Reliability is a quality of computer network that consistently performs according to its specifications. It has long been considered one of three related attributes that must be considered when planning network. Redundancy is one of the key factors to maintain network reliability. Multiple physical connections between network devices provide redundant network paths. The network can continue its operation when a single link or port has failed. Redundant links also share the traffic load and increase capacity and speed.
The Spanning Tree Protocols are the best to use and manage Layer 2 redundancy. Without a proper protocol, the layer 2 loops are created which cause network breakdown. The spanning tree protocol chooses the best paths and also chooses an alternate path immediately when the primary path fails.
The network devices provide the capability of redundancy, such as multilayer switches or routers; provide the facility for a user to use an alternate gateway when the primary gateway fails. A client may now the ability to connect more than one possible default gateway. First Hop Redundancy Protocols manages the client’s default gateway, and also the ability to use an alternate default gateway should the primary default gateway fail.
Redundancy at OSI Layers 1 and 2
The three-tier hierarchical network attempts to eliminate a single point of failure on the network. Multiple cabled connections between switches provide physical redundancy in a switched network. Redundancy improves the reliability and availability of the network. With alternate physical connections for data to pass through the network makes it possible for users to access network resources, regardless of any interference.
Nowadays, the availability of the network is very necessary for every organization to complete their business needs. So, the network infrastructure plan is a serious component. Path redundancy eliminates the possibility of failure and continues the services without any disruption.
A hierarchical network design that uses core, distribution, and access layers with redundancy, attempts to eliminate any disruption of network services to users. Redundant networks need physical connection including logical redundancy. However, there are possibilities of physical and logical layer 2 loops in the redundant paths in a switched Ethernet network.
When multiple paths exist between two switches on a network, and there is no spanning tree protocol between both, a Layer 2 loop occurs. A Layer 2 loop can result in three primary issues:
- MAC database instability
- Broadcast storms
- Multiple frame transmission
MAC Database Instability (Layer 1 Redundancy)
Ethernet frames require a mechanism to block continued propagation of broadcast frames on a switched network. The Ethernet frames propagate between switches endlessly, or until a link is disrupted and breaks the loop because the Ethernet frames have no TTL value in their frame headers. This nonstop propagation between switches can result in MAC database instability.
The MAC Address instability occurs due to broadcast frames forwarding which forwarded out to all switch ports, except the original ingress port. The broadcast ensures that all devices in a broadcast domain can receive the frame. So, if there is a redundant network path between networks switches which cause an endless loop result. When a loop occurs, it is possible for the MAC address table on a switch to constantly change with the updates from the broadcast frames, which results in MAC database instability. Now look at the below topology PC-1 and PC has a redundant path without any proper logical mechanism. The process of packet sending is as under:
- When PC1 sends a broadcast frame to Switch-1. Switch-1 will receive the broadcast frame on F0/1. When Switch-1 receives the broadcast frame, it updates its MAC address table that PC1 is available on port F0/1.
- Due to broadcast frame, Switch-1 forwards the frame out to all its ports, including both Trunks. When the broadcast frame arrives at Switch2 and Switch-4, the switches update their MAC address tables to indicate that PC1 is available on port F0/8 on Switch-2 and F0/11 on Switch-4.
- So, when the broadcast frame received on Switch-2 and Switch-4, they forward the frame out all ports, except the incoming ports. Both switches send the broadcast frame to Switch-3. If Switch-3 received the broadcast frame from Switch-2 first, it maintains the MAC address table that PC-1 is available on F0/2, and flood the frame out to all its port accept the port 0/2. Meanwhile, it receives the frame from switch-4 on the interface F0/9, the switch-3 overwrite its MAC address table again and flood the same packet to all its interfaces except F0/9
- Now again the same packet will circulate into the entire network. Each switch forwards the broadcast frame out all of its ports, except the ingress port. Each time the MAC address table is updated with the last entry received from the other two switches.
This process continuous nonstop until the loop is broken. The loop can be broken by disconnecting the connections physically or powering down one of the switches in the loop. The loop between switches put a high load on the CPU load of all switches which slows down performance on the switch when genuine traffic arrives.
A host participating in the network loop cannot access to other hosts on the network. At the last, due to the constant changes in the MAC table, the switch does not know which port to forward unicast frames.
Broadcast Storms (Layer 1 Redundancy)
When so many broadcast frames caught in a Layer 2 loop that consumes all available bandwidth and there is no bandwidth available for legitimate traffic and the network becomes unavailable for data communication, the broadcast storm occurs. This is also known as an effective denial of service (DoS).
A broadcast storm is expected on a looped network because more devices send broadcasts traffic over the network, which causes more broadcast traffic, is caught in the loop and consumes network resources. This finally creates a broadcast storm that causes the network to fail.
There are other disadvantages of the broadcast storm because traffic is forwarded out every port on a switch; all the connected devices must process all the broadcast traffic that is flooded endlessly around the looped network. This can cause the end device to malfunction.
Duplicate Unicast Frames (Layer 1 Redundancy)
Broadcast frames are not only the type of frames that are affected by loops. Unicast frames sent onto a looped network can result in duplicate frames arriving at the destination device.
Most upper-layer protocols cannot recognize duplicate frames. This protocol generally makes use of a sequence-numbering mechanism assume that the transmission has failed and that the sequence number has recycled for another communication session. Ethernet protocol (Layer 2), require a mechanism to recognize and eliminate endlessly looping frames.
Layer 3 implement a TTL mechanism for eliminating the loops because TTL limits the number of times a Layer 3 networking device can retransmit a packet. Layer 2 devices do not have a mechanism like TTL, so they continue to retransmit looping traffic again and again.
STP, a Layer 2 loop-avoidance mechanism, was developed to solve these problems. To stop these issues from occurring in a redundant network, some type of spanning tree must be enabled on the switches. By spanning tree is enabled in all the Cisco switches to stop Layer 2 loops from occurring.