How Broadcasts work?

I was reading out of my Microsoft Series Server 2008 Admin book and it was talking about DHCP IP assignment.

Well it says that the first thing a client does to retrieve an IP is that it sends out a broadcast for "DHCPDISCOVER" so that the DHCP Server or Servers can respond back with a "DHCPOFFER".

Then I was reading out of my CCNA book about broadcasts. It says specifically that Routers seperate all broadcasts into seperate broadcast domains.
These 2 lines Quoted right straight from Todd Lammle's CCNA book in Chapter 1.
"Routers dont forward broadcasts by default."
"Routers break up broadcast domains."

Now how can a router forward a broadcast for DHCPDISCOVERS when routers dont forward broadcasts, because they break them up.

This sounds like a contradiction.. Or do routers create an exception specifically for DHCPDISCOVER broadcasts?

Anyone explain this?

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Comments

dynamik

As you wrote, that's the default behavior of routers. You can change that. You can also setup DHCP relay agents.

DerekAustin26

Thats great that you can change it and all, but if this action is default by routers, then how does the DHCPDISCOVER ever reach the DHCP Servers if you dont change this? Every router I've ever touched, I've never changed them, so the DHCPDISCOVER broadcasts must be getting through somehow?

kalebksp

Either your DHCP server was on the same subnet as the client, the DHCP server was running on the router, or there was another server/device acting as a DHCP relay agent.

EDIT: To clarify, if the DHCP server was running on the router it would be running on the same subnet as well.

DerekAustin26

DHCP Servers can be running on the routers? That makes sense, I remember in the NOC that one of our Tier 2's would always setup pools of IP's for the DHCP, but i was always curious why, cuz all we managed was switches & routers.

jason_lunde

Check out the command within some Cisco documentation:
ip helper-address <ip address> ....

Hopefully this will clarify some of this for you.

Cheers!

jason_lunde

DerekAustin26 wrote: »

Every router I've ever touched, I've never changed them, so the DHCPDISCOVER broadcasts must be getting through somehow?

What type of "router" are you referring to? Linksys, and other brands like this often "route" between a built-in 4 port switch and a WAN port. So if you have a host...as others have said...on the same subnet than the router will still pass the traffic.

DerekAustin26

[QUOTE=So if you have a host...as others have said...on the same subnet than the router will still pass the traffic.[/QUOTE]

In response to my "broadcasts are getting through somehow"

that response about my "DHCP Server being on the same Subnet" was giving to me after that question.. So I dont know why your begging the question and repeating what someone else said when I never followed up with a 2nd question in regards to that statement. And lastly, you cant say "others" have told me so, because only 1 person has. So please dont respond anymore if all your going to do is regurgitate what someone else already told me. Thanks.

jason_lunde

jason_lunde wrote: »

What type of "router" are you referring to? Linksys, and other brands like this often "route" between a built-in 4 port switch and a WAN port.

I wasn't regurgitating, I was adding to. Some peoples idea's of routers are warped. It's best not to go attacking when others are trying to help...

kalebksp

Wow DerekAustin26, you're being incredibly rude. There was no reason whatsoever to attack jason_lunde like that, whether his clarification was helpful or not (and I believe it was). I think I'm going to refrain from helping you in the future.