How DHCP different than VLAN

6. How is it different than VLANs?

DHCP and VLANs, which are very different in concept, are sometimes cited as different solutions to the same problem. While they have a goal in common (easing moves of networked computers), VLANs represent a more revolutionary change to a LAN than DHCP. A DHCP server and forwarding agents can allow you to set things up so that you can unplug a client computer from one network or subnet and plug it into another and have it come alive immediately, it having been reconfigured automatically. In conjunction to Dynamic DNS, it could automatically be given its same name in its new place. VLAN-capable LAN equipment with dynamic VLAN assignment allows you to configure things so a client computer can be plugged into any port and have the same IP number (as well as name) and be on the same subnet. The VLAN-capable network either has its own configuration that lists which MAC addresses are to belong to each VLAN, or it makes the determination from the source IP address of the IP packets that the client computer sends. Some differences in the two approaches:

DHCP handles changes by reconfiguring the client while a VLAN-capable network handles it by reconfiguring the network port the client is moved to.
DHCP dynamic reconfiguration requires a DHCP server, forwarding agent in each router, and DHCP capability in each client’s TCP/IP support. The analogous capability in VLANs requires that all hubs throughout the network be VLAN-capable, supporting the same VLAN scheme. To this point VLAN support is proprietary with no vendor interoperability, but standards are being developed.
DHCP can configure a new client computer for you while a VLAN-capable network can’t.
DHCP is generally aimed at giving “easy moves” capability to networks that are divided into subnets on a geographical basis, or on separate networks. VLANs are generally aimed at allowing you to set up subnets on some basis other than geographical, e.g. instead of putting everyone in one office on the same subnet, putting each person on a subnet that has access to the servers that that person requires.
There is an issue with trying to use DHCP (or BOOTP) and VLANs at the same time, in particular, with the scheme by which the VLAN-capable network determines the client’s VLAN based upon the client computer’s source IP address. Doing so assumes the client computer is already configured, which precludes the use of network to get the configuration information from a DHCP or BOOTP server.

7. What protocol and port does DHCP use?
DHCP, like BOOTP runs over UDP, utilizing ports 67 and 68.

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