Serial & Ethernet Interface Interoperablity

Unanswered Question
Mar 7th, 2009

I am configuring Cisco 3845 Router. My customer is having Cisco 1841 Router with the following parameters


Interface : serial 0/0/0

IP address:

encapsulation: ppp


Interface : Fa 0/0

IP address:

Since I do not have serial interface in my router at present, I have connected V.35 to Ethernet Interface to fa 0/0 Interface of my Router.

But, I am not able to reach the other end IP address I suspect since the other end interface encapsulation is PPP (Serial) and this end is Ethernet, it may the issue.

Can you please help me what will be the solution for this

I have this problem too.
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scottmac Sat, 03/07/2009 - 07:14

Ethernet and serial protocols are not compatible at any level.

The voltages, frequencies, modulation, connectors ... everything is different.

You don't say if you are connecting through a T1, DSL, or other ... or if there's any other equipment involved.

Explain what you have for equipment at each end, and what kind of connection you have between the sites and perhaps some suggestions can be made for getting them talking together.

hclisschennai Sat, 03/07/2009 - 08:52

Hi scottmac,

Thanks for your reply.

In remote end Router is connected to Leased Line (E1-2 Mbps) with a Modem(CSU/DSU).

In my end i am using a Modem (CSU/DSU) connected to "V.35 to Ethernet convertor" and connected to fa0/0 interface of the router.

I am attaching a Diagram for your reference.

Hope this will help you to provide suitable solution

Giuseppe Larosa Sat, 03/07/2009 - 11:01

Hello Hclisschennai,

be aware that this kind of boxes are thought to be deployed in a pair to act as LAN extender over an E1 or T1.

The setup that you have built cannot work because the other router expects to receive ip packets inside HDLC or PPP frames not ethernet frames within HDLC or PPP frames: this is the way it works.

You need a serial interface on your router or another box like that on the remote end

Hope to help


hclisschennai Sun, 03/08/2009 - 09:22

Hi Giuseppe,

Thanks for your comment.

By this time i have got a document which describes the configuration changes to be done on Cisco Routers which makes this to work.

This interface command is "ppp bridge ip".

But I have not yet tested this. Just thought of sharing with you and to get your opinion also

Expect your valuable opinion


Giuseppe Larosa Sun, 03/08/2009 - 13:59

Hello R.B. Kumar,

it looks like possible to use only one of these boxes and to connect it to a Cisco serial interface.

On the Cisco you can take advantage of IRB: Integrated Routing and Bridging.

I tested a device like this some years to transport CLNS PDUs with the exact objective to save a Cisco router.

The lab tests were good.

something like

bridge 1 protocol ieee

bridge 1 route ip

int eth0

no ip addr

bridge-group 1

int ser0

no ip addr

enc ppp

bridge-group 1

int BV1

ip address

the BVI# must match the bridge-group number.

you cannot bridge ip over an interface with an ip address so you need to use IRB as I've descrived above.

So in my opinion the suggestions are not totally correct but the setup can work as in the tests I did.

Hope to help



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