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Just a dumb question..

I have 1 core switch and 5 access switches.

Just wondering if it would be best to setup the IP helper on all access switches rather than just on the core switch?

Does this take a lot of processing resources for about 400 desks?

Thanks!

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Most probably your core switch has the gateway ip of the subnet like 192.168.1.1. If you want to enable ip-helper on your access switches, then you need to assign ip to the vlan on access switches something like from 192.168.1.2 to 192.168.1.6.

In this case all 6 switches will translate broadcast DHCP discover to unicast and send to DHCP server. DHCP server will receive 6 Discover and reply with 6 Offer. These 6 DHCP Offer will be relayed by all switches to the host so host PC will receive 6 offer. This means a lot of process for everyone without a benefit.

DHCP relay is not something cause high cpu load. It is always best practice to enable ip-helper where your gateway ip is configured for the subnet.

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The ip helper address goes on the layer 3 (vlan) interface.

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IP Helper (in configuration mode: ip helper-address) is functioning on network layer, because it forwards dhcp clients to the dhcp server on another network (lan segment or VLAN).

SO if you are talking about core switch as Layer 3 switch then configure IP Helper Addressing only on the CORE SWITCH, not on all the switches (btw, for layer 2 switches you cannot even turn on the ip routing)

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IP Helpers are used to translate a DHCP Broadcast into a Unicast message to be forwarded outside of a Broadcast domain. As a result, this helper needs to be configured on the edge of the Broadcast domain which will be a Layer 3 interface.

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If you want to set IP helper configuration on layer-2 switches, that switch needs to have an SVI IP address on that VLAN. Layer 2 switch will still use the core layer-3 switch to send the request to the DHCP server. The only thing layer-2 switch will do is to convert the broadcast request to unicast. You won't achieve anything good and you will have additional security concerns for having switch IP addresses on user VLAN.

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