The default gateway entry on VLAN interface disappears when the real interface is restarted by ifdown and ifup.

Issue

• The default gateway entry on VLAN interface disappears when the interface is restarted by ifdown and ifup.

• Create VLAN interface, and set default gateway on the interface

# ifconfig | grep -A1 eth
eth0      Link encap:Ethernet  HWaddr 00:1A:4A:40:DC:F6  
          inet addr:10.64.221.199  Bcast:10.64.221.255  Mask:255.255.255.0
--
eth1      Link encap:Ethernet  HWaddr 00:1A:4A:40:DC:F9  
          inet addr:10.64.100.20  Bcast:10.64.100.255  Mask:255.255.255.0
--
eth1.110  Link encap:Ethernet  HWaddr 00:1A:4A:40:DC:F9  
          inet addr:10.64.110.3  Bcast:10.64.110.255  Mask:255.255.255.0
# route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
10.64.100.0     0.0.0.0         255.255.255.0   U     0      0        0 eth1
10.64.110.0     0.0.0.0         255.255.255.0   U     0      0        0 eth1.110
10.64.221.0     0.0.0.0         255.255.255.0   U     0      0        0 eth0
169.254.0.0     0.0.0.0         255.255.0.0     U     0      0        0 eth1.110
0.0.0.0         10.64.110.1     0.0.0.0         UG    0      0        0 eth1.110

• After ifdown and ifup the real interface, the entry for the default gateway disappears.

# ifdown eth1; ifup eth1
# route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
10.64.100.0     0.0.0.0         255.255.255.0   U     0      0        0 eth1
10.64.110.0     0.0.0.0         255.255.255.0   U     0      0        0 eth1.110
10.64.221.0     0.0.0.0         255.255.255.0   U     0      0        0 eth0
169.254.0.0     0.0.0.0         255.255.0.0     U     0      0        0 eth1

• Restarting network instead of using ifdown and ifup, the entry for the default gateway will be retrieved.

# service network restart
Shutting down interface eth1.110:  Removed VLAN -:eth1.110:-
                                                           [  OK  ]
Shutting down interface eth0:                              [  OK  ]
Shutting down interface eth1:                              [  OK  ]
Shutting down loopback interface:                          [  OK  ]
Bringing up loopback interface:                            [  OK  ]
Setting 802.1Q VLAN parameters:  Set name-type for VLAN subsystem. Should be visible in /proc/net/vlan/config
                                                           [  OK  ]
Bringing up interface eth0:                                [  OK  ]
Bringing up interface eth1:                                [  OK  ]
Bringing up interface eth1.110:  Added VLAN with VID == 110 to IF -:eth1:-
                                                           [  OK  ]
# route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
10.64.100.0     0.0.0.0         255.255.255.0   U     0      0        0 eth1
10.64.110.0     0.0.0.0         255.255.255.0   U     0      0        0 eth1.110
10.64.221.0     0.0.0.0         255.255.255.0   U     0      0        0 eth0
169.254.0.0     0.0.0.0         255.255.0.0     U     0      0        0 eth1.110
0.0.0.0         10.64.110.1     0.0.0.0         UG    0      0        0 eth1.110