Adding ethernet ports in single virtual interface

Unanswered Question
Dec 18th, 2007


We are having 1841 modular router and ISP has provided us MPLS links on ethernet , we need to put both ethernet ports in a single group and give IP address to that group. Can it be done ? This is similar to multilink and adding serial interface in it.

Any link on ?

Link is highly appreciated.

Thanks in advance


I have this problem too.
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Nagendra Kumar ... Tue, 12/18/2007 - 02:24

Hi Subodh,

Port Aggregation (etherchannel) is not possible with onboard ethernet interfaces. You may configure it on Etherswitch network module. But 1841 doesnt support the module.

So I feel you cannot configure port aggregation with 1841.



Richard Burts Tue, 12/18/2007 - 05:05


If I understand your post correctly you want to put both FastEthernet interfaces into a group and give a single IP address to that group. You can do this with IRB. In Integrated Routing and Bridging you configure bridging on both FastEthernet interfaces (in the same bridge group) and you configure a virtual interface (BVI) and the IP address is assigned to the BVI.

This link has helpful information about IRB:



Nagendra Kumar ... Tue, 12/18/2007 - 05:15

Hi Rick,

IMHO, IRB is to forward frames from one interface to other interface, whereas cannot be used for aggregation (similar to multilink in serial interface). So I guess IRB may not help him to aggregate the ethernet interfaces.



Richard Burts Tue, 12/18/2007 - 05:28


We seem to be interpreting the requirements of the original post is different ways. I see this line of the post as the requirement:

we need to put both ethernet ports in a single group and give IP address to that group.

If that is the requirement then IRB would work. If aggregation (especially aggregation in the sense of increasing capacity by combining interfaces) is the requirement then you are correct that IRB will not work. This is especially true since in using IRB the Spanning Tree should have one of the interfaces in blocking mode. So IRB would provide redundancy but not increased capacity. I believe that we need clarification from Subodh about what the real requirement is.



bapatsubodh Tue, 12/18/2007 - 06:00


Thanks for ur updates.

I am trying to put my requirement as clearly as possible.

1. Our ISP has provided us MPLS wan connectivity on ethernet. ( 2 links )

2. Generally this connectivity is given on serial interfaces ( v.35 modems ).

3. If we had connectivity on serial ports , definitely multilink would resolve my problem of failover.

3. Now my problem is if I add these two ethernet ports in IRB and my service provider does the similar configuration at it's end ( hopefully it will be multilink ). Will failover work ?

4. That means first link connected to fa0/0 fails will all traffic be automatically be routed to second ethernet port to fa0/1.

5. At other end service provider has already clubbed these interfaces together so it shoud

work at it's end. Simple static routing is used as ip route next_hop_at_SErvice_Provider_Address

I think i have made my point clear.

Please guide.


Richard Burts Tue, 12/18/2007 - 08:19


Thank you for clarifying your requirements. I am still not sure that I fully understand the ISP providing 2 Ethernet connections and a single IP address for the links. But given that is what is happening and the rest of your explanation then I believe that configuring IRB should give you what you need for failover.

If you configure IRB then both physical interfaces will be bridging in the same bridge group and the single IP address will be associated with the BVI. Spanning Tree will have one interface active and the other interface should be blocking. If the active interface fails then the blocking interface should become active and your failover will work and if fa0/0 fails then traffic should flow through fa0/1.



bapatsubodh Tue, 12/18/2007 - 16:30


Thanks for ur input. As you have said about single IP address, ISP is ready to give seperate pair of IP for the second link. But we have only one pool of other IP address which are routed on first link. So i need clarification from ISP that if first link fails then can it route the same pool on second link. so that failover will take without any delay. At my router I will put a secondary route with higher metric pointing to second ethernet port.

IP address looks as follows.

Primary link fa0/0

IP address pair : A.B.C.D ( wan connectivity over ethernet )


Now ISP will give us a new pair of IP Address say ( W.X.Y.Z fro second failover link. But we need the same IP Pool subnet routed on this new address if Primary link fails. ( that is P.Q.R.S

Can you guide / share any experience.

Thanks in advance. ( When we are done I will post the final solution , till then let this thread be open only and pls keep posting / suggessions )


Richard Burts Wed, 12/19/2007 - 08:03


As you say much of this depends on what your provider can do and is willing to do. In my experience it is common for providers to supply a second link, supply a second set of WAN addresses, and to route your address pool over both links.

In my experience it is uncommon for the provider to supply 2 links and to use a single address on both links. I would be surprised if the provider is willing to implement the solution that has a single address shared by 2 Ethernet interfaces.

Let us know what your provider tells you and what solution you do implement.




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