet Lan Device. Reboot and try "lspci" to see if the PCI Ethernet Card is recognized. If Yes, try "ifconfig -a" or "ifconfig". If No PCI Ethernet Card in system, enable the BIOS or the Marvell on-board LAN. Reboot and see if it works. If Not, it may be worth the time to re-install YOUR Distro. Networking is easiest to configure if done during the installation. With the router/modem/switch/etc. all plugged in and turned on. If using DHCP, do not set the DNS Hostname. Unless the name was provided by YOUR ISP. Leave it blank. If YOU get the chance to supply YOUR Primary DNS Nameserver and YOUR Secondary DNS Nameserver, do so, it really smooths the way later. If running Static IP's, do not use DNS.