Redundancy in a hierarchical network
The hierarchical design model was introduced in Chapter 1. The hierarchical design model addresses issues found in the flat model network topologies. One of the issues is redundancy. Layer 2 redundancy improves the availability of the network by implementing alternate network paths by adding equipment and cabling. Having multiple paths for data to traverse the network allows for a single path to be disrupted without impacting the connectivity of devices on the network.
Layer 2 Loops
Redundancy is an important part of the hierarchical design. Although it is important for availability, there are some considerations that need to be addressed before redundancy is even possible on a network.
When multiple paths exist between two devices on the network and STP has been disabled on those switches, a Layer 2 loop can occur. If STP is enabled on these switches, which is the default, a Layer 2 loop would not occur.
Ethernet frames do not have a time to live (TTL) like IP packets traversing routers. As a result, if they are not terminated properly on a switched network, they continue to bounce from switch to switch endlessly or until a link is disrupted and breaks the loop.
Broadcast frames are forwarded out all switch ports, except the originating port. This ensures that all devices in the broadcast domain are able to receive the frame. If there is more than one path for the frame to be forwarded out, it can result in an endless loop.
Loops result in high CPU load on all switches caught in the loop. Because the same frames are constantly being forwarded back and forth between all switches in the loop, the CPU of the switch ends up having to process a lot of data. This slows down performance on the switch when legitimate traffic arrives.
A host caught in a network loop is not accessible to other hosts on the network. Because the MAC address table is constantly changing with the updates from the broadcast frames, the switch does not know which port to forward the unicast frames out to reach the final destination. The unicast frames end up looping around the network as well. As more and more frames end up looping on the network, a broadcast storm occurs.
Broadcast Storms
A broadcast storm occurs when there are so many broadcast frames caught in a Layer 2 loop that all available bandwidth is consumed. Consequently, no bandwidth is available bandwidth for legitimate traffic, and the network becomes unavailable for data communication.
A broadcast storm is inevitable on a looped network. As more devices send broadcasts out on the network, more and more traffic gets caught in the loop, eventually creating a broadcast storm that causes the network to fail.
There are other consequences for broadcast storms. Because broadcast traffic is forwarded out every port on a switch, all connected devices have to process all broadcast traffic that is being flooded endlessly around the looped network. This can cause the end device to malfunction because of the high processing requirements for sustaining such a high traffic load on the network interface card.
Loops in the Wiring Closet
Redundancy is an important component of a highly available hierarchical network topology, but loops can arise as a result of the multiple paths configured on the network. You can prevent loops using the Spanning Tree Protocol (STP). However, if STP has not been implemented in preparation for a redundant topology, loops can occur unexpectedly.
Network wiring for small to medium-sized businesses can get very confusing. Network cables between access layer switches, located in the wiring closets, disappear into the walls, floors, and ceilings where they are run back to the distribution layer switches on the network. If the network cables are not properly labeled when they are terminated in the patch panel in the wiring closet, it is difficult to determine where the destination is for the patch panel port on the network. Network loops that are a result of accidental duplicate connections in the wiring closets are a common occurrence.
Loops in the Cubicles
Because of insufficient network data connections, some end users have a personal hub or switch located in their working environment. Rather than incur the costs of running additional network data connections to the workspace, a simple hub or switch is connected to an existing network data connection allowing all devices connected to the personal hub or switch to gain access to the network.
Wiring closets are typically secured to prevent unauthorized access, so often the network administrator is the only one who has full control over how and what devices are connected to the network. Unlike the wiring closet, the administrator is not in control of how personal hubs and switches are being used or connected, so the end user can accidentally interconnect the switches or hubs.


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