Simple Question-Configure additional subnet / network expansion 1

[bristolxyz]

I have a cisco 1841. Currently I have a network configured as 10.20.36.xxx . I would like to add another range of addresses to be used (I am running out of hosts) as 10.20.37.XXX . I would like all network traffic to travel freely across these subnets. Pretty simple, but what would be the most effective way to configure it so i will not have too much trouble in the future?

Warm regards,
Bris

CTO
MSCE, CCNA, Novell, Symantec SS, Helicopter Pilot
Don't let what you can't do stand in the way of what you can

 

[unclerico]

assuming your 1841 is the only L3 device on your network you can:
1) do a router-on-a-stick config by using sub-interfaces on the 1841 along with a second VLAN on your switches. this will require your switch(es) to support VLANs and dot1q trunks
2) add a secondary ip address to your LAN facing FE interface. this is what most lazy admins do, but it will wreak havoc on your network if you have a lot of chatty or broadcast/multicast based traffic

with the first option of using ROASt you have the option of either creating two subinterfaces or using your main physical interface as part of your native VLAN and the subinterface as a dot1q tagged interface.

[b]1841 config using the physical interface in the native VLAN and creating only a single subinterface:[/b]
Router(config)# int fa0/1.37 [b]<--- the .37 after the interface number identifies your subinterface[/b]
Router(config-subif)# encap dot1q 37 [b]<--- the 37 signifies your dot1q tag that all ingress/egress traffic will be tagged with[/b]
Router(config-subif)# ip add 10.20.37.1 255.255.255.0

[b]Switch config - obviously if you don't use Cisco switches the switch config will be different[/b]
Switch(config)# vlan 37
Switch(config-vlan)# name some_name
Switch(config-vlan)# exit
Switch(config)# int f0/1 [b]<--- this is the the interface that connects your switch to your 1841. it will now be configured as a static trunk link to transport your native (10.20.36/24) traffic and your tagged (10.20.37/24) traffic[/b]
Switch(config-if)# switchport encap dot1q
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport nonegotiate
Switch(config-if)# switchport trunk allowed vlan 1,37 [b]<--- manually pruning VLANs from the link since only your native VLAN and your newly created VLAN need to traverse the trunk[/b]

[b]1841 using two subinterfaces - cleaner config:[/b]
Router(config)# default interface fa0/1
Router(config)# int fa0/1.36 [b]<--- the .36 after the interface number identifies your subinterface[/b]
Router(config-subif)# encap dot1q 36 [b]<--- the 36 signifies your dot1q tag that all ingress/egress traffic will be tagged with[/b]
Router(config-subif)# ip add 10.20.36.1 255.255.255.0
Router(config-subif)# exit
Router(config)# int fa0/1.37 [b]<--- the .37 after the interface number identifies your subinterface[/b]
Router(config-subif)# encap dot1q 37 [b]<--- the 37 signifies your dot1q tag that all ingress/egress traffic will be tagged with[/b]
Router(config-subif)# ip add 10.20.37.1 255.255.255.0

[b]Switch config[/b]
Switch(config)# vlan 36-37 [b]<--- create your vlans[/b]
Switch(config-vlan)# exit
Switch(config)# int f0/1 [b]<--- this is the the interface that connects your switch to your 1841. it will now be configured as a static trunk link to transport all of your tagged traffic[/b]
Switch(config-if)# switchport encap dot1q
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport nonegotiate
Switch(config-if)# switchport trunk allowed vlan 36,37 [b]<--- manually pruning VLANs from the link since only vlan 36 and 37 need to traverse the trunk[/b]

 

[bristolxyz]

unclerico,
Thnaks very much for your reply and help.
I'm always one for trying to do the difficult right verses the easy wron gr using lazy shortcuts.
Currently, we don't use cisco switches, but they do support vlan configuration, so should be ok.
Thanks