Tag - LAN

What Routers include DHCP servers

DHCP requires disk storage (or some other form of reliable non-volatile storage), making the task of DHCP service more compatible with servers than with dedicated routers. The large-scale routers (i.e., those of Cisco, Bay, Fore) don’t an will probably never will have a DHCP server function.

But there are a number of types of servers that can be configured to route and serve DHCP. This includes Novell servers and computers running Unix. There are also units designed to handle two or more aspects of your Internet connection, e.g. routing between a LAN and a leased line as well as doing other functions to allow computers on the LAN to reach the Internet (or corporate intranet as the case may be). One example is Farallon’s Netopia Internet Router mentioned above under commercial servers.

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how can addresses be served on subnets other than the primary one

24. If a single LAN has more than one subnet number, how can addresses be served on subnets other than the primary one?

A single LAN might have more than one subnet number applicable to the same set of ports (broadcast domain). Typically, one subnet is designated as primary, the others as secondary. A site may find it necessary to support addresses on more than one subnet number associated with a single interface. DHCP’s scheme for handling this is that the server has to be configured with the necessary information and has to support such configuration & allocation. Here are four cases a server might have to handle:

Dynamic allocation supported on secondary subnet numbers on the LAN to which the server is attached.
Dynamic allocation supported on secondary subnet numbers on a LAN which is handled through a DHCP/BOOTP Relay. In this case, the DHCP/BOOTP Relay sends the server a gateway address associated with the primary subnet and the server must know what to do with it.
The other two cases are the same capabilities during manual allocation. It is possible that a particular server-implementation can handle some of these cases, but not all of them. See section below listing the capabilities of some servers.

25. If a physical LAN has more than one logical subnet, how can different groups of clients be allocated addresses on different subnets?
One way to do this is to preconfigure each client with information about what group it belongs to. A DHCP feature designed for this is the user class option. To do this, the client software must allow the user class option to be preconfigured and the server software must support its use to control which pool a client’s address is allocated from.

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DHCP support and BOOTP

13. Can DHCP support statically defined addresses?

Yes. At least there is nothing in the protocol to preclude this and one expects it to be a feature of any DHCP server. This is really a server matter and the client should work either way. The RFC refers to this as manual allocation.

14. How does DHCP and BOOTP handle multiple subnets?

For the situations where there is more than one LAN, each with its own subnet number, there are two ways. First of all, you can set up a seperate server on each subnet. Secondly, a feature of some routers known as “BOOTP forwarding” to forward DHCP or BOOTP requests to a server on another subnet and to forward the replies back to the client. The part of such a router (or server acting as a router) that does this is called a “BOOTP forwarding agent”. Typically you have to enable it on the interface to the subnet to be served and have to configure it with the IP address of the DHCP or BOOTP server. On a Cisco router, the address is known as the “UDP Helper Address”.

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