Thursday 26 August 2010

BSCI - Route Redistribution - #ip helper-address cmd

I'm due to sit my BSCI exam (courtesy of being a Network Academy Student) and I thought I'd just clarify the use of this very helpful cmd.

If you have DHCP host sat on one side of a router but the DHCP server is sat on the other side of the router then without further configuration the Host will not be able to communicate with the server as routers, by default, do not forward broadcast traffic.

To get round this you use the #ip helper-address [ip address of DHCP or Broadcast address of network where multiple DHCP reside] cmd.

Applied to the default gateway interface for the Host do the following:
R1(config)#int e0
R1(config-if)#ip helper-address
R1(config)#int e1
R1(config-if)#ip directed-broadcast

In the example above the admin knows there are multiple DHCP servers on the network connected to int e1. So instead of specifying a single DHCP server the router is configured to pass the DHCP request to the broadcast address of that network.

On interface e1, the #ip directed-broadcast cmd is used to convert the unicast traffic to a link-layer broadcast. a further explanation is here (taken from

An IP directed broadcast is a datagram which is sent to the broadcast address of a subnet to which the sending machine is not directly attached. The directed broadcast is routed through the network as a unicast packet until it arrives at the target subnet, where it is converted into a link-layer broadcast. Because of the nature of the IP addressing architecture, only the last router in the chain, the one that is connected directly to the target subnet, can conclusively identify a directed broadcast

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