VLAN DHCP Question

[coolboarderguy]

Hi All,

just some general questions about VLANs and DHCP etc. When you create a number of different vlans, across different switches, does each vlan require a dhcp server, for those nodes to obtain their IPs.? Or, is this achieved another way.? Also, what is the difference bewteen an access switch and distribution switch.? Cheers.

coolboarderguy...

[paddyxdoyle]

Hi,

Each VLAN will require a different DHCP scope, multiple DHCP scopes can be created on a single DHCP server.

In a typical Cisco design an access switch is the entry point into the network where all the edge devices connect. Its typically a layer two switch only and here users are assigned into VLANs, port based authentication can occur.

The distribution layer is typically a fast layer 3 switch providing interVLAN routing, aggregation of Access layer switches, traffic filtering, address summarisation. The distribution layer should perform all layer3 and policy function allowing the network core to act as a high speed layer2 backbone switch between all the other distribution layer switches.

There are a few different variants though to these layered building blocks, it depends on the size of your network as to how the layers function.

HTH

PJD

[coolboarderguy]

Hi All,

ok, so how does the DHCP know which PC is in which VLAN..? Does the ICND book cover this stuff. This sounds like fundamental stuff to study to work in Network Engineering, yet, both my Cisco Press Intro and Que ExamCram2 mention nothing of this kind of stuff. Any good links on this.? Cheers.

coolboarderguy...

[paddyxdoyle]

Hi,

When the DHCP server is residing on a different network to the client, the clients local gateway needs to have a helper-address configured. This allows the router to pass the original DHCP broadcast onto the destination DHCP server.

The client uses the IP address of this local gateway in a field called GIADDR so the DHCP server knows which subnet the client is on and can allocate the correct IP address.

Its probably best to have a look on the Microsoft site for this kind of thing, the following links should help:

http://support.microsoft.com/default.aspx?scid=kb;en-us;169289

http://support.microsoft.com/default.aspx?scid=kb;en-us;120932

HTH

PJD

[coolboarderguy]

Hi All,

ok, well, that was straight forward. Still, though, it only partially answers my query. Does ICND cover this area, or, does this kind of stuff come later, like CCNP.? What are some example cisco devices that can handle this scenario of using GIADDR.? Cheers.

coolboarderguy...

[paddyxdoyle]

Hi,

Its not really a Cisco thing as such so parameters such as GIADDR that are part of a protocol (DHCP) may not be specifically covered in CCNA, CCNP etc..

Configuring DHCP on Cisco devices may come somewhere but i can't remeber if this was part of ICND etc.

For examples, you need to look up using DHCP in routed networks or more specifically using the "ip-helper" command. Also have a read through the RFC's which are a sure place to find out information about the inner workings of protocols.

Without sounding arrogant, these kind of areas normally crop up working in networked environments with multivendor products and are not specific to Cisco so you may not learn specifically about various paramaters on Cisco courses/books but i'm sure at some stage you will find out about them one way or another.

HTH

PJD

[paddyxdoyle]

Actually whilst seeking information on Multicast i came across this link which does talk about GIADDR :)

http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_configuration_guide_chapter09186a00800ca75c.html

Good luck

Paddy

[dennisopiso]

Hi,

one dhcp server is enough to cater different vlans in your network. you just have to create scopes/superscopes. scopes represents the each vlan. then define the ip address of the dhcp server in the dhcp relay agent (which is your workstation's gateway/router). So when your workstation tries to find an ip address it forwards the request to your relay agent, then it proxies the request to the dhcp server defined on your switch. so when the request is received by the dhcp server, it checks the subnet/vlan of the requesting relay agent then assigns the appropriate network for that vlan

[sushilk]

Hi,

Hope the following link helps you understanding how DHCP works for multiple VLANs:

http://tcpmag.com/qanda/article.asp?EditorialsID=285

Regards,

Sushil

[camac2005]

Related to this issue i have following problem

I have multiple VLAN, one DHCP server, ip helper-address and all is OK but problem is with 2000 Server. The following error messages are added to my server's system log (as seen with the Event Viewer) approximately every 12 minutes:

The master browser has received a server announcement from the computer MyBDC that believes that it is the master browser for the domain on transport NetBT_NETFLX1. The master browser is stopping or an election is being forced.

More details on:

http://support.microsoft.com/kb/135464/EN-US/

http://support.microsoft.com/kb/190930/EN-US/

What i could do? I am thinking about replacing DHCP Server on Windows 2000 Server with one on Cisco 3560 Switch.

Any help or idea?