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New Member

router

Hey Folks,

I have one question to ask from the expert, I have two building connected together with fiber. Also I have two DHCP server on both building. How can I separate the leasing of IP ADDRESS so that building one will not get IP from Building two. And building two would not get IP address also from Building one. Please help.

thanks,

ZAldy

12 REPLIES
Silver

Re: router

If the buildings are two separate broadcast domains, i.e. separated by a router/swouter it would be straightforward.

Clients in building one obtain a lease from the DC for instance. Clients in building two optain a lease from the DC, but because they reside in another subnetwork the DC will assign an address based upon the router interface attaching to that network.

The router/swouter needs additional configuration regarding IP helper-address to forward DHCP request

broadcasts.

HTH

Leon

* Please rate useful posts.

New Member

Re: router

thanks a lot, follow-up question what if the connectivity of the two building is a cross cable connected directly to building one switch. so it connect the two building first before it pass to the router of each building. what should be the proper connectivity of this devices.

thanks,

Silver

Re: router

In that case you would have a single broadcast domain and the DHCP server cannot tell the difference between hosts in building one or building two.

You can use different VLANs to address this problem. For instance use VLAN 10 in building one and VLAN 20 in building 20. If you have a L3 switch in building one you can route between VLANs and still have a L2 connection to building two.

It wouldn't be a great design but it will work.

Are you in the planning stage or have you already expanding your network? And what hardware are you using?

HTH

Leon

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New Member

Re: router

sir,

it is already installed in our building. building one have cisco 1700 router and 10 cisco catalyst 2950G-48 switch and 1 DHCP server. Building two has 3640 router and 4 cisco 2950G-48 switch and 1 DHCP server. Building one I activate DHCP server but in Building two I de-activate and use static IP because of the leasing if IP problem. If I use VLAN can u give me sample on how to configure on my router and switch.

thanks a lot

Silver

Re: router

Let see, if I can picture your network.

[C2950G]<--->[C1700]<-------->[[C3640]<--->[C2950]

I have no knowledge of your addresing plan, but I assume you are using two separate networks. If that's not the case you're bridging your traffic accross the link between the routers - correct me if I am wrong.

Forget about the VLANs I mentioned earlier, you have a different topology. I would not recommend it in this particular scenario.

If you configure your C3640 router to forward DHCP requests to the IP address of the DC in building one you would be OK.

router(config)#interface Ethernet0/0

router(config-if)#ip helper-address 192.168.1.1

router(config-if)#end

This would forward the DHCP request to building one. If you have a scope configured for the lan in building two you should obtain the right IP address.

If you do have bridging than you need to slit the network in two separate broadcast domains. Enable IP routing on the C1700 and C3640 configure two networks and use the ip helper-address in building two.

HTH

Leon

Let's say building one

New Member

Re: router

sir,

how to enable the IP routing on c1700 and c3640 so that i can slit the broadcast domain

thanks again

zaldy

Silver

Re: router

Hello Zaldy,

I understand that you do have a bridged connection between the two routers.

IP routing can be enabled by entering the command "ip routing" in global configuration.

You need to address the link between the two routers aswell.

Here is a brief example

C1700

c1700(config)#ip routing

c1700(config)#interface Ethernet0/0

c1700(config-if)#description LAN BUILDING 1

c1700(config-if)#ip address 192.168.1.1 255.255.255.0

c1700(config-if)#interface Ethernet0/1

c1700(config-if)#description LINK TO ROUTER C3640

c1700(config-if)#ip address 192.168.2.1 255.255.255.0

c1700(config-if)#end

C3640

c3640(config)#ip routing

c3640(config)#interface Ethernet0/0

c3640(config-if)#description LAN BUILDING 2

c3640(config-if)#ip address 192.168.3.1 255.255.255.0

c3640(config-if)#ip helper-address 192.168.1.10

c3640(config-if)#interface Ethernet0/1

c3640(config-if)#description LINK TO ROUTER C1700

c3640(config-if)#ip address 192.168.2.1 255.255.255.0

c3640(config-if)#end

HTH

Leon

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New Member

Re: router

Sir,

This my current connection now:

(Bldg1:[C2950G]<-->cross cable to<-->[C2950G]Bldg2)

" "

" "

(C1700) (C3640)

DC1=172.20.1.0 DC2=172.16.1.0

how can i separate the broadcast domain.

thanks a lot,

zaldy

Silver

Re: router

I seem to have trouble understanding diagram. Can you draw it and attach it to this thread.

Also if you would like to communicate between a host in bldg 1 and bldg 2 do you pass the router first, then the switch followed by the switch in bldg 2 and finally the router that attaches to the LAN in bldg 2?

If that is the case and your netmask is greater than 16 bits you already have two separate networks. Then you just need to add ip helper to the router in bldg 2.

Leon

New Member

Re: router

sir I attached visio drawing of the network connections. thanks a lot for the help

zaldy

Silver

Re: router

The situation becomes clear.

What I can conclude from this drawing is that both networks are within the same switched LAN. To communicate between host they used the attached router. This is the reason why DC in bldg 1 can assign a DHCP lease to a host in bldg 2.

What you need is to use VLANs to separate both networks physically.

You need to do the following.

* Create a VLAN for bldg 1 i.e. VLAN 10.

* Create a VLAN for bldg 2 i.e. VLAN 20.

* Add ports that belong to bldg1 to VLAN10.

* Add ports that belong to bldg2 to VLAN20.

* Create a VTP domain i.e. AGH

* Create a 802.1Q trunk between switch bldg1 and switch bldg2 on the ports that are connected with the crosscable.

* Configure a 802.1Q trunk between switch and router. Configure the router to route between VLAN 10 and VLAN 20. Do this for both routers. Keyword "router-on-a-stick" on Cisco.com.

Study the concepts carefuly as outlined in the 2950 software configuration guide.

http://www.cisco.com/en/US/products/hw/switches/ps628/products_configuration_guide_chapter09186a0080476470.html

HTH

Leon

* Please rate useful posts.

New Member

Re: router

thank you sir, i will try this one and keep you update....

thanks,

zaldy

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