Access Layer; Distribution Layer - Cisco RJ-45-to-AUX Brochure

Weighted fair queuing (WFQ)—Allows for multiple queues so that no one queue can starve another of
all its bandwidth. WFQ is enabled by default on all serial interfaces that run at or below 2Mbps,
except for those interfaces with Link Access Procedure, Balanced (LAPB), X.25, or Synchronous
Data Link Control (SDLC) encapsulations. Most networks fail when their design creates unstable
network links, hardware failures, or routing loops. When a failure occurs in such a network, and then
the network does not converge in time to prevent a major problem for network processes or users,
redundancy must be built in.
When designing a network using the Cisco campus hierarchical switching model, you create redundancy;
doing so aids in the case of a network failure by providing logical points to aggregate and summarize network
traffic. This setup prevents a failure in one part of the network from affecting the entire enterprise network.
This model divides the network into three distinct layers:
Access layer—The first layer, which is the first point of access for the end user interface. This layer
passes traffic from the end user interface to the rest of the network. Security at this layer is port−based
and provides verification of an authentic MAC address, local device security, and access lists.
Distribution layer—The second layer, which serves to combine the traffic of the Access layer,
summarize traffic, and combine routes. This layer also processes data traffic and applies security and
queuing policies, allowing data traffic to be filtered and providing a guarantee of bandwidth
availability for certain traffic.
Core layer—Reads headers and forwards traffic as quickly as possible through the network. This is its
only function. This layer needs to have high reliability and availability because any losses at this layer
can greatly affect the rest of the network.
The Cisco campus hierarchical switching model is depicted in Figure 2.1.
Figure 2.1: The Cisco campus hierarchical switching model.

Access Layer

The Access layer provides some important functionality, such as shared bandwidth, switched bandwidth,
Media Access Control (MAC) layer filtering, and microsegmentation. Two goals of this layer are to pass
traffic to the network for valid network users and to filter traffic that is passed along.
The Access layer switch connects the physical wire from the end user interface, thereby providing the means
to connect to the devices located on the Distribution layer. It provides connections to both the local LAN and
remote devices. The Access layer is the entry point to the network. This layer makes security and policy
decisions and becomes the logical termination point for virtual private networks (VPNs).

Distribution Layer

The Distribution layer is the demarcation point between the Access and Core layers. This layer terminates
network traffic that originates in the Access layer and then summarizes the traffic before passing it along to
the highest Core layer. The Distribution layer also provides policy−based network connectivity, such as
queuing and data termination.
32