router two fast ethernets port connected to switch

Unanswered Question
Sep 21st, 2008

hi every body!

Can we connect two fastethernet ports of the same router to a switch using same subnet i.e



thanks a lot and have a nice weekend!

I have this problem too.
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Giuseppe Larosa Sun, 09/21/2008 - 07:58

Hello Sarah,

you cannot configure overlapping IP addresses on different L3 interfaces or subinterfaces a message error is shown when configuring the second interface and the command is not accepted.

The reason for this is that a router is an internetworking device that routes traffic between different IP subnets: but for each subnet a single input/exit point is needed: otherwise when there is a packet for PC out which interface should go out f0 or f1 ?

Some years ago I made tests of this to demonstrate that a network design had to be reviewed for this reason.

Hope to help


sarahr202 Sun, 09/21/2008 - 11:12

thanks for your reply Giuseppe! That means ifwe have(multilayer switch) distribution switches d1 and d2,connected to layer 2 core switch c1 by f0,f1 i.e

d1f0---------c1------------f0 d2

f1------- ------------f1

Then we can't use single vlan say 2 to put d1 and d2 interfaces f0,f1,f0,f1 because:

1)f0, f1 both require different ip subnet, 2)the best design practise dictates one to one correspondence between vlan and ip subnet.our options are:

1) remove the redunant link say f1 from switches d1,d2 and use single subnet for f0 on both d1 and d2 and use one vlan to put f0 on both d1 and d2.


2) use two vlans and put f0 on switches in one vlan say vlan 1 and use other vlan say vlan 2 to put f1 on both d1 and d2 in that vlan and use two subnets, one for each port f0,f1.

Am i right?

thanks a lot!

Giuseppe Larosa Sun, 09/21/2008 - 11:32

Hello Sarah,

your understanding is correct.

Actually the correction I had suggested was this: two vlans, two subnets, two interfaces on each router

Hope to help


Joseph W. Doherty Sun, 09/21/2008 - 11:34

Sarah, when you discuss multilayer switches, rather than your initial post's reference to router, your options may change. Most multilayer switches can have the same IP address appear on multiple ports. You also have, with most mutlilayer switches, the option of using L2 or L3 connections between devices. What is it you're really trying to accomplish and with what type of devices?

sarahr202 Sun, 09/21/2008 - 12:26

Thanks for your reply Josephoherty!

I was actually reading a cisco guide on switching( i am preparing for ccnp) about how the access layer , distribution layer and core layer are implemented . It says how distribution switches can be connected to layer 3 core and layer 2 core. I was just wondering about the layer 2 core as to how distribution switch, a mulilayer switch can be connected to layer 2 core.

thanks a lot!

Joseph W. Doherty Sun, 09/21/2008 - 08:30

Don't believe you be able to do that, as Giuseppe explains.

However, you might be able to channel the two fast Ethernet ports, so they function as a single 200 Mbps port.

lamav Sun, 09/21/2008 - 09:17


Giuseppe and Joseph are 100% correct. See below:

NJLEO2CH-WR02#sh run int GigabitEthernet0/0

Building configuration...

Current configuration : 159 bytes


interface GigabitEthernet0/0

ip address

duplex auto

speed auto




NJLEO2CH-WR02#conf t

Enter configuration commands, one per line. End with CNTL/Z.

NJLEO2CH-WR02(config)#int GigabitEthernet0/1

NJLEO2CH-WR02(config-if)#ip address

% overlaps with GigabitEthernet0/0



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