Role of Distribution Switch

[anis_cisco]

Hello all,

Can any one explain me what is the role of distribution switch in netwok environment? How the network should be designed proper having it.

My confusion is, having distribution switch in network, should we configure layer3 interfaces on distribution or on core ? how the connected users at access layer switch should communicate with other vlans ? does intervlan routing should be enabled on distribution switch or at core ? what should be the gateway configured for access layer switches or for end users ?

Regards,

[Reza Sharifi]

Hi,

Most distribution layer switches are layer-3 and can route between your vlans.  There is really no need to extended the vlans all the way to the core, as if you have redundant devices, you need to run STP. If you terminate your vlans on the distro switches, you need to have layer-3 links between the distro and core switches.  Also, these days most people used what is called a collapsed core, which means the distro and core switches are the same and so no need for distro layer at all.

Here is a good doc to look at

http://www.cisco.com/c/en/us/td/docs/solutions/Enterprise/Campus/campover.html

HTH

[anis_cisco]

Hi Reza,

In my scenario, i have a pair of core 6509(VSS) one distribution in each floor then access layer switches. So i will have etherchannel between core & 3750 (10G) distribution switch & from distribution switch to access switch (3750-1G) would be 802.1Q trunking.

Other then Collapsed core designed, do you mean in general distro sw will have vlans + layer3 interfaces for access switches & core switch will have separate vlans + layer3 interfaces for distro sw ? In this scenario if end user would have to communicate with other vlan it would be routed at distro ( no need to send traffic on core to re-route or intervlan routing ) ? if end user would have to go out of its environment then it will go to core ? in this case we will have trunks between core to distro & trunks between distro to access ? so distro will have only trunks + layer 3 interfaces for access switches and core switch would have vlans + layer 3 interfaces for distro switches ?

If above is right then in my scenario i cant see much utilization of core (in routing prospective) other then handling servers + applications that are directly connected on core ?

One new question here is can i connect 3750 (10G) switch with 3750 (1G) switch via matrix cable ?

Regards

[Jon Marshall]

edited

[devils_advocate]

There are lots of ways to do things but not all of them are best practice.

The general consensus (and best practice) is to terminate your Vlans on the Distribution switches. As they will all be 'Directly Connected' subnets, there is no need for a routing protocol as long as you enable IP Routing on the Distribution switch.

Having a seperate Management Vlan for your access switches (and possibly other network devices) is also best practice as it seperates User and Management traffic from each other.

The core layer is responsible for high speed packet transfer so you want to avoid giving it any tasks that the Distirubtion switch should be responsible for such as Spanning Tree, Intervlan routing, Access Lists etc. I would be looking to run L3 routed links between the Core and Distribution layers and run a Routing Protocol such as EIGRP or OSPF between them. The core layer should know when a subnet is no longer reachable to prevent it sending packets needlessly so having Dynamic Routing will help achieve this.

Some people choose to run L3 all the way to the access Layer but this brings about challenges of its own so I am on the fence as to whether this is a good thing. I can see both the benefits and the downsides to this approach and I tend to run L2 Port Channel Trunks between Distribution and Access switches.

Hope this helps...

[Jon Marshall]

As the others have said the core is generally used to simply interconnect your distribution switches, usually using L3 routed links ie. no need for vlans between distro and core.

That said you seem to be talking about a building rather than a campus setup. In a campus set you can have multiple buildings with access switches per floor connecting to a distro pair that do inter vlan routing for the buildings vlans and then each distro pair is connected to core set of switches that route between buildings. This is a standard Cisco 3 tier design but within a single building it might not be the best approach.

In a single building design, as Reza mentioned, a more common design is a collapsed distribution/core where the same pair of switches do both. Each floor really doesn't need a distribution switch unless there is an abnormally high amount of traffic between vlans on the same floor which is generally not the case.

So your access switches on each floor connect back to core/disto pair of switches which handle all the inter vlan routing. As devils advocate mentioned. you can extend L3 back to the access layer but there are limitations with this and with the advent of VSS/MEC on the 6500s and MEC on the 3750s a lot of the advtantage of running L3 to the access layer has gone, and you still get the flexibility that L2 can give you ie. the ability to have a vlan on multiple access switches.

Ideally you do not want servers connecting directly into the core/distro pair (and certainly not the core if you had a dedicated core pair of switches). You would want a separate pair of switches for the servers that then connect back to the core/distro pair. But this often comes down to cost and you often see servers connected to the core/distro pair.

If it is one building i would use the 6500s as a core/distro pair and run MEC from the switches on each floor back to the 6500s. Inter vlan routing would be done on the 6500s, again with the proviso that most of the traffic is between clients on the floors and servers and not between clients and other clients on the same floor.

There is no need to have distribution switches per floor. If you do then be aware that is in effect a L3 routed access layer and the major restriction is that if you have vlan/IP subnet on one switch you cannot then have the same vlan/IP siubnet on another switch on a different floor.

Jon