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2 LAN interface in Cisco 1841 Router

hi all,

i have a cisco 1841 router with a DSL module for WAN link and set up FE0/1 for the LAN interface. i have a spare FE0/0 port that i would like to use as an additional LAN interface. would it be possible so that i would not disconnect my LAN switch currently connected to FE0/1? do i just need to unshut FE 0/0 port and connect a device with static ip address?

1841#sh ver
Cisco IOS Software, 1841 Software (C1841-BROADBAND-M), Version 12.4(15)T7, RELEASE SOFTWARE (fc3)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2008 by Cisco Systems, Inc.
Compiled Wed 13-Aug-08 15:42 by prod_rel_team

ROM: System Bootstrap, Version 12.4(13r)T, RELEASE SOFTWARE (fc1)

SILVACO-SG-GW uptime is 16 hours, 8 minutes
System returned to ROM by power-on
System restarted at 17:03:25 SGT Mon Nov 22 2010
System image file is "flash:c1841-broadband-mz.124-15.T7.bin"

Cisco 1841 (revision 7.0) with 115712K/15360K bytes of memory.
Processor board ID FHK1321263D
1 DSL controller
2 FastEthernet interfaces
1 ATM interface
DRAM configuration is 64 bits wide with parity disabled.
191K bytes of NVRAM.
31360K bytes of ATA CompactFlash (Read/Write)

Configuration register is 0x2102

---------

1841#sh ip int bri
Interface                  IP-Address      OK? Method Status                Protocol
FastEthernet0/0            unassigned      YES NVRAM  administratively down down   
FastEthernet0/1            203.X.X.X   YES NVRAM  up                    up     
ATM0/0/0                   202..X.X.X  YES NVRAM  up                    up    

----------

1841#sh run int f0/1
Building configuration...

Current configuration : 129 bytes
!
interface FastEthernet0/1
description LAN Network
ip address 203..X.X.X 255.255.255.240
speed 10
full-duplex
end

-----

1841#sh run int f0/0
Building configuration...

Current configuration : 83 bytes
!
interface FastEthernet0/0
no ip address
shutdown
duplex auto
speed auto
end

  • LAN Switching and Routing
1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Silver

Re: 2 LAN interface in Cisco 1841 Router

John

It is not quite clear to me but it looks like what you are trying to do is to have both FastEth0/0 and FastEth0/1 to be in the same subnet (the subnet assigned by the provider). In general this is not a good thing to try to do. And specifically if you are configuring both interfaces as layer 3 interfaces (which is the default behavior) then it does not work - as you have discovered.

I said that in general it is not a good thing to do. Note that I did not say that it can not be done. There is a way to do it if you really want to. If you really want both FastEth0/0 and FastEth0/1 to be active in the same IP subnet then you need to configure Integrated Routing and Bridging. In IRB you configure both interfaces as layer 2 interfaces (no ip address) and you configure a virtual interface (interface BVI) which becomes the layer 3 interface with the IP address in the subnet assigned by the provider. This is more complex to do, and I believe that it will require at least a short outage of connectivity for the devices connected on the FastEth0/1 interface.

In my experience many customers have configured both FastEthernet interfaces to be active, but they are active in separate subnets. Perhaps it would help if you would explain what you are trying to accomplish in activating interface FastEth0/0. If we understood that then perhaps we could give better advice.

HTH

Rick

9 REPLIES
VIP Super Bronze

Re: 2 LAN interface in Cisco 1841 Router

Hi,

Yes, do "no sh" and assign an IP address to  FastEthernet0/0.  Now you can connect for example your laptop to this port and give it an IP address in the same segment. You should be able to ping the router and vice versa

HTH

Reza

Re: 2 LAN interface in Cisco 1841 Router

hi reza,

it's unable to accept the public ip address. after doing a no shut on FE 0/0, it's still down.  i got an error that it overlaps with the current ip address on FE0/1. any alternatives?

1841(config-if)#ip add 203.X.X.X 255.255.255.240
1841(config-if)#no shut
% 203.X.X.X overlaps with FastEthernet0/1
FastEthernet0/0: incorrect IP address assignment

VIP Super Bronze

Re: 2 LAN interface in Cisco 1841 Router

You can not use the same subnet for both interfaces.  Each interface needs to be in a different segment. Since you have /27 subnet (255.255.255.240), you can divided it into 2 /28s as follows:

10.10.10.0 255.255.255.248 fa0/0

10.10.10.8 255.255.255.248 fa0/1

But this will require down time since you are changing the mask for for the existing interface (fa0/1).

HTH

Reza

Re: 2 LAN interface in Cisco 1841 Router

hi reza,

my objective here is to assign public LAN ip address provided by the ISP and not private IP. would this still be possible and any workaround? i don't want to have a downtime on the FE 0/1 and that's why i want to use the spare FE 0/0 port for testing.

Hall of Fame Super Silver

Re: 2 LAN interface in Cisco 1841 Router

John

It is not quite clear to me but it looks like what you are trying to do is to have both FastEth0/0 and FastEth0/1 to be in the same subnet (the subnet assigned by the provider). In general this is not a good thing to try to do. And specifically if you are configuring both interfaces as layer 3 interfaces (which is the default behavior) then it does not work - as you have discovered.

I said that in general it is not a good thing to do. Note that I did not say that it can not be done. There is a way to do it if you really want to. If you really want both FastEth0/0 and FastEth0/1 to be active in the same IP subnet then you need to configure Integrated Routing and Bridging. In IRB you configure both interfaces as layer 2 interfaces (no ip address) and you configure a virtual interface (interface BVI) which becomes the layer 3 interface with the IP address in the subnet assigned by the provider. This is more complex to do, and I believe that it will require at least a short outage of connectivity for the devices connected on the FastEth0/1 interface.

In my experience many customers have configured both FastEthernet interfaces to be active, but they are active in separate subnets. Perhaps it would help if you would explain what you are trying to accomplish in activating interface FastEth0/0. If we understood that then perhaps we could give better advice.

HTH

Rick

Re: 2 LAN interface in Cisco 1841 Router

hi richard,

thanks for clarifying! i think my testing and your explanation enlightened me on this issue. no IRB required for this case. i was thinking earlier to set a VLAN interface to act as a LAN default gateway so that i could use both FE ports but the router also doesn't accept this command.

Purple

Re: 2 LAN interface in Cisco 1841 Router

john,

You can configure a SVI only on switches not on routers.

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Cisco Employee

Re: 2 LAN interface in Cisco 1841 Router

Hello,

Actually, even on routers, you can (and usually have to) configure SVIs if they have switching modules such as HWIC-4ESW or HWIC-9ESW installed. Of course, this does not appear to be the case - as I see it, John simply tried to use both Fa0/0 and Fa0/1 ports on the router as if they were two switched ports. Rick has already explained why that is not the best way to go.

Best regards,

Peter

New Member

hello everyone , i am need of

hello everyone , i am need of help regarding configuring of   FE 0/1 port. our company have a cisco 1841 router. The serial 0/0/0 is connected with VSAT for internet. The FE 0/0 is connected to switch(LAN) through which net connectivity is provided to all users. Recently a new VSAT has been installed at our site,with different IP series. So every time we want to switch between the two net connectivity we need to change the entire IP configuration of all users, which in turn prohibits the users from accessing the printers,data servers etc which are been set to our existing IP series. So, my idea was to configure the FE 0/1, so that just by changing the DNS will help us providing internet along with all other devices without changing the entire IP series. The new VSAT modem has a lan cable which can be connected to FE 0/1. Can any one help out in solving the problem. Our existing IP series is 192.168.3.1..... and the new VSAT series is 10.205.74.1......

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