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DHCP backup over Layer 3 Interfaces

Hello experts,

i have a question implementing a DHCP backup over Layer 3.

We have a couple of branch offices which are conected via layer 3 (ospf) to the main office. In each branch office and main office there is a DHCP server and on the vlan interfaces there is a ip helper also configured:

|Main|-------L3--------->|Branch|-------->VLAN (helper)--------------->DHCP Server

  |

  |

L3|

  |

  |

  |

|Branch|

  |

  |

VLAN (helper)

  |

  |

DHCP Server

In case of a DHCP failure in one of the branches i would like to add the ip helper to the layer 3 interfaces of the branch router to act like a backup so that the DHCP Server in the main office can take over. Is this a correct approach?

Thank you

1 REPLY
Hall of Fame Super Blue

Re: DHCP backup over Layer 3 Interfaces

==cisco@akdb.de

Hello experts,

i have a question implementing a DHCP backup over Layer 3.

We have a couple of branch offices which are conected via layer 3 (ospf) to the main office. In each branch office and main office there is a DHCP server and on the vlan interfaces there is a ip helper also configured:

|Main|-------L3--------->|Branch|-------->VLAN (helper)--------------->DHCP Server

  |

  |

L3|

  |

  |

  |

|Branch|

  |

  |

VLAN (helper)

  |

  |

DHCP Server

In case of a DHCP failure in one of the branches i would like to add the ip helper to the layer 3 interfaces of the branch router to act like a backup so that the DHCP Server in the main office can take over. Is this a correct approach?

Thank you


If i understand you correctly you can do this by having multiple ip helper-addresses under the vlan interfaces. however it is important to understand that you cannot use one as active and one as backup. If there are multiple ip helper-addresses under the vlan interface then they will both be queried for every DHCP request. So you cannot have the same IPs being handed out by both DHCP servers. What you need to do is split each DHCP scope between the DHCP servers eg.

you have a DHCP scope 192.168.5.0 255.255.255.0 in the branch office. The default-gateway for this scope is 192.168.5.1

you now want to use the HQ DHCP server as well as the branch DHCP server for this scope to add some redundancy. So what you would do is allocate half the scope to the branch DHCP server and half to the HQ server -

HQ

==

IPs - 192.168.5.1 - 127

Subnet mask - 255.255.255.0

default-gateway - 192.168.5.1

Branch

=====

IPs - 192.168.5.128 - 254

subnet mask - 255.255.255.0

default-gateway - 192.168.5.1

note that the subnet mask and the default-gateway are the same as they were before. You also need to exclude the other half of the addresses on each DHCP server eg.

in HQ exclude 192.168.5.128 - 254

in Branch exclude 192.168.5.1 - 127

Jon

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