Module 3 Notes
- Two known UDP port numbers
- Server Port: 67
- Client Port: 68
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DORA
- Discover: Client attempts to discover a DHCP server
- Offer: IP lease offer from server to client
- Request: Client requests to use the IP lease sent by the server
- Acknowledgement: Server sends acknowledgement to client that the lease was accepted.
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DHCP renewal: Process for client to request continued use(renewal) of its lease.
- By default renewal happens 50% of the way through the current IP lease
- Client sends DHCP request directly to server
- If the server respond with a DHCP Acknowledgement, the IP lease is renewed and its time clock restarts.
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DHCP Rebinding: If the server does not respond to the client's renewal requests, we eventually reach the rebinding phase.
- By default rebinding happens 87.5% through the current IP lease
- The client begins sending its DHCP Request packets as broadcasts to see if any DHCP server will allow them to continue using their IP.
- If a server responds, the lease is renewed and the timer restarts.
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DHCP Expiration: If no server responds before the lease ends, the lease expires and the IP is released.
- All TCP/IP communication stops.
- The client must go through the DORA initialization process once again to get a new IP address.
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DHCP Relay
- Unconfigured clients have no IP configuration so they don't have a subnet or gateway, they can only broadcast.
- Broadcasts are layer two so if the network doesn't have local DHCP, layer 3 devices can be configured as DHCP relays
- They forward broadcasts to the DHCP server
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DHCP Relay/Helper on Cisco
ip helper-address