DeVry University Network Design

DeVry University Network Design

Introduction

This paper provides practical steps in reducing the effects of a Layer 2 loop on a Cisco Catalyst 6500. Before delving into details, let's first define what a Layer 2 loop is. A Layer 2 loop occurs in a campus network when more than one Layer 2 forwarding path exists between two given switches. In this scenario, a switch that receives a broadcast frame sends it to all its trunk ports and access ports (same VLAN) (Gopalan & Chiueh, 2006). In the presence of a loop, when campus switches forward broadcast frames to all their ports, this creates an amplification phenomenon for broadcast frames trapped indefinitely within the loop. This phenomenon is also known as a broadcast storm. It leads to an exhaustion of bandwidth, and CPU overutilization due to the presence of large volumes of broadcast frames. A broadcast storm brings a network to an unusable state, and in certain cases network administrators may lose the capability to access devices by console.

The Spanning Tree Protocol (STP) was designed to ensure loop-free Layer 2 topologies. Despite the use of STP, some situations can create Layer 2 loops such as wiring mistakes, misconfigured hosts (bridged interfaces), switch configuration mistakes, and loss of BPDU keepalives. It is important to have a good campus network design, but human errors are still possible. It is critical to look beyond what can be prevented, and ask yourself the following question (Clos, 1953). If a Layer 2 loop were to occur, would my campus network be able to sustain it? If the answer to this question is no, or if you are not sure, I encourage you to explore a suggested solution presented in this article to mitigate the impact of a Layer 2 loop.

I got involved in exploring solutions to protect the Catalyst 6500 because one of our customers had experienced Layer 2 issues, and they wanted to leverage Cisco existing tools to alleviate the effects of Layer 2 loops on their Cisco Catalyst 6500 distribution switches. Their campus network consists of a three-layer model with Layer 2 connections between access and distribution switches, and Layer 3 connections between distribution and core switches. This paper presents the steps I took to develop a solution to protect the Catalyst 6500 in the presence of a Layer 2 loop (Bottor & Haddock, 2007).

Discussion

The loop is a path for data flows that can repeat data again  for  sender , means when sender (PC , Hub ,...) send data from a port to other devices after few moment receives the data again and also tries to repeat this data again and all bandwidth and CPU in device are busy to repeat this data . This is a definition in simple word . The protocol to avoid the loops in layer 2 (switching) is STP, and is similar to the OSPF protocol in routing protocols (Al-Fares & Radhakrishnan, 2010).

In this figure we can see from switch D to A two paths can be used and ...