Breaking up the supernet at each end

Posted on 2008-10-28
Medium Priority
Last Modified: 2011-10-19
Hello all,  

I am a little stumped here and have been reading for an answer all night with no luck, so I thought I would post.

We moved to a new MPLS network with our provider.  We have 4 subnets at each location.  All /24 bit masks.


So our isp just routed the supernets to each location on the mpls network.

my question is, how do i break them back apart?  I have cisco 3725s with 5 FE interfaces at each location.  (We are also using ASA5510 for the internet that comes on a different port from our isp, but this is a different topic.)

I know I can assign each subnet to 4 of the FE interfaces, but the transport interface (interface connected to port provided by isp) is where i get confused.  do i just assign that an IP address with a 22bit mask (i.e. or will it get confused and think its on the same subnet as the other interfaces?  i have considered EIGRP, etc, but I don't know if my isp is running any of those protocols and am concerned about the other interfaces being part of the same supernet.  The ip address of my providers router at my main site is (they did this to route my networks to me).  So it becomes part of the same network of what I have inside.

Any help would be appreciated.



Question by:digiandy
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Accepted Solution

inrouted earned 2000 total points
ID: 22820057
Ok.  Lets see how we can get ourselves to a position where we can easily navigate what are sure to be some quite complex waters.  I will assume you had a reason for moving to MPLS and that we dont need to go into the benefits of it here.  

1.  Are you running iBGP at any CE device?
2. What is your interior routing protocol?
3. Are you using VRFs?  
4. Do you have a simple network map of how you have the networks deployed.  I know you stated that you have the 4 /24s but how are they allocated now?  With MPLS, VLANs and VRFs..you could have all 4 subnets in every location and it would be a valid design.


Expert Comment

ID: 22820062
Oh and to answer one of your questions.  The act of taking your subnets and globbing them together is called route summarization, and is typical of provider transports.  Prefixes take up memory space and memory is precious on a router.

Expert Comment

ID: 22820092
Actually, I thought about this..and i dont want to bog you down in what might be way too much info.  If you want to consider this case strictly as static routing..the provider probably gave you an ip to assign to your interface that connects to their PE device.  This could be (as you indicated above) in a totally different subnet than any of your others.  They will have their routing table setup to point your blocks to the ip address they gave you.

As soon as you assign one of your interfaces an ip using a /24, the router will put that into its global routing table. So in other words, just assign your interfaces with the /24s as you wanted to. Point the default gateway to the PE ip address.  You should be good.  If you are running a routing protocol, this changes though.

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Author Comment

ID: 22825945

I will attach a diagram in a minute.  The problem is as you stated in your last post the providers IP that they gave me is which overlaps my network.  It seems as though they took my network plan and put it on their routers instead of using a different network and just routing traffic.

Thanks for looking at this with me.

Author Comment

ID: 22826012
Here is our network map that we gave our providers. They were supposed to summarized the supernets as routed.


Expert Comment

ID: 22840732

Sorry for the delay in responding.  Thanks so much for the network diagram (are you really based in Austin?  Small world..as that is where I live :) )

So, given what I can determine now..are each of the subnets only used on their respective interfaces?  And when you say ISP, you are not talking about the ASA right?  Just the routing and the CE devices.



Author Comment

ID: 23033752
-route, sorry for abandoning you. I unexpectedly got sick and went into the hospital. I am ok, but was taking it easy the last few weeks and took some time off work, so I didn't mean to abandon you after your help. From what I can tell another guy got the working with vrf. I haven't seen it myself yet but I hear it's all good, and again thanks. And yes I moved to Austin 2 years ago from Chicago, it's great down here.  

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