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distribution layer


in network design, when do i apply a distribution layer? Is this only dependant on physical distance? Or what other factors do i have to consider?



Re: distribution layer

When designing a network, it is ideal to try and sepaerate as much as possible into the 3 layers. However, this is not always possible. They different layers deal more with traffic filtering and policing more than they do physical distance. Its best to try to push your acls, rate-limits, QoS markings, etc. as far to the edge as possible (access layer). Final policing of traffic will occur at the distro layer which should be an aggregation of multiple similar access layer devices, and trying to step up the speed to pass into the core. The core should be as unfiltered (within reason) as possible. The idea is to move traffic in the core as fast as possible since you have already policed and marked at the other two layers. The core will be an aggregation for multiple similar distro devices. For a potential service provider/datacenter example:

Access Layer - 3550s used to connect various different customers. QoS trust/overide applied to the port, queues setup on the port, rate-limits pushed to the port depending on type of service, copper ethernet would be the handoff to each customer with speeds ranging from 1 to 100Mbps. Each 3550 would use dual fiber uplinks to connect to two 6500s for redundancy.

Distro Layer - Dual 6500s with sfp fiber blades to terminate the uplinks from the access layer. Layer 3 termination for customers is handled with SVIs and HSRP is used for logical redundancy. Layer 3 marking of QoS, acls, and rate-limiting occur before passing into the core. Each 6500 would dual up link to dual 6500s in the core using 10gigE modules.

Core Layer - Here the core has a connection to every other module in the network so the traffic is forwarded as fast as possible to where it needs to go.

Hope this helps.

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