Both servers running keepalived become master and have a same Virtual IP

Solution 1:

Packets are not passing between machines on the em1 interface (causing a split brain scenario as Mike states).

• check your firewall to ensure packets aren't being caught
• check your networking to ensure em1 is the same network on both machines

Here's an example of what one of the packets looks like:

Frame 2: 54 bytes on wire (432 bits), 54 bytes captured (432 bits)
    Arrival Time: Jun  1, 2013 03:39:50.709520000 UTC
    Epoch Time: 1370057990.709520000 seconds
    [Time delta from previous captured frame: 0.000970000 seconds]
    [Time delta from previous displayed frame: 0.000970000 seconds]
    [Time since reference or first frame: 0.000970000 seconds]
    Frame Number: 2
    Frame Length: 54 bytes (432 bits)
    Capture Length: 54 bytes (432 bits)
    [Frame is marked: False]
    [Frame is ignored: False]
    [Protocols in frame: eth:ip:vrrp]
Ethernet II, Src: 00:25:90:83:b0:07 (00:25:90:83:b0:07), Dst: 01:00:5e:00:00:12 (01:00:5e:00:00:12)
    Destination: 01:00:5e:00:00:12 (01:00:5e:00:00:12)
        Address: 01:00:5e:00:00:12 (01:00:5e:00:00:12)
        .... ...1 .... .... .... .... = IG bit: Group address (multicast/broadcast)
        .... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
    Source: 00:25:90:83:b0:07 (00:25:90:83:b0:07)
        Address: 00:25:90:83:b0:07 (00:25:90:83:b0:07)
        .... ...0 .... .... .... .... = IG bit: Individual address (unicast)
        .... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
    Type: IP (0x0800)
Internet Protocol Version 4, Src: 10.0.10.11 (10.0.10.11), Dst: 224.0.0.18 (224.0.0.18)
    Version: 4
    Header length: 20 bytes
    Differentiated Services Field: 0x00 (DSCP 0x00: Default; ECN: 0x00: Not-ECT (Not ECN-Capable Transport))
        0000 00.. = Differentiated Services Codepoint: Default (0x00)
        .... ..00 = Explicit Congestion Notification: Not-ECT (Not ECN-Capable Transport) (0x00)
    Total Length: 40
    Identification: 0x8711 (34577)
    Flags: 0x00
        0... .... = Reserved bit: Not set
        .0.. .... = Don't fragment: Not set
        ..0. .... = More fragments: Not set
    Fragment offset: 0
    Time to live: 255
    Protocol: VRRP (112)
    Header checksum: 0x4037 [correct]
        [Good: True]
        [Bad: False]
    Source: 10.0.10.11 (10.0.10.11)
    Destination: 224.0.0.18 (224.0.0.18)
Virtual Router Redundancy Protocol
    Version 2, Packet type 1 (Advertisement)
        0010 .... = VRRP protocol version: 2
        .... 0001 = VRRP packet type: Advertisement (1)
    Virtual Rtr ID: 254
    Priority: 151 (Non-default backup priority)
    Addr Count: 1
    Auth Type: No Authentication (0)
    Adver Int: 1
    Checksum: 0x3c01 [correct]
    IP Address: 10.0.0.254 (10.0.0.254)

Solution 2:

For my case I had to allow multicast traffic through the firewall to 224.0.0.18, for ufw:

ufw allow from 224.0.0.18
ufw allow to 224.0.0.18

This helped me.