[CentOS] Network configuration issue with second public ip on CentOS 6

Hi folks,

I have a server at Soyoustart.com (which is a brand of OVH). I run 
CentOS 6.7 and have problems with network configuration.

I want to add a second public ip (failover ip) to the server and did 
follow the instructions in the CentOS-section of 
http://hilfe.ovh.de/AdministrationIpAliasHinzufuegen

After executing the required steps I'm not able to reach the host via 
the second ip from an external host. I tried it with ping and with ssh 
commands.

Here the content of the config files (I anonymized the server ip with 
xx.xx.xx.xx and the failover ip with yy.yy.yy.yy):

$ cat /etc/sysconfig/network
NETWORKING=yes
HOSTNAME=<name-of-my-host>
NOZEROCONF=true
GATEWAY=xx.xx.xx.254

$ cat /etc/sysconfig/network-scripts/ifcfg-eth0
DEVICE=eth0
BOOTPROTO=static
IPADDR=xx.xx.xx.xx
NETMASK=255.255.255.0
ONBOOT=yes
GATEWAY=xx.xx.xx.254

$ cat /etc/sysconfig/network-scripts/ifcfg-eth0:0
DEVICE=eth0:0
BOOTPROTO=static
IPADDR=yy.yy.yy.yy
NETMASK=255.255.255.255
ONBOOT=yes

When I restart the network I get a message (two times) saying "RTNETLINK 
answers: Operation not supported".

$ service network restart
Schnittstelle eth0 beenden:                                [  OK  ]
Loopback-Schnittstelle beenden:                            [  OK  ]
Loopback-Schnittstelle hochfahren:                         [  OK  ]
Schnittstelle eth0 hochfahren:  Determining if ip address xx.xx.xx.xx is 
already in use for device eth0...
RTNETLINK answers: Operation not supported
RTNETLINK answers: Operation not supported
                                                            [  OK  ]

I can see two interfaces eth0 and eth0:0

$ LANG="" ifconfig
eth0      Link encap:Ethernet  HWaddr ..:..:..:..:..:..
           inet addr:xx.xx.xx.xx  Bcast:xx.xx.xx.255 Mask:255.255.255.0
           UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
           RX packets:11122 errors:0 dropped:0 overruns:0 frame:0
           TX packets:14371 errors:0 dropped:0 overruns:0 carrier:0
           collisions:0 txqueuelen:1000
           RX bytes:1362900 (1.2 MiB)  TX bytes:3462327 (3.3 MiB)
           Interrupt:20 Memory:fe500000-fe520000

eth0:0    Link encap:Ethernet  HWaddr ..:..:..:..:..:..
           inet addr:yy.yy.yy.yy  Bcast:yy.yy.yy.yy Mask:255.255.255.255
           UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
           Interrupt:20 Memory:fe500000-fe520000

lo        Link encap:Local Loopback
           inet addr:127.0.0.1  Mask:255.0.0.0
           UP LOOPBACK RUNNING  MTU:65536  Metric:1
           RX packets:3096 errors:0 dropped:0 overruns:0 frame:0
           TX packets:3096 errors:0 dropped:0 overruns:0 carrier:0
           collisions:0 txqueuelen:0
           RX bytes:274948 (268.5 KiB)  TX bytes:274948 (268.5 KiB)


I'm able to reach the host from another host (outside OVH network) via 
xx.xx.xx.xx (I use ping and ssh), but not via yy.yy.yy.yy.

Just to be sure that there isn't an issue with the firewall I did 
"service iptables stop" and ran the "ping" and "ssh" commands again with 
the same result. For ip xx.xx.xx.xx it works, but not for yy.yy.yy.yy.

Not sure if this is important, just in case it matters, some additional 
information:

IPv6 is disabled by kernel parameter "ipv6.disable=1" in /etc/grub.conf

The two mac addresses I see for eth0 and eth0:0 with the "ifconfig" 
command are the same. In the OVH/Soyoustart.com GUI I created a virtual 
mac for the failover ip, I'm not sure if that virtual mac should come 
into play somewhere?

$ uname -a
Linux <name-of-my-host> 2.6.32-573.12.1.el6.x86_64 #1 SMP Tue Dec 15 
21:19:08 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux

$ LANG="" route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref Use Iface
xx.xx.xx.0      *               255.255.255.0   U     0 0        0 eth0
default         vss-gw-6k.fr.eu 0.0.0.0         UG    0 0        0 eth0

Why is yy.yy.yy.yy not reachable from outside? What am I doing wrong or 
what can I do to narrow down the problem?

Regards,

Meikel