Explain Tiered Switching Architecture

Objectives

1.7 Explain basic corporate and datacenter network architecture. 2.3 Given a scenario, configure and deploy common Ethernet switching features. (STP only) 5.5 Given a scenario, troubleshoot general networking issues.

Three Tierd Topology

Many companies follow Cisco’s three tierd method with Access Edge Layer, Distribution/Aggregation Layer, and Core Layer.

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Edge/Access Layer Layer

Allow endpoint systems such as desktops, laptops, phones, IoE devices, security systems, etc.

Distribution Layer

Distribution layer contians traffic polocies such as routing rules, firewalls, Quality of Service (QoS), VLANs, etc. which can help divert traffic more effectively.

Core Layer

Core layer is a fault tolerant and redudant layer that maintains connections between an Internet Service Proider (ISP) and the internal network and distribution layer.

Spanning Tree Protocol

Spanning Tree Protocol (STP) allows for bridges/switches to self identify in a tree of other nodes and understand where they live inside of a network. Systems use something called a Bridge Protocol Data Unit (BPDU) multicast frames to fetch resposnes from other nodes on the network to understand their place.

Configuration

Not all switches support configuration. Some that do should be setup by an administrator to be identified as the root switch node for all other switches.

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Broadcast Storm

A broadcast store occurs when switches are looping frames to each other an unable to determine the exact place to which they should be forwarding frames. Because layer 2 switches do not have the concept of a Time to Live (TTL) the frames will continue to be forwarded.

If a broadcast storm occurs on a network where STP is already enabled, you should investigate the following potential causes:

  • Verify compatible versions of STP are enabled on all switches.
  • Verify the physical configuration of segments that use legacy equipment, such as Ethernet hubs.
  • Investigate networking devices in the user environment and verify that they are not connected as part of a loop. Typical sources of problems include unmanaged desktop switches and VoIP handsets.

Review Activity

  1. Which two topologies are used in the three-tier hierarchical model?

This is a hybrid topology with mesh and star elements. The core layer is a mesh and the links between core and distribution and distribution and access are also a mesh or partial mesh. The access switches use a star topology to connect end systems.

  1. Spanning tree has been deployed without the administrator setting a priority value. Which of the following switches will be selected as the root?

    • Switch A with base MAC f062.81ff.0001 and a 10 Gbps uplink
    • Switch B with base MAC f062.81ff.0002 and a 40 Gbps uplink
    • Switch C with base MAC f062.81ff.0003 and a 40 Gbps uplink

Switch A. The switch with the lowest value MAC address is selected if priority values are equal.

  1. In what STP-configured state(s) are all ports when a network running STP is converged?

Forwarding and blocking

  1. True or false? A broadcast storm can only be resolved by investing interface configurations.

False. A broadcast storm could be caused by a physical layer issue, such as improper cabling.