Distribution layer switches have a very important role on the network. They collect the data from all the access layer switches and forward it to the core layer switches. As you will learn later in this course, traffic that is generated at Layer 2 on a switched network needs to be managed, or segmented into VLANs, so it does not needlessly consume bandwidth throughout the network. Distribution layer switches provides the inter-VLAN routing functions so that one VLAN can communicate with another on the network. This routing typically takes place at the distribution layer because distribution layer switches have higher processing capabilities than the access layer switches. Distribution layer switches alleviate the core switches from needing to perform that task since the core is busy handling the forwarding of very high volumes of traffic. Because inter-VLAN routing is performed at the distribution layer, the switches at this layer need to support Layer 3 functions.
Security Policies
Another reason why Layer 3 functionality is required for distribution layer switches is because of the advanced security policies that can be applied to network traffic. Access lists are used to control how traffic flows through the network. An Access Control List (ACL) allows the switch to prevent certain types of traffic and permit others. ACLs also allow you to control which network devices can communicate on the network. Using ACLs is processing-intensive because the switch needs to inspect every packet and see if it matches one of the ACL rules defined on the switch. This inspection is performed at the distribution layer, because the switches at this layer typically have the processing capability to handle the additional load, and it also simplifies the use of ACLs. Instead of using ACLs for every access layer switch in the network, they are defined on the fewer distribution layer switches, making management of the ACLs much easier.
Quality of Service
The distribution layer switches also need to support QoS to maintain the prioritization of traffic coming from the access layer switches that have implemented QoS. Priority policies ensure that audio and video communications are guaranteed adequate bandwidth to maintain an acceptable quality of service. To maintain the priority of the voice data throughout the network, all of the switches that forward voice data must support QoS; if not all of the network devices support QoS, the benefits of QoS will be reduced. This results in poor performance and quality for audio and video communications.
The distribution layer switches are under high demand on the network because of the functions that they provide. It is important that distribution switches support redundancy for adequate availability. Loss of a distribution layer switch could have significant impact on the rest of the network because all access layer traffic passes through the distribution layer switches. Distribution layer switches are typically implemented in pairs to ensure availability. It is also recommended that distribution layer switches support multiple, hot swappable power supplies. Having more than one power supply allows the switch to continue operating even if one of the power supplies failed during operation. Having hot swappable power supplies allows you to change a failed power supply while the switch is still running. This allows you to repair the failed component without impacting the functionality of the network.
Finally, distribution layer switches need to support link aggregation. Typically, access layer switches use multiple links to connect to a distribution layer switch to ensure adequate bandwidth to accommodate the traffic generated on the access layer, and provide fault tolerance in case a link is lost. Because distribution layer switches accept incoming traffic from multiple access layer switches, they need to be able to forward all of that traffic as fast as possible to the core layer switches. As a result, distribution layer switches also need high-bandwidth aggregated links back to the core layer switches. Newer distribution layer switches support aggregated 10 Gigabit Ethernet (10GbE) uplinks to the core layer switches.
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