03-18-2009 09:05 AM - edited 03-04-2019 03:59 AM
We have 2 distinct vlans in our office.
Vlan154 is currently on dhcp via a router
vlan 153 is handled by a windows box.
We are now moving dhcp for vlan 154 to a windows box.
My question...do i need to do anything
besides remove the dhcp group and put in
a helper address?
here is our config
ip dhcp pool blt-pool
network 192.168.154.0 255.255.254.0
default-router 192.168.154.1
netbios-node-type h-node
dns-server 192.168.153.5 192.168.9.5
netbios-name-server 192.168.153.7 192.168.153.5
lease 0 7
interface GigabitEthernet0/0.153
encapsulation dot1Q 153
ip address 192.168.153.252 255.255.255.0
no snmp trap link-status
standby 1 ip 192.168.153.1
standby 1 track Serial1/0
!
interface GigabitEthernet0/0.154
encapsulation dot1Q 154
ip address 192.168.155.252 255.255.254.0
no snmp trap link-status
standby 2 ip 192.168.154.1
standby 2 track Serial1/0
is it simply a matter of adding a helper address under the subinterface and removing the dhcp pool config?
I notice we do not a helper address configured for the 153 dhcp network, even
though a windows box is handling the task
also how will the dhcp server know if it should hand out addresses to the .153 network
or .154 network to a specific host
Solved! Go to Solution.
03-18-2009 12:09 PM
Richard
When the router forwards the packet on to the DHCP server it uses the interface IP address on which the request was received and includes this in the request to the DHCP server. It's actually the giaddr (gateway address) field that is filled in.
So if a request came in on the 192.168.153.x network the router would forward the request on as a unicast packet to the DHCP server and would put it's 192.168.153.x interface address into the giaddr field.
So the DHCP then reads the giaddr field and can then determine which scope to use when offering an IP address.
Jon
03-18-2009 09:15 AM
Richard
"is it simply a matter of adding a helper address and removing the dhcp pool config?"
Yes altho obviously you need to timeout the leases handed from the router.
" notice we do not a helper address configured for the 153 dhcp network, even
though a windows box is handling the task"
This is probably because the DHCP server is on vlan 153 ?. An ip helper-address is only need when the DHCP server is on a different vlan.
"also how will the dhcp know what network/vlan to give out to specific ports"
The router will do all that for you. The packet that gets to the DHCP server will have the information of the router subnet.
Jon
03-18-2009 11:39 AM
thx jon.
another question..
the new dhcp server is given an address
for ex
192.168.154.10 and this would handle all
dhcp requests from the both vlans and
thus give out 2 diff networks.
is the decision on what ip/subnet to
assign a given host based upon the trunk
information on the router /gateway interface?
03-18-2009 12:09 PM
Richard
When the router forwards the packet on to the DHCP server it uses the interface IP address on which the request was received and includes this in the request to the DHCP server. It's actually the giaddr (gateway address) field that is filled in.
So if a request came in on the 192.168.153.x network the router would forward the request on as a unicast packet to the DHCP server and would put it's 192.168.153.x interface address into the giaddr field.
So the DHCP then reads the giaddr field and can then determine which scope to use when offering an IP address.
Jon
03-18-2009 12:33 PM
thx!
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