reserved-only keyword in dhcp pool

[carl_townshend]

Hi All

I have seen the below on a router under the dhcp pool

reserved-only
address 172.1.1.1 hardware-address e0cb.bc8f.4839

Is this the same as a dhcp reservation using the client-identifier command or does it do something else?

Cheers

[Kasun Bandara]

Hi,

To restrict address assignments from the Dynamic Host Configuration Protocol (DHCP) address pool only to the preconfigured reservations, use the reserved-only command in DHCP pool configuration mode.

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipaddr/command/ipaddr-cr-book/ipaddr-r1.html#wp1449898080

[Georg Pauwen]

Hello,

consider the DHCP pool below. The 'reserved-only' keyword makes sure that only the two clients with client-id reservations get an IP address (the IP addresses specified). All other DHCP requests from other clients will be denied.

Hope that makes sense...

ip dhcp pool LAN
network 192.168.1.0 255.255.255.0
default-router 192.168.1.1
reserved-only
address 192.168.1.2 client-id 0100.5079.6668.00
address 192.168.1.11 client-id 0100.5079.6668.01

[carl_townshend]

Hi Georg

Is that really the case? Paul's comment below says different, there is not much about this command on the forums.

[Georg Pauwen]

Hello,

I have tested with several clients on the same switch, only those with reservations get an IP address.

[paul driver]

Hello

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@Georg Pauwen wrote:

Hello,

I have tested with several clients on the same switch,

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It looks like you are not using port-based allocation, just reserving client ids?
Also you don’t have multiple dhcp pools on multiple switches supporting the same subnet for which the reserved-only keyword is use for.

[Georg Pauwen]

@paul driver In your extended (lab I guess) setup, do the clients without reservations, on other switches, get an IP address ?

Better yet, if possible, post the configs of your lab switches...

[paul driver]

Hello @Georg Pauwen 

I’m not in a position to lab anything presently as for the configuration to test port based dhcp use a couple of switches and a few hosts  create pools with the same subnet on both switches and test 

[paul driver]

Hello Carl

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@carl_townshend wrote:

there is not much about this command on the forums.

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Please review here

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[paul driver]

@carl_townshend wrote:

reserved-only
address 172.1.1.1 hardware-address e0cb.bc8f.4839

Looks like this feature is used in relation to port based dhcp allocation
(local devices attached to specific ports that require the same ip address from the same access port)
I would say most applicable in large manufacturing sites where port based dhcp is active and you have multiple dhcp pools on multiple switches for localised ip address allocation.

The reserved-only keyword allows dhcp to service clients that are directly attached to that specific switch which is running a dhcp pool thus it negates any local client obtaining ip allocation off a neighbouring switch also running a local dhcp pool for its locally attached hosts.