Hello, and welcome back once again to my video series. In the previous videos on IP routing, we configured static and default routing. In this video, we begin looking at dynamic routing by considering concepts such as administrative distance, route metric, and types of routing protocols.

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Further reading:

• CCNA Cheat-Sheet Study Guide: IP Routing: http://resources.intenseschool.com/ccna-cheat-sheet-study-guide-ip-routing/

• Cisco Networking Academy’s Introduction to Routing Dynamically:http://www.ciscopress.com/articles/article.asp?p=2180210

Video Transcript:

Welcome back to the video series where we cover the practical aspects of the CCNA routing and switching exam.

In the last video, we discussed static and default routing. In this video, we will begin our discussion on dynamic routing by looking at concepts such as Administrative distance, metric and types of routing protocols.

Static routing cannot automatically adjust to changes in the network. FOr example, if a part of the network goes down and there is still a static route for that network in the routing table of a router, That router will still forward traffic to that destination only to be dropped later.

This is one of the advantages of using dynamic routing protocols in your network. Keep in mind that not all networks require the use of routing protocols and sometimes, static or default routing is enough.

Before we begin configuring dynamic routing protocols, let us discuss some concepts. First on the list is Administrative distance. An analogy will help here: even though you receive news from different outlets, there are some you trust more and you tend to believe news from these ones more than others

In the same way, when a router receives information about the same destination from multiple sources, it ranks some sources higher than others. The Administrative distance can range from between 0 and 255 with 0 being the most trusted and 255 being the least trusted.

A couple of common default ADs include: Directly connected routes with an AD of 0; static routes have an AD of 1; EIGRP has an AD of 90, OSPF has an AD of 110. This means that if there is the same route, say, from EIGRP and OSPF, then the one from EIGRP will be chosen and installed in the routing table because it has a better AD.

Keep in mind that Administrative distance is not the same as route metric. AD relates to trustworthiness of the same route received from different routing protocols or sources. Metric on the other hand has meaning inside a particular routing protocol.

For example, if two different routers in an EIGRP AS advertise the same route to another EIGRP router and one of those routes has a better metric than the other, then the one with the better metric will be preferred for forwarding traffic.

Generally speaking, routing protocols can be of two types: Interior routing protocols (IGP) and Exterior routing protocols (EGP). EIGRP and OSPF are IGPs while BGP is an EGP. For the CCNA exam, you should only be concerned with IGPs.

Routing protocols can also be classified by the way they share routing information such as distance vector, link-state or path vector. OSPF is an example of a link-state routing protocol. EIGRP is more like a hybrid protocol, both distance vector and link-state although Cisco classifies it as an advanced distance vector.

In distance vector routing protocols, routes are advertised based on both distance and direction. Distance means how far the destination route is. This distance is based on metrics such as hop count, bandwidth and so on. RIP for example bases its metric on hop count. Direction specifies what interface or next-hop router through which the destination route can be reached.

One of the problems with distance vector protocols like RIP, which use hopcount as a metric is that they may not always choose the best path to reach a network. In the diagram, even though the path R1-R2-R3 has much better bandwidth, RIP will still choose the direct path R1-R3 because it has a shorter hop count

On the other hand, routers using link-state routing protocols maintain a complete map of the entire network and can calculate the best path to reach every destination on the network.

In this video, we have discussed concepts such as administrative distance, route metric and types of routing protocols. You can read more about what we have covered in the further reading list. In the next video, we will start discussing EIGRP. I hope you have found this video insightful.