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I am new here and very interested in computer networks. From a book, I have some questions, for which I need help.

question1: Can there be only one working DHCP server in one VLAN?

question2: Can there be only one subnet in one VLAN?

I know that a VLAN can have ports which belongs to several different subnets. So, why can't a VLAN have more than one DHCP server and more than one subnet?

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    Commented Jan 5, 2021 at 22:52

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You can have multiple DHCP servers serving a VLAN. The client will use OFFER that comes in first if receiving multiple.

ISC has support for DHCP redundancy while Microsoft was a bit lacking in this department. They have recommended split scopes in the past but it's not really a good solution.

You can have multiple subnets per VLAN. On a Cisco device you would use:

ip address x.x.x.x y.y.y.y secondary
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  • Thanks.More question , On a Cisco device , the ip address and the secondary ip address means the ip of a switch port or the client?
    – wuchang
    Commented Jun 24, 2013 at 8:19
  • Not sure what you mean. The IP belongs to the device. You can put it on SVI or interface or subinterface depending on platform.
    – Daniel Dib
    Commented Jun 24, 2013 at 8:56
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Question1:

No.

Multiple DHCP servers in one VLAN is possible. There can be operational issues if they are not under the same administrative control.

Usually there are more than one DHCP servers per VLAN for redundancy. Considerations need to be made to avoid DHCP conflicts. If DHCP servers share the same DHCP pool, something needs to be implemented to make sure the DHCP server does not offer an ip address that's already in use.

One approach is to use DNS forwarder on the access VLAN, the forwarder forward DHCP packets between hosts and DHCP server cluster. The DHCP server cluster are stateful (knowing which ip addresses have already been assigned).

Another approach is to use DHCP options to distinguish clients. For example, one DHCP server works for IP phones and they only offer IP addresses when certain DHCP options are set in the client's DHCP discover. This DHCP server uses a dedicated pool.

The short answer is, yes (as long as they are properly configured)

question2:

No.

VLAN is a layer 2 concept. A VLAN separates layer 2 broadcast domain. You can think of a non-managed switch as a switch with all ports in one VLAN. (no VLAN support is effectively every port in one VLAN). You can have two hosts in 192.168.0.0/24 and 2 hosts in 172.16.0.1/24 connected to the switch. Hosts in the same subnet can directly communicate with each other through the switch.

The problem with this approach is that everything is in the same broadcast domain. Packets destinated 255.255.255.255 will be sent to every port in the VLAN, which may be a security issue, a waste of bandwidth, processing power etc.

Even if you assign each port into their own VLAN, multiple subnets can still exist on one port (multiple IP addresses on one NIC, secondary IP addresses etc.).

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  • Thanks , after read you reply time and time again ,I get what you mean.If I say that the relationship between VLAN and subnet it many-to-many , is it right?
    – wuchang
    Commented Jun 24, 2013 at 8:17
  • Vlan is the underlying layer for subnets, one to many
    – sdaffa23fdsf
    Commented Jun 24, 2013 at 8:54

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