another router-on-a-stick question

dannan lin · ‎03-27-2012

hi:

today i was asked a question about router-on-a-stick. it is quite interesting.

the situation is like this , two PCs connect to each other via a switch, and a router is connected on another side of switch( typical router on a stick setup)

pc0 ip:1.1.1.1/24 vlan10

pc0 gateway:1.1.1.254

pc1 ip:1.1.1.2/24 vlan20

pc1 gateway:1.1.1.1.254

router f0/0:1.1.1.254/24

pc0 is a part of vlan 10 and pc1 is a part of vlan 20 and they share the same gateway.

in this case how do i make pc1 communicate with pc0 ? is it even doable ? if so how do i do this?

thanks in advance.

vijay.swaminathan · ‎03-27-2012

This is bad design. It is not advisable to share a subnet across vlans.

Cisco recommends to stick to one subnet per vlan.

-Vijay

dannan lin · ‎03-27-2012

thanks.

glen.grant · ‎03-27-2012

Both are in the same subnet but in 2 different layer 2 vlans. Change vlan on one pc to match the other seeing that the address range is in the same in both vlans. Can't imagine why you would want to have the setup in your drawing .

dannan lin · ‎03-27-2012

i just wanted to know if it is doable. i know this kind of setup is insane though. thanks

Jan Hrnko · ‎03-27-2012

Hi Dannan,

As gentlemen have said - there is something wrong. First of all you should use one subnet per vlan and second thing is that this is not how you setup Router-On-A-Stick.

Router-On-A-Stick would have for example 2 subinterfaces - one for vlan 10, another for vlan 20. Both vlans would have been in different subnets. Something like this:

Router:

for vlan 10

fa0.0/10

encapsulation dot1q 10

ip address 1.1.1.254 /24

for vlan 20

fa0.0/20

encapsulation dot1q 20

ip address 1.1.2.254 /24

PCs

for vlan 10

ip address 1.1.1.1 /24

default gateway 1.1.1.254

for vlan 20

ip address 1.1.2.1 /24

default gateway 1.1.2.254

Best regards,

Jan

dannan lin · ‎03-27-2012

thanks

RUSTOM BILLIMORIA · ‎03-28-2012

Hi,

I think the question is insane but there is a solution. We had once tried routing this way on a Cisco 6500 and used routing between two different VRF's. dont quite recall much but i guess it is possible. As for a CCNA kinda question, not possible.

Cheers

Rustom BIllimoria.