DHCP ?

Just getting into the DHCP chapter in the MS book and something caught my attention. Is it true the DHCP server must have a Nic on the subnet of which it is dishing out IPs for? This seems crazy, but the book states this in the begining of chapter 7.

maybe the nic could be connected to a port that is tagged for all the vlans you are dishing out ips in??

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gojericho0

I know in my workplace we have dhcp servers at each remote office handing out ip's to hosts on that same subnet lets say 192.168.5.x/22. If that server fails however, the dhcp requests get redirected back to our central office which is on a different network 192.168.0.x. The backup dhcp server hands out ip's on the same subnet as the remote office but will hand out a different scope 192.168.6.x so that there is no duplicate dhcp info

eurotrash

yes, it must. unless you use superscopes.

kalebksp

DHCP uses MAC broadcasts to initially assign clients IP addresses, so any device that creates seperate broadcast domains will drop the packet. This can be worked around by the use of RFC 1542 (I think) compliant routers, or a DHCP relay agent, these devices will detect a DHCP request and forward it to a network where there is a DHCP server.

12thlevelwarrior

cool, thanks

eurotrash

kalebksp wrote:

DHCP uses MAC broadcasts to initially assign clients IP addresses, so any device that creates seperate broadcast domains will drop the packet. This can be worked around by the use of RFC 1542 (I think) compliant routers, or a DHCP relay agent, these devices will detect a DHCP request and forward it to a network where there is a DHCP server.

that's if you want to hand out addresses from the same scope to different subnets. if you want to assign a different network address to each subnet you would need more than one NIC, as it won't assign a 10.0.0.0/24 address through a 192.168.0.0/24 interface.
unless i am very much mistaken.

12thlevelwarrior

that is really hard for me to believe, we have a linux box handling dhcp and we have 10 or more vlans, and this server handles it all and is a Virtual Machine.

eurotrash

chuchuchu wrote:

that is really hard for me to believe, we have a linux box handling dhcp and we have 10 or more vlans, and this server handles it all and is a Virtual Machine.

does it use superscopes?

12thlevelwarrior

got the info, our core 8600 router is setup to allow dhcp on certain vlans and not allow on others. this makes sense now. thanks!

12thlevelwarrior

OK, getting hung up on this question?

If you have a router or routers that have dhcp relay agents, why would you need superscopes? I am envisioning the DHCP server with all applicable scopes setup and the router side handles delivering dhcp requests to the DHCP server. Why the need for superscopes?

DHCP server recieves request from scope 192.165.3 and responds with a correct IP from that scope because the routers correctly routed the ruquest....

eurotrash

ok you're right, i had imagined the scenario a little differently.

gojericho0

ok now i have a questions... when a client directs a broadcast for dhcp info to a server that is on another subnet and has multiple scopes, how does the dhcp server know know what scope to use to send the correct info back to that client?

seuss_ssues

Well to start off you dont "direct" a broadcast to a server.

A broadcast is sent to all hosts on its segment and if one of them is a dhcp server or a relay agent it will respond with an IP.

If a client sends a broadcast for an ip and reaches a relay agent that relay agent communicates with the actual DHCP server and issues an ip.

The ip it gives is based on the information setup on the dhcp server for that relay agent.

gojericho0

i'm sorry when i said direct broadcast i meant multicast. wasn't sure if dhcp request was a multicast address or broadcast. I found out a bunch of good info here if guys reading this thread want to check it out

http://www.comptechdoc.org/os/windows/win2k/win2kdhcp.html

12thlevelwarrior

so if server can handle multiple scopes and the router/relay agent handles connecting the two parties where do superscopes fit in with this scenario?