subnet on layer 3 switch

Unanswered Question
Mar 3rd, 2008

The ISP provided one 28bit subnet. One IP address has been configured as the default gateway on router. One layer 3 switch connects to this router. I'm gonna separate this 28bit subnet to two 29bit subnets on layer 3 switch. one for the L3 routing port connects to uper level route. Another one for the VLAN. How can I configure it? Thanks.

I have this problem too.
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Richard Burts Mon, 03/03/2008 - 15:05


Perhaps I am not understanding well: you have already put 1 address on the router (which logically means that there is 1 subnet on the router) and it connects to your layer 3 switch (which will have the same subnet as the router). And if you want to put the other /29 on the VLAN then you can do so. But putting the /29 on the VLAN means that there will be a limitation to how many host addresses you have available in the VLAN. Is that what you want to do? If so it is pretty straightforward. If the ISP gave you then you will use and

on layer 3 switch connection to the router

ip address

on the vlan interface

ip address

I might suggest a different approach. Do not put the ISP addresses on the layer 3 switch. do all the addressing on the switch in private address space ( or whatever). The create a NAT pool on the router using the ISP addresses and do address translation on the router.



David Lin Mon, 03/03/2008 - 20:07

Thanks, Rick.

The topology is: ISP-Router-Switch-Servers

I want some Servers stay in an VLAN on the Switch even it will waste some IP address.

I did the configuration as your example,

Router's E0/1:


interface GigabitEthernet1/1

no switchport

ip address

interface Vlan200

ip address

ip route

I can ping anywhere from Switch. But from a Server in VLAN200, I can't ping either or other public IP address. Vice versa, I can't ping the IP address in VLAN 200 from outside Internet.

When I disable the Layer 3 feature on the Switch and configured the server with and gateway which are functioning properly.

I am aware the problem should be the difference of subnet between the Router and Switch B. But I don't know how to figure it out. Any further advice would be great appreciated.

aijaz802 Mon, 03/03/2008 - 22:24


I think you mis configured the subnet mask on the router....make it /29 instead of /28.

Router's E0/1:

I hope this helps....



David Lin Tue, 03/04/2008 - 06:30


But it doesn't help as I tried before. And I think the broadcast for will not go to the Swtich like this way.


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