During my review of the Infrastructure Security section of the 300-115 Cisco exam, I was a little confused about the purpose of DHCP Option 82. Why would the controller (or other relay agent) need to check that option before forwarding the DHCP request to the server? This didn't click until I read Cisco's configuration guide on this. Here's how it works: DHCP Option 82 Step 1: Configure Option 82 in the DHCP server Step 2: Configure the relay agent with appropriate Option 82 Step 3: Client requests a DHCP address Step 4: The DHCP relay agent adds option 82 information before encapsulating the DISCOVER into a Unicast packet to the DHCP server Step 5: The DHCP server receives the relayed request, checks Option 82 and sends OFFER accordingly. Step 6: The relay agent receives the OFFER, strips Option 82 info (deploys policy if-applicable), then forwards IP address assignment to the client. If you're studying the topic, I hope this helps.
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