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Cisco IOS DHCP Server Address Reclaim

erickafabiana

The DHCP Server Leases an address for a (Usually a number of days) and then
the client can ask to renew the lease. If the client does not renew the Server can reclaim the ip address and put it back in the pool. So my Question is what if a network admin goes to the End Device Host, and Statically configure an Ip address within the DHCP Servers Pool? I assume that creates a DHCP Conflict when the server tries to lease that address to a DHCP client. The server removes that address from the Pool until a network admin does a Clear ip DHCP Conflict*. Can The DHCP Server still reclaim a statically configured IP address this way even if it's removed from the pool from a Conflict?

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Comments

MAC_Addy

Typically if configured on a server and then statically configured on an end-point, it'll know that IP is used. When a new client comes online, it'll figure it out during the SYN/ACK phase. Though, it isn't best practice to configure an end-point with an IP statically. This is usually done from the server itself. It's just messy that way.

erickafabiana

Can the DHCP Server Still reclaim that Static IP

dontstop

erickafabiana wrote: »

Can The DHCP Server still reclaim a statically configured IP address this way even if it's removed from the pool from a Conflict?

If the address is removed from the conflict list but is still in conflict with a static address, it will be placed back in the pool until the next attempt at assigning it occurs. Which at that point in time it will be added back to the conflict list if the conflict occurs again.

The DHCP server only knows it's a conflict at the time it tries to allocate the address, so if you clear the conflict the DHCP server will happily add all the addresses back to the pool. Only at the time it tries to hand out the address the Router will try to ping the Address and/or the Host will try to ARP for that address to see if it's in use.

MAC_Addy

erickafabiana wrote: »

Can the DHCP Server Still reclaim that Static IP

Only if the IP is not statically assigned anymore. It'll query to see if the IP is being used or not. Just like with DHCP. Through ARP it'll know it's already taken.

Daughterofrnbsoul

MAC_Addy wrote: »

Only if the IP is not statically assigned anymore. It'll query to see if the IP is being used or not. Just like with DHCP. Through ARP it'll know it's already taken.

Configuring a Static IP at the End Device vs Static Allocation in the DHCP Server by CLIENT Mac-address are 2 different concepts right ?

dontstop

Daughterofrnbsoul wrote: »

Configuring a Static IP at the End Device vs Static Allocation in the DHCP Server by CLIENT Mac-address are 2 different concepts right ?

That's correct! With the end device static assignment the Router is totally unaware of the assignment until it tries to allocate the address from it's local pool and this results in a conflict.

erickafabiana

dontstop wrote: »

That's correct! With the end device static assignment the Router is totally unaware of the assignment until it tries to allocate the address from it's local pool and this results in a conflict.

Thankyou for the clear and concise information.