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New Member

how to setup a router to see more than one subnet on 1 interface

Can anyone help me out with setting up a router interface with more than one subnet. We have 4 buildings all on different subnets. We want all of the subnets to be able to talk and see each other. Each building has fiber to it and we are upgrading from 4 routers to 1, we bought a 2821 router and can't figure it out.

10 REPLIES
Hall of Fame Super Bronze

Re: how to setup a router to see more than one subnet on 1 inter

What type of switch each building has? You should be able to accomplish inter-vlan routing with Layer3 switches. It's more efficient than having a router perform that function.

If you have a 3560, 3750, 4500 or 6500 switch in your network - you have the ability to configure those devices as routers.

__

Edison.

New Member

Re: how to setup a router to see more than one subnet on 1 inter

We have HP Procurve switches.

New Member

Re: how to setup a router to see more than one subnet on 1 inter

If you are planning to use sub-interfaces, here is sample config;

interface FastEthernet0/0.1

description building-1

encapsulation dot1Q 1 native

ip address 1.1.1.1 255.255.255.0

no snmp trap link-status

!

interface FastEthernet0/0.2

description building-2

encapsulation dot1Q 2

ip address 2.2.2.2 255.255.255.0

no snmp trap link-status

!

interface FastEthernet0/0.3

description building-3

encapsulation dot1Q 3

ip address 3.3.3.3 255.255.255.0

no snmp trap link-status

HTH

Reza

New Member

Re: how to setup a router to see more than one subnet on 1 inter

will i still need to create a vlan?

New Member

Re: how to setup a router to see more than one subnet on 1 inter

yes, you do.

Example:

Switch in building-1 VLAN 1

Switch in building-2 VLAN 2

Switch in building-3 VLAN 3

etc

New Member

Re: how to setup a router to see more than one subnet on 1 inter

and my router will know which vlan to go over by the ip correct?

New Member

Re: how to setup a router to see more than one subnet on 1 inter

do i need to setup an ip just for the interface of just for the sub interface?

New Member

Re: how to setup a router to see more than one subnet on 1 inter

you need 1 subnet per sub-interface

New Member

Re: how to setup a router to see more than one subnet on 1 inter

Do i need to make all of the native?

Hall of Fame Super Bronze

Re: how to setup a router to see more than one subnet on 1 inter

You are only allow to have one native Vlan in dot1q trunking. The native vlan is defined as the 'untagged vlan'. You need to match the untagged vlan from your procurve switch with the one configured at the router. The router will know what vlan the packet is coming from based on the tag on the packet header.

Keep in mind, on this design - you will have a bottleneck at the router LAN interface and your inter-vlan throughput will be limited to the speed of the LAN interface on this router.

I really recommend getting some Layer3 switches, they don't need to be Cisco brand. HP also sells Layer3 switching equipment.

HTH,

__

Edison.

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