what must change when adding multiple subnets to a flat network

Darrell_Milam
Darrell_Milam used Ask the Experts™
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We have ran out of ip address in our subnet. we have recieved another that we can configure on our flat network. I can configured the dhcp server to hand out from this other subnet, but do not understand how or what is needed to configure to allow workstations from the second subnet to connect to the servers and devices on the first subnet without going through the router.  Can anyone point me in the right direction
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you will have to configure a different vlan for second subnet on the switch. If you have an L2 switch then you will have to use "router on stick" configuration. Otherwise you can use L3 switch and apply below command:

conf t
ip routing
exit
You will have to create vlans in L3 switch as well...

Author

Commented:
Is there any other way other than setting up vlans.  
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Another solution is to renumber the network in its entirety changing the gateway and all the static devices to a larger subnet.  Or you need to route the networks somehow.
if you have multiple subnets then you will have to have vlans in order for them to communicate with each other....
I agree, if they are going to be connected to the same switch, they should be on different VLANs and trunking and L3 routing device will be needed, whether it be a router or a L3 switch.

Author

Commented:
the devices only connect/communicate to the servers and the internet, if this matters. the routers has the second subnet setup and workstations that are configured will access the internet correctly. my problem is that when the workstation with the 2nd subnet communicates to the servers, dns tells the they are in the other subnet and they walk across the router and back to the server in subnet 1 which will swamp the router and internet connection.  Is there a way to route the 2 subnet at the core switch?
core switch has a routing capabilities...you can create two vlans and apply below command:

conf t
ip routing
exit


You will have to create one management vlan for connecting to router...
and you issue will be solved...

Author

Commented:
the switch is a 3com 5500g and does not recognise that command
I am not conversant with 3com switches...you must be knowing how to enable routing in 3com switches....

Author

Commented:
what sould a generic route between 2 subnets look like configured on a switch?
In Cisco, when you create two vlans and apply ip routing command...inter vlan routing is enabled...
A quick browse of that switch shows that it can route vlans you just need to create an VLAN interface in each vlan and set that as the default gateway for each client in each respective vlan, no need for a command to turn on ip routeing seems to be on by default, unlike cisco.

Author

Commented:
how do you get the dhcp server to only hand out ip address from a certain subnet to certain vlans.
The command in cisco is IP helper address but the concept is DCHP relay, the DCHP server knows what gateway the request comes from if there is some sort of relay occurring so it will give an address to the correct subnet.

Author

Commented:
so when setting up vlans
1. configure core switch with multiple vlans lets say 3, one fore each subnet
   a. configure static routes between vlans "if not done by default"
   b. configure dhcp relay between vlans
   c. tag vlan ports on core
2. configure connecting switches for vlan
     b. configure connecting switches vlan to correspond to connecting vlans on core

Is that pretty much it or am I missing something
Everything looks ok, that last part you just need trunk links they carry all VLANs between switches.

Author

Commented:
does the vlan ports that connect switches have to be tagged on both sides?
example
switch 1 "core" port 5
switch 2    port 1

sw1 port 5 should be a trunk link-type that is tagged to vlan1
sw2 port 1 just tagged to vlan1

Author

Commented:
Thanks for all your help. I have been able to accomplish this on my test enviroment. my steps were
1. create vlans on core switch
2. create vlan interface for each vlan with ip address on individual subnets
3. configure trunk for contecting switch ports on core and associate to corresponding vlans
4. configure trunks on connecting ports for otlying switches
5. configure valns on outlying switches
6. tag ports
7. configure static route to internet router
8. configure dhcp relay on core switch
9. configure dhcp to hand out 2nd subnet and corresponding gateway of vlan interface

will be trying this on our production enviroment in small steps

Author

Commented:
never got a follow up on the last question. but the primary question on what needs to be done was answered.

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