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

Unanswered Question
Feb 21st, 2008

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.

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Edison Ortiz Thu, 02/21/2008 - 10:38

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.



rsharifi Thu, 02/21/2008 - 10:41

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

no snmp trap link-status


interface FastEthernet0/0.2

description building-2

encapsulation dot1Q 2

ip address

no snmp trap link-status


interface FastEthernet0/0.3

description building-3

encapsulation dot1Q 3

ip address

no snmp trap link-status



rsharifi Thu, 02/21/2008 - 10:53

yes, you do.


Switch in building-1 VLAN 1

Switch in building-2 VLAN 2

Switch in building-3 VLAN 3


brownmw.bsd Thu, 02/21/2008 - 10:50

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

Edison Ortiz Thu, 02/21/2008 - 11:42

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.





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