Question about DHCP

I never thought about this but how does a router acting as a DHCP server 'knows' which pool to take address out from and assign to a requesting DHCP client?

Lets say you have several pools all for different vlans; The routes gets DHCP request and then which pool he looks at?

Find more posts tagged with

SAVE $250 on Your CISCO Boot Camp — Use Code "EXAM26"

Exclusively for TechExams members for Infosec Boot Camps starting before April 30, 2026

View CISCO Boot Camps

Comments

mikem2te

acidsatyr wrote: »

I never thought about this but how does a router acting as a DHCP server 'knows' which pool to take address out from and assign to a requesting DHCP client?

Lets say you have several pools all for different vlans; The routes gets DHCP request and then which pool he looks at?

It knows because all the interfaces have an ip address assigned, so if a dhcp discover message comes in to a routers interface it matches up the ip address of the interface to the available dhcp pools and hands out correct addresses.

Work just the same for vlan subinterfaces as well, a tagged dhcp discover message comes in, the router then matches this up against the 'encapsulation dot1q' lines under the sub interfaces to find the correct subinterface , it then knows subinterface IP to find the correct dhcp pool.

acidsatyr

But, what if a request is relayed accross a network?

Thanks for reply

Forsaken_GA

the broadcast is converted to unicast and forwarded to the configured server

edit: I should elaborate on that.

Assuming the router for the subnet has it's interface configured with an ip helper-address, the broadcast is converted to unicast and forwarded to the IP specified in the helper-address command.

If your host needs access to more than one service via broadcast (ie, it needs DHCP and TFTP, but they reside on different servers) you need to configure directed broadcast forwarding

acidsatyr

Ah, so, the source IP of that unicast would be DHCP relay agent and based on that DHCP will deduce which pool to look at.

Thanks!