the question about the dhcp relay deployment

Simon Hobson dhcp1 at
Sun May 8 08:01:26 UTC 2011

Glenn Satchell wrote:

>There is a third option (and the more general case): the relay agent 
>box has two interfaces, one connected to the subnet of the dhcp 
>client, and the other connected to a different subnet that still 
>needs to route to the dhcp server.
>In all cases the dhcp relay box needs to be able to route to the 
>dhcp server, whether this is through either network interface, or 
>because the relay box has an interface on the same subnet as the 
>dhcp server.
>When the two are on the same subnet there is an implicit route for 
>the local subnet, so you don't need to add a specific route.
>A simple test is "can the relay box ping the dhcp server?"

Just to expand on that ...

The questions here aren't really specific to DHCP, they are simple IP 
routing questions. Basically, every divice on the network need to 
have route(s) available to other devices it needs to communicate. In 
the absence of special requirements, the normal means of achieving 
this is to ensure that all routers have correct routes to all other 
subnets in the network.

At the device level, it is normal to simply specify a default gateway 
(router) on the subnet, and let that router take care of getting the 
packets where they are needed. In that way, any device needing to 
communicate off it's local network simply fires a packet off to the 
default gateway, and need know nothing more about what happens to it 
after than.

Now, for DHCP we have one or two extra requirements. Because the 
client cannot communicate off it's local network until it's fully 
configured it's network, then it cannot communicate with an off-net 
dhcp server to get it's network config. Therefore we need a dhcp 
relay *anywhere* on the clients local network - as Glenn says, this 
can be in any device, but they tend to be in the router for the 
simple reason that the router is "always on" anyway, but they don't 
need to be.
The only requirements for the relay agent are a) it can communicate 
via unicast with the dhcp server, and b) it has an interface directly 
attached to the client's network. You can perm any combination of 
interfaces etc as long as these two conditions are met. As Glenn says 
- can you ping the DCHP server ?

There is one other requirements that can be overlooked. It's not 
likely to be an issue in most networks, but the client must also be 
able to "ping the server" as it will attempt to renew it's lease via 
unicast directly to the server.
Simon Hobson

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