switching question

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hi! i understand that the ideal setup of a new network infra is in the Hierarchy Network Model. (Core--->Distribution--->access).

My question is that since, all the routing is done in the distribution layer (eg. 45xx L3 switch) with the L2 switches connected to these L3 switches, why do i still need a core layer switches (eg. 65xx)?

From reading, i understand that, it's a good pratice to have the core to do the switching and L3 switch to do the routing, but i'm not sure how's the config like. Please advise. Thanks.

I have this problem too.
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szahid Mon, 09/10/2007 - 12:32

One reason for having switches in core layer is to be able to aggregate distribution layer switches. and if you do not have more then 2 aggregation layer switches , then you can do w/o the core switches. At that point , it just becomes a matter of future scalability.



hi! what do you mean by "aggregate distribution layer switches"? we are planning to have 2 core>>>multiple L3 switches(probably 4 or more)>>>about 60x L2 switches. Is that consider more than 2 aggregation layer switches?

most probably you can show me the brief L2 and L3 switches config, then i can understand more. Thanks


Jon Marshall Tue, 09/11/2007 - 01:15


What Salman means is if you have more than a pair of distribution switches ie.

If you have only 2 distribution switches you could treat them as a pair with responsibility for all the inter-vlan routing and then you would have no real need for a core switch layer. The distribution switches would be connected via a Layer 2 trunk and run HSRP between them.

But what if you then want to add another pair of distribution switches ?. How do you connect these switches to the exsiting pair of distribution switches. You would do this by connecting each pair of distribution switches to a core pair of switches. This also allows you to keep adding pairs of distribution switches as and when you need.

There is no reason why you should not connect your distribution switches to your core switches with layer 3 links as long as you carefully plan your vlan placement. The advantage of this is that you remove spanning-tree from the core of your network and layer 3 switching is not generally considered a performance hit anymore.

You can obviously also use layer 2 if you want to.

A core switch network is primarily used in a campus setting where you are connecting multiple buildings together or as previously mentioned where you have multiple ( more than 2 ) distribution swithes.

Using a collapsed core/distribution is quite common in single building deployments where you have no need for more than 2 distribution switches.

For config example you might want to look at some of Cisco design docs which give examples. Start here




Joseph W. Doherty Tue, 09/11/2007 - 04:55

Another way to look at this, you note you have 4 or more distribution routers. Without any core, how would you interconnect them? You could do a full mesh, but that is one connection from each distribution router to every other. Besides the problem of physical wiring topology, especially if the distribution routers are not near each other, it also adds to the complexity of the logical topology. (For instance, more routes to pass around within the routing protocol.)

If instead we have a core, we can run one link from each distribution router to the core. This avoids the full mesh complexity as you add additional distribution routers. It also simplifies bandwidth measurement, instead of watching bandwidth on every link between all the distribution routers, we only need to watch the bandwidth to the core.

If you can see the advantages to having a core with multiple distribution routers, the other important questions might be should the core be L2 or L3 and whether it should be dual core.

Traditionally, switching was much faster than routing, and if the distribution is routing, routing in the core seems a bit redundant and might slow the network down. So, the older design philosophy was to use L2 in the core.

Modern L3 switches usually approach L2 in speed, negating that consideration. Second, having all your distribution routers peer on a L2 segment does raise some other issues, one being multicast. The latter has been addressed by Cicso with technology such as PIM snooping (not ICMP snooping), but routing within the core also addresses the problem. Today, routed cores are recommended.

On the issue of core redundancy, since the core is so critical, we don't want a single point of failure. Hence the common dual core approach. However, it can also be addressed by a router with very few, if any, single points of failure. Examples would be 4500 or 6500 series with duals sups and power supplies and multiple line card links, or by the 3570 and 3750-E series as a stack. Using this type of equipment, one physical core might be adequate.

Lastly, I often remind people that your logical topology doesn't always have to be your physical topology. A logical core is a good thing, but does it have to be on its own physical platform in all cases? Sometimes it's not really necessary. You could place your logically core on one or two of your distribution routers. You even have the choice of it being L2 or L3. I sometimes suggest the server distribution routers for the co-location. The reason for placing it there, those routers already usually pass all traffic except client-to-client, which is often light.


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