Why is routing protocol selection is important

The original version or RIPv1 determines network paths based on the IP destination and the hop count of the journey. RIPv1 interacts with the network by broadcasting its IP table to all routers connected to the network.

RIPv2 is a little more sophisticated than this and sends its routing table on to a multicast address. RIPv2 also uses authentication to keep data more secure and chooses a subnet mask and gateway for future traffic.

The main limitation of RIP is that it has a maximum hop count of 15 which makes it unsuitable for larger networks. See also: LAN Monitoring tools. IGRP was designed to build on the foundations laid down on RIP to function more effectively within larger connected networks and removed the 15 hop cap that was placed on RIP. IGRP uses metrics such as bandwidth, delay, reliability, and load to compare the viability of routes within the network.

IGRP is ideal for larger networks because it broadcasts updates every 90 seconds and has a maximum hop count of This allows it to sustain larger networks than a protocol like RIP. IGRP is also widely used because it is resistant to routing loops because it updates itself automatically when route changes occur within the network. The SPF routing algorithm is used to calculate the shortest path spanning-tree to ensure efficient data transmission of packets.

OSPF routers maintain databases detailing information about the surrounding topology of the network. LSAs are packets that detail information about how many resources a given path would take. OSPF also uses the Dijkstra algorithm to recalculate network paths when the topology changes. This protocol is also relatively secure as it can authenticate protocol changes to keep data secure.

Topology changes are tracked and OSPF can recalculate compromised packet routes if a previously-used route has been blocked. Exterior Gateway Protocol or EGP is a protocol that is used to exchange data between gateway hosts that neighbor each other within autonomous systems.

In other words, EGP provides a forum for routers to share information across different domains. The most high profile example of an EGP is the internet itself. The routing table of the EGP protocol includes known routers, route costs, and network addresses of neighboring devices.

The EGP protocol works by keeping a database of nearby networks and the routing paths it could take to reach them. This route information is sent on to connected routers. Once it arrives, the devices can update their routing tables and undertake more informed path selection throughout the network. Neighbors are queried for a route and when a change occurs the router notifies its neighbors about the change. This has the end result of making neighboring routers aware of what is going on in nearby devices.

Packet transmissions are made more effective because routes are recalculated to speed up the convergence process. Border Gateway Protocol or BGP is the routing protocol of the internet that is classified as a distance path vector protocol. However many administrators choose to change routing decisions to criteria in line with their needs.

The best routing path selection algorithm can be customized by changing the BGP cost community attribute. BGP only sends updated router table data when something changes. As a result, there is no auto-discovery of topology changes which means that the user has to configure BGP manually.

In terms of security, BGP protocol can be authenticated so that only approved routers can exchange data with each other. IS-IS uses a modified version of the Dijkstra algorithm. An IS-IS network consists of a range of components including end systems, user devices , intermediate systems routers , areas, and domains. Under IS-IS routers are organized into groups called areas and multiple areas are grouped together to make up a domain. Routers within the area are placed with Layer 1 and routers that connect segments together are classified as Layer 2.

Routing protocols can also be categorized as classful and classless routing protocols. The distinction between these two comes down to how they go about executing routing updates. An example of exterior gateway protocols is the Border gateway protocol.

Administrative distance: AD can be defined as the reliability of routing updates received from the neighboring router. For example, if two updates are received for the same path from two routing protocols, the router will check the best AD value to choose the optimistic path. The AD with the lowest value will be given more preference. Metric: If two routing updates have the same Ad value, then the metric will come in to picture to calculate the best path. Similar to AD, different routing protocols use different metric values.

The routing protocol with the lowest value will be selected. The routing protocols can be mainly classified into three types. Each of these has its own importance in data transfer. Let us now discuss in detail each of these protocols in detail. This mainly uses distance as the metric value and direction as a vector to select the optimal path to the destination network.

Basically, the router receives the routing information from neighboring routers, which receives this information from their neighboring router until the destination network. Each neighboring router in the path of a destination network is called a hop. Every time a data packet goes through a router, the hop value increases, and the route with the least hop value will be chosen. The link-state routing protocol uses a complex metric table to choose the best path for the destination network.

As the name itself indicates that it works in linked format. It uses three tables. Hybrid routing protocols can be defined as the mix of distance vector and link-state protocol. A hybrid routing protocol uses aspects from both distance vector and link-state protocol to locate a more accurate path. Using various parameters such as the Autonomous system and metric locates a better path, and all types of protocols have their own pros and cons.

They take different approaches to share routing updates and locating the best path. This has been a guide to Routing Protocol. Here we discussed the introduction, understanding, working, mechanism, and types of a routing protocol. You can also go through our other suggested articles to learn more —. If there are multiple protocols with registered backup routes, the preferred route is chosen based on administrative distance. The default administrative distance might not always be right for your network; you may want to adjust them so that RIP routes are preferred over IGRP routes, for instance.

Before explaining how to adjust the administrative distances, we need to look at the implications of changing the administrative distance. Changing the administrative distance on routing protocols can be very dangerous! Changing the default distances can actually lead to routing loops and other oddities in your network. We recommend you change administrative distance with caution, and only after you have thought through what you want to achieve, and all the consequences of your actions.

For entire protocols, changing the distance is relatively easy; simply configure the distance using the distance command in the routing process sub-configuration mode.

You can also change the distance for routes learned from one source only in some protocols, and you can change the distance on just some routes. For static routes, you can change the distance of each route by entering a distance after the ip route command:. Routes are chosen and built in the routing table based on the routing protocol's administrative distance. The routes learned from the routing protocol with the lowest administrative distance are installed in the routing table.

If there are multiple paths to the same destination from a single routing protocol, then the multiple paths would have the same administrative distance and the best path is selected based on the metrics. Metrics are values associated with specific routes, ranking them from most preferred to least preferred.

The parameters used to determine the metrics differ for different routing protocols. The path with the lowest metric is selected as the optimal path and installed in the routing table. If there are multiple paths to the same destination with equal metrics, load balancing is done on these equal cost paths.

Let's look at another scenario to see how the router handles another common situation: varying prefix lengths. Assume, again, that a router has four routing processes running on it, and each process has received these routes:.

Which of these routes will be installed in the routing table? Since EIGRP internal routes have the best administrative distance, it's tempting to assume the first one will be installed.

However, since each of these routes has a different prefix length subnet mask , they're considered different destinations, and they will all be installed in the routing table.

Let's see how the forwarding engine uses the information from the routing table to make forwarding decisions. Let's look at the three routes we just installed in the routing table, and see how they look on the router. If a packet arrives on a router interface destined for It depends on the prefix length, or the number of bits set in the subnet mask.