Today's video lesson on Distance Vector and Link State routing protocols precedes one of the most important topics of the CCNA course – OSPF and EIGRP routing protocols. This topic will span the next 4 to 6 video lessons. Therefore, today I will briefly cover several concepts that are essential to know before diving into OSPF and EIGRP.
In the last lesson, we covered section 2.1 of the ICND2 material, and today we will study sections 2.2 'Similarities and Differences between Distance Vector (DV) Routing Protocols and Link State (LS) Protocols' and 2.3 'Similarities and Differences between Internal and External Routing Protocols.'
As I mentioned, in the next 4 to 6 videos, we will discuss 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 a routing protocol is and how it differs from a routed/routable protocol.
A routing protocol is used by a router, such as RIP, EIGRP, OSPF, BGP, and others. The routing protocol is a means of communication between routers, allowing them to exchange information about the network and populate their routing tables. Based on these tables, they make routing decisions.
After routers 'communicate' with each other and fill their routing tables, having done all this through the routing protocol, they make decisions about sending traffic to other networks. This involves a routed protocol that allows routers to forward or route traffic. These protocols include IPv4 and IPv6.
Thus, the routing protocol ensures that routing tables are populated with information, while the routed protocol facilitates traffic routing based on that information. Through IPv4 or IPv6, transmitted data is encapsulated and provided with IP headers, which is reflected in the names of these protocols – IP.
The next question concerns the differences between Interior Gateway Protocols and Exterior Gateway Protocols. Don't be misled by the term 'gateway.' Typically, routers are used within an autonomous system. Let's assume your company has 50 routers utilizing any IP protocol. All of them form an autonomous system, meaning they are used and managed by a single company or organization.
So, the protocols used to facilitate routing within such an autonomous system are called interior gateway protocols, while protocols that perform routing outside the system are known as exterior gateway protocols. An exterior gateway protocol enables routing between different autonomous systems. One such system could be your ISP, which may consist of 200 routers. Autonomous systems use exterior gateway protocols to communicate with each other.
Interior gateway protocols include RIP, OSPF, and EIGRP, while currently, there is one exterior gateway protocol in use: BGP.
The next two definitions you should understand are Distance Vector and Link State. These are two types of interior gateway routing protocols.
Suppose we have 3 routers connected to each other and to the network 192.168.10.0/24. Let's label them A, B, and C. From the ICND1 course, we know what happens when using RIP.
Since router B is closest to the network 192.168.10.0/24, it is the first to send an announcement about this network to router A and router C. Router C also forwards this announcement to router A. Router A receives information about the network 192.168.10.0/24 through two of its interfaces – f0/0 and f0/1. Because the RIPv2 protocol uses a Hop Count metric, it will tell the router that the optimal route to reach this network is via router B, as the network can be reached in one hop. If interface f0/1 is used to connect to the network 192.168.10.0/24, it will require 2 hops. Thus, from router A's perspective, the optimal choice will be to use interface f0/0. Router A makes this decision because it uses RIP, which is a distance-vector protocol.
According to the diagram, we see that this is the correct solution because the distance between A and B is the shortest. But what if I say there is a line with a bandwidth of 64 kbps between A and B, and a line with 100 Mbps is located between C and B, and the same line is between C and A?
What route would be the most optimal under these conditions?
Of course, a 100 Mbps line is much better than a 64 kbps line, even if the route through it takes 2 hops instead of one. However, the Distance Vector Routing Protocol RIP does not take into account the transmission speed of the traffic, as it is guided by the minimum number of hops when choosing the optimal route. In this case, it is better to use a Link State protocol, such as OSPF. This protocol checks the cost of routes, and by finding the least costly one, directs traffic along the path Router A – Router C – Router B.
Compared to RIP, the OSPF protocol is much more complex; it considers many factors in determining the optimal route and finds the shortest path in terms of metrics.
EIGRP was once a proprietary routing protocol from Cisco and is now an open standard. It combines the best features of a distance vector protocol and a link state protocol. It considers both bandwidth and network delays. As is known, the longer the route, meaning the more hops, the longer the delays. Therefore, the EIGRP protocol selects a route with the maximum bandwidth and minimum total delay by comparing route metrics. The displayed bandwidth and delays are parts of the formula used to make routing decisions.
This is the difference between Distance Vector and Link State protocols. Distance Vector protocols only consider the distance of the route, while Link State protocols examine the state of the network along the route, such as speed and bandwidth.
EIGRP is a hybrid routing protocol as it combines aspects of both of the aforementioned protocols. From Cisco's perspective, it is the best routing protocol, which is why all of the company's engineers prefer to use it; however, the most widespread protocol in the world is OSPF. The reason is that EIGRP has only recently become an open standard, so third-party manufacturers are uncertain about its compatibility with their networking equipment.
Let’s examine what the trust level in a protocol means. When Router A receives routing information from two different sources, it uses a formula to decide which of the two routes to place in the routing table. This is straightforward because it looks at the parameters of routes B-A and A-C-B, compares them, and makes the optimal decision. Of course, OSPF also balances the load, meaning if two routes have the same cost, it performs load balancing. We will explore this issue in more detail in the following videos; today, I simply want you to be aware of it.
Let’s take a 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, while the second uses OSPF.
Of course, you can reconfigure your network to use OSPF, or migrate the network of the acquired company to the EIGRP protocol, but that's a lot of administrative work. For a small company, this might be manageable, but for a large company, it involves a huge amount of effort. In this case, you could implement route redistribution, meaning take EIGRP routes and distribute them into OSPF, while EIGRP also receives redistributed OSPF routes. This is quite feasible. For this, one of your company's routers must operate on both protocols – EIGRP and OSPF. Let's assume this is Router B. It will have a routing table where some routes are sourced from EIGRP and others from OSPF. Suppose we have another network that connects both companies. The first company will use EIGRP table routes to connect with it, while the second will use routes from the OSPF protocol, and matching these routes from different sources will be very challenging, as each selects the optimal route based on its own metrics.
In this case, the concept of Administrative Distance is used. It helps the router choose the most optimal route from multiple routes obtained from different routing protocols. For instance, if Router B is directly connected to Router C, the administrative distance will be 0, making this the most trusted route. Let's say Router A informs Router B that it also has access to Router C; in this situation, Router B will respond: "thank you for your information, but Router C is directly connected to me, so I choose the option with the lower administrative distance rather than the one through you."
Administrative distance indicates the level of trust in a 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 level of trust for the EIGRP protocol is characterized by an administrative distance value of 90, for OSPF it is 110, and for RIP it is 120.
Therefore, if both EIGRP and OSPF are representing the same network, the router will trust the routing information received from EIGRP because this protocol has an administrative distance of 90, which is lower than that of OSPF.
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Source: habr.com
