Cisco Systems DHCP relay agent versus Option 3; Routers Option
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Subject Author Date
DHCP relay agent versus Option 3; Routers Option lcorrigan 09-25-06
Posted by lcorrigan on September 25, 2006, 10:20 am
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Could someone explain to me the difference between using a router as a
DHCP relay agent (option 82) and configuring option 3 (routers option)
on the DHCP server?
I currently need to set up a server to use option 3 to reproduce an
issue in a test environment but I need a good "real-life" scenario to
assist. Would it be something like this for option 3?

DHCPclient >>>>>>> Remote router >>>>>>> DHCP Server
Where the "DHCP Server" would send the "Remote Router" client-side IP
address in the option 3 section of the DHCP Offer ? Or does the the
remote router need to be set as a relay-agent to have this make sense?

Would it ever make sense to have:

DHCPclient>>>>>>>DHCP Server, where Option 3 is still necessary in the
DHCP Offer? ie: Option 3 contains the DHCP Server's client side
address?

I already know how to set up option 3 and relay agents / helper ip
address but just need more on the theory of the option 3 and the
scenarios where it would be used.
Any help on this would be appreciated.


Posted by Roman Nakhmanson on September 25, 2006, 6:02 pm
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Hi

here is real-life scenario:

host-----10.1.1.0/24--router1---router2----router3---router4-------10.1.12.0/24----DHCP-server

router1 has ip-helper dhcp-server-ip

make sure ALL routers from host to server DON'T have "no service dhcp"
(usually it's enabled by default)

on the server (here is part of the config file for isc-dhcpd):
subnet 10.1.1.0 netmask 255.255.255.0 {
range 10.1.1.101 10.1.1.200;
option routers 10.1.1.1;
}

hope that helped
Roman Nakhmanson




Icorrigan wrote:
> Could someone explain to me the difference between using a router as a
> DHCP relay agent (option 82) and configuring option 3 (routers option)
> on the DHCP server?
> I currently need to set up a server to use option 3 to reproduce an
> issue in a test environment but I need a good "real-life" scenario to
> assist. Would it be something like this for option 3?
>
> DHCPclient >>>>>>> Remote router >>>>>>> DHCP Server
> Where the "DHCP Server" would send the "Remote Router" client-side IP
> address in the option 3 section of the DHCP Offer ? Or does the the
> remote router need to be set as a relay-agent to have this make sense?
>
> Would it ever make sense to have:
>
> DHCPclient>>>>>>>DHCP Server, where Option 3 is still necessary in the
> DHCP Offer? ie: Option 3 contains the DHCP Server's client side
> address?
>
> I already know how to set up option 3 and relay agents / helper ip
> address but just need more on the theory of the option 3 and the
> scenarios where it would be used.
> Any help on this would be appreciated.


Posted by lcorrigan on September 27, 2006, 1:18 pm
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Roman Nakhmanson wrote:
> Hi
>
> here is real-life scenario:
>
>
host-----10.1.1.0/24--router1---router2----router3---router4-------10.1.12.0/24----DHCP-server
>
> router1 has ip-helper dhcp-server-ip
>
> make sure ALL routers from host to server DON'T have "no service dhcp"
> (usually it's enabled by default)
>
> on the server (here is part of the config file for isc-dhcpd):
> subnet 10.1.1.0 netmask 255.255.255.0 {
> range 10.1.1.101 10.1.1.200;
> option routers 10.1.1.1;
> }
>
> hope that helped
> Roman Nakhmanson
>
>
>
>
> Icorrigan wrote:
> > Could someone explain to me the difference between using a router as a
> > DHCP relay agent (option 82) and configuring option 3 (routers option)
> > on the DHCP server?
> > I currently need to set up a server to use option 3 to reproduce an
> > issue in a test environment but I need a good "real-life" scenario to
> > assist. Would it be something like this for option 3?
> >
> > DHCPclient >>>>>>> Remote router >>>>>>> DHCP Server
> > Where the "DHCP Server" would send the "Remote Router" client-side IP
> > address in the option 3 section of the DHCP Offer ? Or does the the
> > remote router need to be set as a relay-agent to have this make sense?
> >
> > Would it ever make sense to have:
> >
> > DHCPclient>>>>>>>DHCP Server, where Option 3 is still necessary in the
> > DHCP Offer? ie: Option 3 contains the DHCP Server's client side
> > address?
> >
> > I already know how to set up option 3 and relay agents / helper ip
> > address but just need more on the theory of the option 3 and the
> > scenarios where it would be used.
> > Any help on this would be appreciated.


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