Today's video tutorial on the Distance Vector and Link State routing protocols prefaces one of the most important topics of the CCNA course - OSPF and EIGRP routing protocols. This topic will take 4 or even 6 next video tutorials. Therefore, today I will briefly talk about a few concepts that you need to know before you start learning about OSPF and EIGRP.
In the last lesson, we reviewed section 2.1 of the ICND2 topic, and today we will study sections 2.2 “Similarities and differences between distance vector protocols Distance Vector (DV) and Link State (LS) communication channel protocols” and 2.3 “Similarities and differences between internal and external routing protocols ".
As I said, in the next 4 or 6 videos we will cover the key topics of the entire course - OSPFv2 for IPv4, OSPFv3 for IPv6, EIGRP for IPv4 and EIGRP for IPv6. Students often ask me what the Routing protocol is and how it differs from the Routed/Routable protocol.
The routing protocol used by the router, such as RIP, EIGRP, OSPF, BGP, and others. A routing protocol is a way for routers to communicate with each other in which they exchange information about a network and populate their routing tables with that information. Based on these tables, they make routing decisions.
After the routers have "talked" to each other and filled out the routing tables, having done all this with the help of a routing protocol, they make decisions about sending traffic to other networks. It uses a routable protocol that allows routers to forward or route traffic. These protocols include IPv4 and IPv6.
So, the routing protocol ensures that the routing tables are filled with information, and the routable protocol ensures that traffic is routed in accordance with the information in these tables. Thanks to IPv4 or IPv6, the transmitted data is encapsulated and supplied with IP headers, as the names of these protocols themselves, IP, indicate.
The next question is about the differences between the Interior Gateway Protocol and the Exterior Gateway Protocol. Don't let the word "gateway" fool you. Typically, routers are used in an autonomous system. Suppose you have 50 routers in your company using any IP protocol you like. All of them form an autonomous system, that is, they are used and managed by one company, one organization.
So, the protocols that are used to provide routing within such an autonomous system are called internal gateway protocols, and the protocols for routing outside the system are called external gateway protocols. The External Gateway Protocol provides routing between different Autonomous Systems. One such system could be your ISP, and their system could be up to 200 routers. Autonomous systems use the external gateway protocol to communicate with each other.
Internal gateway protocols are RIP, OSPF, EIGRP, and one protocol is currently used as an external gateway protocol - BGP.
The next two definitions you need to understand are Distance Vector and Link State. These are two types of internal gateway routing protocol.
Suppose we have 3 routers that are connected to each other and to the 192.168.10.0/24 network. Let's call them A, B, and C. From the ICND1 course, we know what happens when you use RIP.
Because Router B is closest to the 192.168.10.0/24 network, Router B sends the advertisement about this network first to Router A and Router C. Router C also forwards this advertisement to Router A. Router A receives information about the network 192.168.10.0. interface - f24/0 and f0/0. Since the RIPv1 protocol uses the Hop Count metric, it will tell the router that the optimal route to reach this network is through Router B, because then the network can be reached in one hop. If you use the f2/192.168.10.0 interface to communicate with the 24/0 network, then 1 hops will be required. Thus, from the point of view of router A, it will be optimal to use the f2 / 0 interface. A makes this decision because it uses RIP, which is a distance vector protocol.
According to the diagram shown, we see that this is the correct solution, because the distance between A and B is the shortest. But what happens if I say that there is a 64 kbps line between A and B, and a 100 Mbps line between C and B, and the same line is between C and A?
What route under such conditions will be the most optimal?
Of course, a 100 megabits per second line is much better than a 64 kilobits per second line, even if the route through it takes 2 hops instead of one. However, the distance vector protocol RIP does not take into account the speed of traffic transmission, since the choice of the optimal route is guided by the minimum number of hops. In this case, it is better to use a Link State protocol such as OSPF. This protocol checks the cost of routes, and finding the “cheapest” one, sends traffic along the path Router A - Router C - Router B.
Compared to RIP, OSPF is much more complex, taking many factors into account when determining the best route, and finding the shortest path in terms of metrics.
EIGRP was once a Cisco proprietary routing protocol and is now an open standard. It is a combination of the best features of the distance vector protocol and the network state protocol. It takes into account both bandwidth and network delays. As you know, the longer the route, that is, the more hops, the longer the delay. Therefore, the EIGRP protocol chooses the route with the maximum throughput and minimum total delay by comparing the route metrics. The shown throughput and latency are part of the formula based on which the routing decision is made.
This is the difference between the Distance Vector and Link State protocols. Distance vector protocols only consider the distance of a route, while Link State protocols consider the state of the network along the route's path, such as speed and throughput.
EIGRP is a hybrid routing protocol as it combines the features of both of the above protocols. From Cisco's point of view, this is the best routing protocol, so it is preferred by all engineers of the company, but the most common protocol in the world is OSPF. The reason is that EIGRP has only recently become an open standard, so third-party vendors are unsure of its compatibility with their network equipment.
Consider what is the degree of trust in the protocol. When router A receives routing information from 2 different sources, it uses a formula to decide which of the two routes to put in the routing table. It's easy because he looks at route parameters B-A and A-C-B, compares them and makes the best decision. Of course, OSPF also load balances, that is, if two routes have the same cost, then it performs load balancing. We will consider this issue in detail in the following videos, but today I just want you to just know about it.
Let's look at the following table. Below I will again draw routers A, B and C, which form an autonomous network system in your company. Suppose your company has acquired another company that has a system with routers A1, B1, and C1. So, you now have two companies, each with its own network. Let's say the first uses the EIGRP protocol, and the second uses OSPF.
Of course, you can reconfigure your network to use OSPF, or switch your acquired company's network to EIGRP, but that's a whole bunch of administrative work. For a small company, this can still be done, but if the company is large, then this is a huge amount of work. In this case, you can redistribute, that is, take the EIGRP routes and distribute them over OSPF, and redistribute the OSPF routes over EIGRP. It is quite possible. To do this, one of your company's routers must work on two protocols - EIGRP and OSPF, suppose it will be router B. It will contain a routing table, where some of the routes are obtained from EIGRP, and some from OSPF. Let's say we have another network that both companies are connected to. In this case, the first company will use the routes of the EIGRP table to communicate with it, and the second will use the routes from the OSPF protocol, and it will be very difficult to compare these routes received from different sources, because each of them chooses the best route according to its own metrics.
In this case, the concept of Administrative Distance, or administrative distance, is used. It helps the router to choose the most optimal route from several routes obtained from different routing protocols. For example, if router B is directly connected to router C, then the administrative distance will be 0, which is the most trusted route. Suppose A informs B that he also has access to C, in which case router B will answer him: “thanks for your information, but router C is connected to me directly, so I choose the option with a smaller administrative distance, and not the option to communicate through you".
The administrative distance indicates the degree of confidence in the protocol. The smaller the administrative distance, the greater the trust. The next most trusted option after a direct connection is a static connection with an administrative distance of 1. The trust level for EIGRP is 90, OSPF 110, and RIP 120.
Therefore, if EIGRP and OSPF both represent the same network, the router will trust the routing information received from EIGRP, because this protocol has an administrative distance of 90, which is less than that of OSPF.
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