Traffic Autoshifting --> Switches

Unanswered Question
Sep 15th, 2009

Hello Netpros,

Need your suggestion to configure the scenerio attached in diagram.

---> Parent Switch-A & Parent Switch-B are connected with Switch-1 & Switch-2 respectively(Both are L2 connectivity).

---> There are some other access connectivities extended from parent switches A & B apart from connectivities shown in diagram.

We are looking for below scenerio :-

1) If the connectivity between Parent Switch-A & Switch-1 fails, the traffic from switch-1 autoshifted or follow other uplink path by proposed connecivity betwen Switch-1 & witch-2 i.e Switch -1 traffic---> Swicth-2 --> Parent Switch-B.

2) Same is applicable for reverse case. i.e if connectivity between Parent Switch-B & Switch-2 fails, all the traffic autoshifted to proposed connectivity between Switch-1 & Switch-2. ie. Switch-2 Traffic --> Switch-1--> Parent Switch-A.

3) Kindly note we are avoiding any config changes on both parent switches as heavy traffic running on both and as mentioned other access L2 connectivity running via both (on some access connectivities same Vlans are running as on this proposed connectivity).

please help...

I have this problem too.
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Jon Marshall Tue, 09/15/2009 - 23:59


The design proposal won't work because there is no connection between parent switch A and parent switch B.

So lets say there is a device connected to Switch 1 that needs to send traffic to a device on a switch that is connected to parent switch A. The link between switch 1 and Parent 1 fails so traffic goes via switch 2 to Parent B but then what ?

Because there is no link between parent switches the traffic can't get to the correct switch.

If you want alternate paths then connect each switch ie. Switch1 and Switch2 to both parent switches. That way if the link between switch 1 and parent 1 failed the traffic would go to switch2 and then to parent A.

Edit - one other point. The overall design in your jpg is not good in terms of redundancy for all the other switches as well. Each switch should really be dual connected to each parent switch and there should be a connection between the parent switches. The connection could be L3 in your case which would allow both uplinks from each switch to be forwarding. Or you could use a L2 connection which is more common but then one of the uplinks would be blocked due to STP.

One other option is that if your switches are all L3 then you could route to the parent switches rather than use L2 connections but that would require significant reconfiguration.


Joseph W. Doherty Wed, 09/16/2009 - 04:24

Echoing what Jon has posted, if this is all being done at L2, you need to "close a loop". Simplest approach might be to trunk VLANs (passing between parent and child swithes to also pass between the parent switches. Of course once you have a physical L2 loop, you'll want something to control it, offen some variant of STP (ideally with STP cost to break the link between the parents).

If you use links between both parents and both child switches (as also described by Jon), believe you might have the option to use Cisco FlexLinks(?). More about L2 design can be found here:


Like Jon, since all your switches are L3 you could consider routing between them. Wed, 09/16/2009 - 08:54

Hii Joseph & Jon,

Agree with Both of you. But seems i need to clear something more. As you both mentioning that "what abouttraffic to other Switches connected to Parent Swutch-1 when connectivity between Parent-1 & Switch-1 failed and traffic flows thru Switch-2"...

I am clearing by one example. Lets say we have 4 different customers on L2 and none of these four connectivities have to do do anything with each other i.e they are not dependent on each other & neither have to access each other.

Here is my proposed scnerio Both Switch-1 & Switch-2 are placed at same remote location or say at same customer location with same Vlans and not to do anything with other access connectivities from Parent Switches. Tthese two switches are placed at single customer/same remote location to make sort of hardware / traffic level redundancy. nothing else special

We just want to avoid major changes of Parent Switches as there is huge traffic on both of these and a bit mistake can make a trouble.

As for as flex links are concerned , this may not support on L3(450 series) switches.

PLease suggest now.

Joseph W. Doherty Wed, 09/16/2009 - 10:45

Unclear, to me, your new question. From your original diagram you show multiple (and same) VLANs (10, 12, .. 22) between both parent switches and switches 1 and 2. Certainly you might not allow or desire traffic between these VLANs (if, for example, they are used by different customers), but if the same VLAN spans all four switches, for redundancy, you need at least one more path. Otherwise failure of switch 1 or 2, or their links will segment the VLANs.

Perhaps you could clarify your question futher? Wed, 09/16/2009 - 22:11

Hello Josehph,

Simply, i just want to acheive what eer i mentioned in my first post ith diagram, ie traffic autoshifting of Switch-1 & Switch-2 to each other, in case of failure of there direct respective switches(Parent-1 & Parent-2) connectivity.


Jon Marshall Thu, 09/17/2009 - 00:17


This is getting a bit confusing :-). If the customers are totally isolated so each does not need to communicate with each other what exactly are the parent switches doing ?

What is the purpose of connecting the customer switches to the parent switches.

Jon Thu, 09/17/2009 - 02:08

Hii Jon,

Parent switches are basically UPLINK devices for Switch-1 & Switch-2. Connected with L2 connectivity with their respective Switch-1 & Switch-2.

Jon Marshall Thu, 09/17/2009 - 02:33


But if you are connecting the 2 switches together then do you need the parent switches as uplink switches ie. what services do the parent switches provide for the customers.

We are trying to help you but your one sentence answers are not giving enough information. Your'e original query was that you wanted to provide redundant paths from the switches switch1 and switch2. As both Joseph and myself pointed out your design would not provide that redundancy and we suggested a number of alternatives.

So exactly what is it you are trying to achieve and what is the purpose of the parent switches for customer traffic. If these are separate customers then if i was a customer i would not be happy that the failure of one parent switch isolates my switch(es). I appreciate this is what you are trying to design around but it seems like we are talking at cross purposes which leads me to believe we are not fully understanding your requirements.

Jon Thu, 09/17/2009 - 08:27


I guess i was very specific that i need and looking for commuinication redundancy.

Both Parent Switches belongs to same network provider. Any any one of the uplink connectivity towards Parent Switched down/failed traffic of directly connected switches (i.e Switch-1 or Switch-2) should auto-shifted on proposed copnnectivity as show in my network diagram .


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