Advanced DHCP
Scopes, exclusions and reservations, the life of a lease, options, the relay agent between networks and redundancy with two servers.
- 29 cards
- 5 topics
- Free
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DHCP: what is it?
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A protocol that automatically gives a host its IP address, mask and other settings, by exchanging four messages with a server: Discover, Offer, Request, Acknowledge.
The client has no address yet: it sends the first message as a broadcast, from UDP port 68 to the server's UDP port 67.
A VLAN 30 host gets no address; the DHCP server is in VLAN 10. Why?
- The Discover is a broadcast that the router does not forward
- The router drops all UDP packets
- The server only answers MAC addresses it already knows
- The client must first get a static address to reach the server
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The Discover is a broadcast that the router does not forward
The client has neither an address nor knowledge of the server: it broadcasts. A broadcast stays inside its broadcast domain. A relay agent is needed on the VLAN 30 router.
The four messages of an assignment are, in order: Discover, …, …, Acknowledge.
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The four messages of an assignment are, in order: Discover, Offer, Request, Acknowledge.
The client broadcasts Discover, the server proposes an address with Offer, the client asks for it with Request (broadcast, to inform other servers) and the server confirms with Acknowledge.
Scope: what is it?
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The range of addresses in a subnet that a server may hand out, with its options; exclusions are carved out of it and reservations are fixed within it.
An exclusion removes addresses from distribution (fixed devices). A reservation ties an address to a MAC address: the same device always receives it.