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can a helper ip address and a ip address of a vlan be the same?

Posted on 2012-04-02
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Last Modified: 2012-04-02
is it possible to use the same ip address for the helper ip and the ip address of a vlan?
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Question by:knfitz
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Jimmy Larsson, CISSP, CEH earned 2000 total points
ID: 37796551
Yes, but it makes no sense.

Lets say that your clients are on subnet 192.168.1.0/24 and your "vlan interface" is 192.168.1.1. What happens when a client wants an ip-address is that he sends a broadcast requesting contact with an dhcp-server. Since broadcasts are on the same subnet only, the dhcp-server needs to be on that subnet. Your router/switch can ofcours be the dhcp-server and since 192.168.1.1 is on the same subnet, all you have to do is to enable the dhcp-scope in the server.

On the other hand, if the dhcp server is elsewhere, what you do then is to put an ip helper in any device on the local subnet (for example you router/switch 192.168.1.1, and tell it to forward all dhcp-requests coming in as broadcasts, and forward them as plain unicasts to the dhcp-server defined. For example:

interface vlan 168
 ip address 192.168.1.1 255.255.255.0
 ip helper-address 192.168.2.10

If you have a config like this:

interface vlan 168
 ip address 192.168.1.1 255.255.255.0
 ip helper-address 192.168.1.1

All you do is to say "take all incoming dhcp broadcasts and forward them to myself" which is totally uneccecary, since the router/switch will catch the requests anyway.

Does this explain what you wanna know?

Best regards
Kvistofta
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Author Comment

by:knfitz
ID: 37796713
So you're saying it will work either way and all I really need is an ip address on the Vlan? My dhcp server is at another site. So I give the vlan a ip address and then create a dhcp scope on my dhcp server for that range and dhcp will know what to do?
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Expert Comment

by:Jimmy Larsson, CISSP, CEH
ID: 37796743
Yes, that´s basically it.

/Kvistofta
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Author Comment

by:knfitz
ID: 37796764
I have this set on my core switch. Do I need to set an ip address on the vlan ids for my closet switches also?
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LVL 17

Expert Comment

by:Jimmy Larsson, CISSP, CEH
ID: 37796786
Nope.

/Kvistofta
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