When building a network, some considerations must be taken from the point of view ofavailability. In the context of networks, availability means that the network must be operable during the whole day regardless of failures. Thus, some techniques must be taken to make the network available all the time.
In order to develop redundant network one must consider the possible failures that must occur during the network operation. These failures are categorized into two types: link failure and network device failures. In link failures, one cable connecting two computers or two any two devices such as routers or switches can be disconnected and make some of the network or all of it to be down. On the other end, device failure means that the network device itself is down which could be router, switch, or a hub.
First let us find how to overcome link failures. The solution in link failures is simpler than device failure because all we need to do is to provide multiple links between devices such that when a link is down, other link takes its role. This is simple of course but can lead to problems. This is because when there are more one link connecting the devices the data will find multiple links and the switch will forward the data to multiple links and the receiving end will receive the data more than one time. In addition, looping can occur because the data will be forwarded across the links forever. This will occur because there is more than one path to the destination and because the switch or the hub forwards the data to all ports.
The solution to looping is to use some protocols that block some ports on the switch and open others so that only one path is existed to the destination. This is achieved by what is called spanning tee protocol, which is available on Cisco switches. Notice that the concept of looping and STP are applied only to devices connected through a LAN and not to routers.
Second let us see the other type of failure which is device failure. This type occurs when a network device fails and thus is unable to forward the packets. The failure here can have a big impact on the network if it occurs in the core layer of the network which connects the whole network together. Therefore the solution is suggested to be applied at the core layer where two network devices are connected to the network to forward the data, one is the primary and the other is the secondary If one device fails, the secondary becomes available.
The above technique is configured on the router or the switch depending on what is used. Some protocols are developed to accomplish this function and are available on Cisco devices. When implementing it, one must connect the device at the core which must be redundant to the proper ports of other devices and connect similar device to the rest of the network in the same manner the primary device is connected. If the primary fails, this secondary takes it role