Weird behavior with multiple pool inside shared networks

Simon Hobson dhcp1 at
Thu Oct 29 11:52:57 UTC 2015

Muhammad Faisal <faisalusuf at> wrote:

> Recently encountered the situation where different clients are required to be segregated for different cities and assigned different subnets from centralized server. For this we have defined required subnets and used "option router" to classify different hosts for respective subnet. Using shared network to achieve the above did not work as explained below.

I assume these different cities were using different subnets, with traffic routed between them ? If so then that is *NOT* a shared network, and you are correct that defining a shared-network would not work. For multiple routed networks, selecting an appropriate address for the client happens automagically based on the GI-Addr (Gateway Interface Address) value inserted into the relayed packet by the BOOTP relay agent serving the remote network.

BTW - for an easy to read and in depth explanation of how it all works, lookup The DHCP Handbook by Ralph Droms and Ted Lemon (two of the key people responsible for the ISC DHCP server, and who really know their stuff). I recommend this book - it covers the "why" (including an explanation of earlier methods), and the "how". The 2nd edition also covers failover.

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