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A Design Dilemma by a net Admin turning into Architect

In a big hierarchical network architecture (Core-Distribution-Access) is right to have ospf routing running on core switches? I mean... should I build an high speed L2 Core or should I connect the Core to the different distributions using small interconnection subnets (interface VLANs) and a routing protocol (OSPF)? In this case is the use of a distributed default route originated by all core switches suggested?


Re: A Design Dilemma by a net Admin turning into Architect

My recommendation would be:

1. Run OSPF between the core switches and the routers. Configure layer3 only on the core switch.

2. Trunk between the core switches and the distribution switches. Leave distribution as a layer 2 switch.

3. If possible, configure access switches to be on just on one Vlan. This will eliminate the need to create a trunk between the access and distribution switch and it will limit the implications of STP problems during outage.

Hope that helps.




Re: A Design Dilemma by a net Admin turning into Architect

If you are starting from scratch consider a High Availability Routed Campus Design completely Layer3 all the way up to the Access Layer. You will be able to have multiple VLANs at each Access switch with no Spanning Trees.

Run EIGRP with Access switches as Stubs. Summarize Access routes on the Distribution to Core, and the Core summarizes Distribution routes to WAN, Datacenter, and Internet Layers.

As the previous poster mentioned, OSPF is an option too, but this requires manual tuning of the timers to match the convergence time of EIGRP.

Do not put dual supervisors in the Distribution or Core layers, only the Access Layer (assuming a chassis based Access). Dual Sups in the Dist/Core slows down convergence in a High Availability Routed Campus design.

Give a read to the linked document.

Please rate all helpful posts.



New Member

Re: A Design Dilemma by a net Admin turning into Architect

I agree with the suggestions given by Brad. I would only add that Cisco nowadays advocates L3 network throughout enterprise campus. The older concept of "high speed L2 links" and "slower L3 routing" are replaced by wire speed L2/L3/L4 switching. So there is no real speed penalty for using L3 wherever possible.

So i would break it down as follows:

if you ever need to span a vlan across multiple access switch (not recommended) then you may have no choice but to use L2 access switches with L2 trunks connecting distribution. In this case too, you can perhaps use MST/RSTP for a very fast convergence that the pure L3 networks enjoy.

For all other purposes, if you have the ability to do L3 at access i.e. All your switches are L3 switches, then you should perhaps consider a 'routed' design rather than a 'switched' one.

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