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dhcp & vlan

pravin.bhavsar
Occasional Contributor

dhcp & vlan

Hi

i have dhcp server & three vlan created

vlan 6 getting ip from dhcp is 192.168.151.0/24
vlan 7 getting ip from dhco is 192.168.152.0/24
vlan 8 getting ip from dhco is 192.168.153.0/24

In dhcp i have configured three scope associated with three vlan.

in core switch for inter vlan routring i have configured svi.

In svi i typed command ip helper-address 192.168.151.2 which is my dhcp server

It works fine but my question is how different vlan getting different scope range from dhcp server.how dhcp know about that?

ip helper-address command is for allowing broadcast.

can someone can describe me about how thing is work?

Thanks,
Pravin
3 REPLIES
Matt Hobbs
Honored Contributor

Re: dhcp & vlan

The ip helper-address changes the broadcast packet into a unicast packet towards to the DHCP server. It also inserts a GIADDR option into the packet which tells the DHCP server which subnet it needs to give an IP address for.
pravin.bhavsar
Occasional Contributor

Re: dhcp & vlan

can u explain more about that.one by one step.
like machine in vlan 6 is broadcast for ip address & it is broadcasting in vlan 6 & vlan 6 has ip address 192.168.151.1 & then What happan?

Regards

Pravin
pkulkarni
Occasional Visitor

Re: dhcp & vlan

The broadcast is converted to a unicast by the switch, directed towards the address mentioned by the ip helper-address. This unicast packet contains the the GIADDR field which tells the DHCP server the scope from which to assign an IP address. This reply is then sent as an unicast to the device which sent the request broadcast initailly.

Hope this answers your question.

Cheers,

Prakash