I have 4 servers running Centos 5. All of them have two ethernet network cards.

I have configured 192.168.1.x IP addresses on their eth0 card. They are all connected to the same switch using their eth0 card and they are all working.

I have configured 10.72.11.x IP addresses on their eth1 card.They are all connected to the same switch - a different one from the switch used with eth0 card - using their eth1 card and they are NOT all working.

Their configuration files is like:


The interface is starting and configured as I need.

[root@sql1 network-scripts]# ifconfig
**eth0**  Link encap:Ethernet  HWaddr 84:2B:2B:55:4B:97
          inet addr:  Bcast:  Mask:
          inet6 addr: fe80::862b:2bff:fe55:4b97/64 Scope:Link
          RX packets:2981 errors:0 dropped:0 overruns:0 frame:0
          TX packets:319 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:386809 (377.7 KiB)  TX bytes:66134 (64.5 KiB)
          Interrupt:36 Memory:da000000-da012800

**eth1**  Link encap:Ethernet  HWaddr 84:2B:2B:55:4B:98
          inet addr:  Bcast:  Mask:
          UP BROADCAST MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:0 (0.0 b)  TX bytes:0 (0.0 b)
          Interrupt:48 Memory:dc000000-dc012800  

I also added a route-eth1 file that looks like: via

Routing looks fine to me:

[root@sql1 network-scripts]# netstat -rn
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface U         0 0          0 eth1   U         0 0          0 eth0     U         0 0          0 eth1       UG        0 0          0 eth1         UG        0 0          0 eth0

But I cannot ping one server from the other.

[root@sql1 network-scripts]# ping
PING ( 56(84) bytes of data.
From icmp_seq=1 Destination Host Unreachable
From icmp_seq=2 Destination Host Unreachable
From icmp_seq=3 Destination Host Unreachable
From icmp_seq=4 Destination Host Unreachable
From icmp_seq=5 Destination Host Unreachable
From icmp_seq=6 Destination Host Unreachable
--- ping statistics ---
7 packets transmitted, 0 received, +6 errors, 100% packet loss, time 6033ms, pipe 3

What am I doing wrong?


It should be noted that your NICs aren't properly 'working'. Your ifconfig output doesn't show the 'RUNNING' required for there to be a properly working link. note the line UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 from your eth0 interface. There's no 'RUNNING' evident in the eth1 output. In my experience, this indicates a low level issue. eg: Bad/disconnected cabling.

Is is possible the NIC you have plugged your cable into is NOT eth1? Servers can get some weird naming going on. Try checking the output of /var/log/messages, to see if it reports an interface getting (dis)connected. Likewise, see if you get any diagnostic info there when you connect/disconnect, or during boot. Check and see if this is occurring on all 5 boxes. You may also want to try the graphical configuration tool, just in case you've got a typo in the configuration file. (system-config-network)

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  • You were right cables were wrong. – Sebas Feb 9 '11 at 0:53

Think a bit about your subnet setup:


In base 2:

236 is 1110 1100
191 is 1011 1111
192 is 1100 0000
128 is 1000 0000

Your IPs are outside your subnet - is not in, is. Neither is but is. Either change your IPs or change your broadcast to .255 and your network to .192. Alternatively (unless it's being dictated to you), run in the lower half of (i.e. or just use a /24 and save yourself the hassle.

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  • It appears you got 2 separate segments going on your proposed single segment switching network. Unless you are running vlans on that switch, it may be confused depending on your configuration and in this specific configuration its going to be. Trying bumping your 10.72.11.X networks all to 24 bits. – Jason B Shrout Feb 7 '11 at 23:38

Try using ipcalc from http://jodies.de/ipcalc, it is a big help when configuring networks and subnetworks.

# ipcalc
Address:         00001010.01001000.00001011.11 101100
Netmask: = 26 11111111.11111111.11111111.11 000000
Wildcard:             00000000.00000000.00000000.00 111111
Network:      00001010.01001000.00001011.11 000000
HostMin:         00001010.01001000.00001011.11 000001
HostMax:         00001010.01001000.00001011.11 111110
Broadcast:         00001010.01001000.00001011.11 111111
Hosts/Net: 62                    Class A, Private Internet


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I've seen this problem before. Not 100% sure of the fix, but I remember there being something in sysctl to tell ARP to consider these different devices - or something along these lines.

There have been a few posts about in on serverfault before if I remember correctly.

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Inbound traffic that goes to eth0 should come back via eth0 and vice versa. Try doing a TCPdump to check where traffic is going.

You also need policy based routing to do this : See Linux 2 network card routing depending on the interface used

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