Cisco Systems ip helper for a specific address

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Subject Author Date
ip helper for a specific address chris.glanville@gmail.com 06-18-07
Posted by chris.glanville@gmail.com on June 18, 2007, 4:21 pm
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Is there a way to specify an "ip helper" rule for a specific address
so that only UDP traffic from THAT ip address is forwarded to a
different subnet?

For example udp from 192.168.1.15 -> 192.168.2.18

I know that you can do 192.168.1/24 -> 192.168.2.18, i was just
hopping there was a way to only forward a single address.

Thanks!


Posted by headsetadapter.com on June 18, 2007, 4:44 pm
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Chris,

IP HELPER forwards DHCP REQUESTS. The workstation sends DHCP request when it
HAS NO IP ADDRESS. It means that you cannot filter IP Helper requests based
on IP addresses.

If you want to have a different DHCP servers for different PCs, you may
install two or more DHCP servers (physical servers), configure certain MAC
addresses to get a specific IP addresses (static DHCP assignment), and
configure multiple "ip helper" addresses on your router. In this case your
DHCP requests will be forwarded to ALL DHCP servers, but the only DHCP
server with specific configuration (assignment) will reply to certain
requests.

Good luck,

Mike
CCNP, CCDP, CCSP, Cisco Voice, MCSE W2K, MCSE+I, Security+, etc.
CCIE R&S (in progress), CCIE Voice (in progress)
------
Headset Adapters for Cisco IP Phones
www.ciscoheadsetadapter.com
www.headsetadapter.com



> Is there a way to specify an "ip helper" rule for a specific address
> so that only UDP traffic from THAT ip address is forwarded to a
> different subnet?
>
> For example udp from 192.168.1.15 -> 192.168.2.18
>
> I know that you can do 192.168.1/24 -> 192.168.2.18, i was just
> hopping there was a way to only forward a single address.
>
> Thanks!
>



Posted by Cliff on June 18, 2007, 5:58 pm
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> Chris,
>
> IP HELPER forwards DHCP REQUESTS. The workstation sends DHCP request when it
> HAS NO IP ADDRESS. It means that you cannot filter IP Helper requests based
> on IP addresses.
>
> If you want to have a different DHCP servers for different PCs, you may
> install two or more DHCP servers (physical servers), configure certain MAC
> addresses to get a specific IP addresses (static DHCP assignment), and
> configure multiple "ip helper" addresses on your router. In this case your
> DHCP requests will be forwarded to ALL DHCP servers, but the only DHCP
> server with specific configuration (assignment) will reply to certain
> requests.
>
> Good luck,
>
> Mike
> CCNP, CCDP, CCSP, Cisco Voice, MCSE W2K, MCSE+I, Security+, etc.
> CCIE R&S (in progress), CCIE Voice (in progress)
> ------
> Headset Adapters for Cisco IP
Phoneswww.ciscoheadsetadapter.comwww.headsetadapter.com
>
>
>
>
>
> > Is there a way to specify an "ip helper" rule for a specific address
> > so that only UDP traffic from THAT ip address is forwarded to a
> > different subnet?
>
> > For example udp from 192.168.1.15 -> 192.168.2.18
>
> > I know that you can do 192.168.1/24 -> 192.168.2.18, i was just
> > hopping there was a way to only forward a single address.
>
> > Thanks!- Hide quoted text -
>
> - Show quoted text -

Couldnt he use the IP helper address then just make an ACL to filter
which packets are forwarded?


Posted by Brian V on June 18, 2007, 11:00 pm
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> Chris,
>
> IP HELPER forwards DHCP REQUESTS. The workstation sends DHCP request when
> it HAS NO IP ADDRESS. It means that you cannot filter IP Helper requests
> based on IP addresses.
>
> If you want to have a different DHCP servers for different PCs, you may
> install two or more DHCP servers (physical servers), configure certain MAC
> addresses to get a specific IP addresses (static DHCP assignment), and
> configure multiple "ip helper" addresses on your router. In this case your
> DHCP requests will be forwarded to ALL DHCP servers, but the only DHCP
> server with specific configuration (assignment) will reply to certain
> requests.
>
> Good luck,
>
> Mike
> CCNP, CCDP, CCSP, Cisco Voice, MCSE W2K, MCSE+I, Security+, etc.
> CCIE R&S (in progress), CCIE Voice (in progress)
> ------
> Headset Adapters for Cisco IP Phones
> www.ciscoheadsetadapter.com
> www.headsetadapter.com
>
>
>
>> Is there a way to specify an "ip helper" rule for a specific address
>> so that only UDP traffic from THAT ip address is forwarded to a
>> different subnet?
>>
>> For example udp from 192.168.1.15 -> 192.168.2.18
>>
>> I know that you can do 192.168.1/24 -> 192.168.2.18, i was just
>> hopping there was a way to only forward a single address.
>>
>> Thanks!
>>
>
>

ip helper does a whole heck of a lot more than just dhcp. Helper by default
forwards 8 different UDP protocols, you can also customize what it sends. To
answer the OP's question, no, you cannot tell one device to go one way,
another to go the other way via the helper command. If you know that there
is one specific device that needs specific information either hard code it
or create another vlan for it and send that vlan where you want it to go.


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