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New subnet on router

I have a Cisco 2811 router that I use as a gateway for a local /24 LAN.  We are shutting down another office and moving the assets and functionality to this office so we are going to need more space than the /24.  I am going to just take the second Fe interface on that router and create another /24 subnet on it.  We will have all servers and printers on the first LAN (192.168.100.0/24) and the all the users and their IP phones on the second LAN (192.168.101.0/24).

I know that since they are on the same router it will automatically see and create the route between the two networks but I would like some confirmation.  Will the computers on the user LAN automatically be able to hit the domain controllers on the server LAN for logging into the domain and DNS?  I don't really need any kind of filtering between the two subnets right now so I wasn't going to do an access-list unless necessary.
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qbakies
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qbakies
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2 Solutions
 
Ernie BeekCommented:
If there are no access lists in between and the machines have the right default gateway, you should be ok.
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mrklaxonCommented:
As long as the 2811 is your default gateway for all devices/servers then all packets will go to it and get dumped to the correct interface.

You could also expand your subnet, may create some work re-addressing but would reduce dependency on the router when accessing resources.
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Otto_NCommented:
The 2811 will be the default gateway if the DHCP Server (which I assume you use) set it so.

If the DHCP Server is also in the Server subnet, you will just have to configure the router to relay DHCP broadcasts to the server by configuring the "ip helper-address" on the client interface.

If you do not use DHCP to assign IP addresses, or use the router as a DHCP Server, this should not be an issue.
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qbakiesAuthor Commented:
The 2811 is set as the default gateway through DHCP.  But the DHCP server is in the server subnet.  So DHCP requests will not be sent from the user subnet to the server subnet automatically?
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Ernie BeekCommented:
Nope, a router doesn't forward broadcasts.
You could use the 'ip helper' command though.
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Ernie BeekCommented:
ip helper-address x.x.x.x
(dhcp) broadcasts will then be forwarded to x.x.x.x (address of the dhcp server).
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qbakiesAuthor Commented:
That is exactly what I wanted to know.  I know switches break up collision domains and routers breakup broadcast domains but I'm not sure of all the relevant network traffic that is broadcast.  So is there anything else that is broadcast but needed?
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Ernie BeekCommented:
Nothing comes to mind right now.......
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Otto_NCommented:
Just make sure that the DHCP Server have a scope for the user subnet, and that it permits BOOTP.  I know this is counter-intuitive, but the relay-action is classified as part of the BOOTP protocol, and is not recognised as DHCP packets, even though the relayed packets contains DHCP information.
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qbakiesAuthor Commented:
"Just make sure that the DHCP Server have a scope for the user subnet, and that it permits BOOTP"


Is this a setting in the DHCP scope or the server?
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mrklaxonCommented:
Server is where the scope is so yes?
Read:
http://technet.microsoft.com/en-us/library/dd759168.aspx
It talks about setting up a Super Scope to handle multiple subnets.
You might want to look at a backup DHCP on the second subnet as well.
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Otto_NCommented:
I'ts been a while since I created a new scope on a Windows DHCP Server, but as far as I recall, you need a scope for each subnet you wish to administer.  You don't really need a superscope.

When you set-up a scope (in Windows), one of the steps ask you to choose the protocol allowed for the scope: "DHCP", "BOOTP" or "Both".  Choose either "BOOTP" or "Both" to allow packets redirected by the router.  But as I said, it's been a while, and perhaps this isn't an issue anymore...
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qbakiesAuthor Commented:
Thank you.
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