I already said that I will update my video tutorials to CCNA v3. Everything that you have learned in previous lessons is fully consistent with the new course. If the need arises, I will include additional topics in new lessons, so you don't have to worry about our lessons being in line with CCNA 200-125.
First, we will fully cover the topics of the first exam 100-105 ICND1. We still have a few lessons left, after which you will be ready to take this exam. Then we will start studying the ICND2 course. I guarantee that by the end of this video course, you will be fully prepared to take the 200-125 exam. In the last lesson, I said that we will not return to the RIP protocol anymore, because it is not part of the CCNA course. But since RIP was included in the third version of CCNA, we will continue to study it.
The topics of today's lesson will be three problems that arise in the process of using RIP: Counting to Infinity, or counting to infinity, Split Horizon - the rules of split horizons and Route Poison, or route poisoning.
To understand the essence of the problem of counting to infinity, let's turn to the scheme. Suppose we have a router R1, a router R2 and a router R3. The first router is connected to the second network 192.168.2.0/24, the second router is connected to the third network 192.168.3.0/24, the network 192.168.1.0/24 is connected to the first router, and the network 192.168.4.0/24 is connected to the third router.
Let's look at the route to the 192.168.1.0/24 network from the first router. In his table, this route will be displayed as 192.168.1.0 with a hop count of 0.
For the second router, the same route will be displayed in the table as 192.168.1.0 with the number of hops equal to 1. In this case, the routing table of routers is updated by the Update timer every 30 seconds. R1 tells R2 that the network 192.168.1.0 is reachable through it with 0 hops. Upon receiving this message, R2 will respond with an update that the same network is reachable through it in one hop. This is how regular RIP routing works.
Imagine a situation where the connection between R1 and the network 192.168.1.0/24 was broken, after which the router lost access to it. At the same time, router R2 sends an update to router R1, in which it reports that the network 192.168.1.0/24 is available to it in one hop. R1 knows that he has lost access to this network, but R2 claims that this network is available through him in one hop, so the first router thinks it must update its routing table by changing the number of hops from 0 to 2.
R1 then sends an update to R2. He says: βok, before that you sent me an update that the network 192.168.1.0 is available with zero hops, now you report that the route to this network can be built in 2 hops. So I have to update my routing table from 1 to 3." On the next update, R1 will change the number of hops to 4, the second router to 5, then to 5 and 6, and this process will continue indefinitely.
This problem is known as the "routing loop" and is referred to in the RIP protocol as the "counting to infinity problem". The network 192.168.1.0/24 is not actually reachable, however R1, R2 and all the other routers on the network believe that it can be reached because the route is constantly looping. This problem can be solved using the mechanisms of split horizons and route poisoning. Consider the network topology with which we will work today.
The network has three routers R1,2,3 and two computers with IP addresses 192.168.1.10 and 192.168.4.10. There are 4 networks between computers: 1.0, 2.0, 3.0 and 4.0. Routers have IP addresses, where the last octet is the router number and the penultimate octet is the network number. You can assign any addresses to these network devices, but I prefer these because it's easier for me to explain.
To set up our network, let's move on to Packet Tracer. I am using Cisco Model 2911 routers and use this scheme to assign IP addresses to both PC0 and PC1 hosts.
You can ignore the switches, because they are βstraight out of the boxβ and use VLAN1 by default. Routers 2911 have two gigabit ports. To make it easier for us, I use ready-made configuration files for each of these routers. You can visit our website, go to the Resources tab and view all of our video tutorials.
It doesn't have all the updates at this time, but for an example, you can take a look at the Day 13 lesson, which has a Workbook link, or "Workbook". The same link will be attached to today's video tutorial, and by clicking on it, you can download the router configuration files.
In order to configure our routers, I simply copy the contents of the R1 configuration text file, open its console in Packet Tracer and enter the config t command.
Then I just paste the copied text and exit the settings.
I do the same with the settings of the second and third routers. This is one of the advantages of Cisco settings - you can simply copy and paste the settings you need into network device configuration files. In my case, I will also add 2 commands to the beginning of the finished configuration files so as not to enter them in the console - these are en (enable) and config t. Then I'll copy the content and paste it in its entirety into the R3 settings console.
So, we configured all 3 routers. If you want to use ready-made configuration files for your routers, make sure that the models correspond to those shown in this diagram - here the routers have GigabitEthernet ports. You may need to change this line in the file to FastEthernet if your router has these ports.
You can see that the router port markers in the diagram are still red. What is the problem? For diagnostics, go to the IOS command line interface of router 1 and type the show ip interface brief command. This command is your "Swiss knife" for solving various network problems.
Yes, we have a problem - you can see that the GigabitEthernet 0/0 interface is in an administratively down state. The fact is that in the copied configuration file, I forgot to use the no shutdown command and now I will enter it manually.
Now I will have to manually add this line to the settings of all routers, after which the port markers will change color to green. Now I will display all three CLI windows of the routers on a common screen, so that it is more convenient to observe my actions.
At the moment, the RIP protocol is configured on all 3 devices, and I will debug it, for which I use the debug ip rip command, after which all devices will exchange RIP updates. After that, I use the undebug all command for all 3 routers.
You can see that R3 has a problem finding the DNS server. In the following, we will discuss the topic of CCNA v3 related to DNS servers, and I will show you how to disable the lookup feature of this server. For now, let's return to the topic of the lesson and look at how the RIP update works.
After we turn on the routers, their routing tables will have entries about networks that are directly connected to their ports. In the tables, these entries are headed with the letter C, and the number of hops in a direct connection is 0.
When R1 sends an update to R2, it contains information about networks 192.168.1.0 and 192.168.2.0. Because R2 already knows about the 192.168.2.0 network, it only puts an update about the 192.168.1.0 network into its routing table.
This entry is titled with the letter R, which means that connection to the network 192.168.1.0 is possible through the f0/0: 192.168.2.2 router interface only via the RIP protocol with the number of hops 1.
Similarly, when R2 sends an update to R3, the third router places an entry in its routing table that network 192.168.1.0 is reachable through router interface 192.168.3.3 via RIP with hop count 2. This is how the routing update works.
To prevent routing loops, or counting to infinity, RIP has a "split-horizon" mechanism. This mechanism is a rule: "do not send a network or route update through the interface on which you received this update." In our case, it looks like this: if R2 received an update about the network 1 from R192.168.1.0 via the f0/0: 192.168.2.2 interface, it should not send an update about this network 0 to the first router via the f0/2.0 interface. It can send through this interface associated with the first router only updates that concern networks 192.168.3.0 and 192.168.4.0. It also should not send an update about the 192.168.2.0 network through the f0 / 0 interface, because this interface already knows about it, because this network is connected to it directly. So, when the second router sends an update to the first router, it should only contain entries about networks 3.0 and 4.0, because it learned about these networks from another interface - f0 / 1.
Here's the simple split-horizon rule: never send any route information back in the same direction it came from. This rule prevents a routing loop or counting to infinity.
If you look at Packet Tracer, you can see that R1 received an update from 192.168.2.2 through the GigabitEthernet0/1 interface on only two networks: 3.0 and 4.0. The second router did not report anything about networks 1.0 and 2.0, because it learned about these networks through this very interface.
The first router R1 sends an update to the multicast IP address 224.0.0.9 - it does not send a broadcast message. This address is something like a specific frequency on which FM radio stations broadcast, that is, only those devices that are tuned to this multicast address will receive the message. In the same way, routers configure themselves to accept traffic for the address 224.0.0.9. So, R1 sends an update to this address via the GigabitEthernet0/0 interface with the IP address 192.168.1.1. This interface should only send updates about networks 2.0, 3.0, and 4.0 because network 1.0 is connected directly to it. We see him doing just that.
It then sends the update through the second f0/1 interface with the address 192.168.2.1. Ignore the F for FastEthernet - this is just an example, as our routers have GigabitEthernet interfaces that should be g. It cannot send an update about networks 2.0, 3.0 and 4.0 through this interface, because it learned about them through the f0 / 1 interface, therefore it sends an update only about network 1.0.
Let's see what happens if the connection to the first network is lost for some reason. In this case, R1 immediately invokes a mechanism called "route poisoning". It lies in the fact that as soon as the connection to the network is lost, the number of hops in the entry about this network in the routing table immediately increases to 16. As we know, the number of hops equal to 16 means that this network is unavailable.
In this case, the Update timer is not used, it is a triggered update that is instantly sent over the network to the nearest router. I will mark it in blue on the diagram. Router R2 receives an update, which says that from now on, network 192.168.1.0 is available with a hop count of 16, that is, it is not available. This is what is called route poisoning. As soon as R2 receives this update, it immediately changes the value of hops in the entry line 192.168.1.0 to 16 and sends this update to the third router. In turn, R3 also changes the number of hops for the unreachable network to 16. Thus, all devices connected via the RIP protocol will know that network 192.168.1.0 is no longer available.
This process is called convergence. This means that all routers update their routing tables to the current state, excluding the route to network 192.168.1.0 from them.
So, we have covered all the topics of today's lesson. Now I will show you the commands that are used to diagnose and troubleshoot the network. In addition to the show ip interface brief command, there is a show ip protocols command. It shows the routing protocol settings and status for devices that use dynamic routing.
After using this command, information about the protocols that are used by this router appears. It says that the routing protocol is RIP, updates are sent every 30 seconds, the next update will be sent in 8 seconds, the Invalid timer starts after 180 seconds, the Hold Down timer after 180 seconds, the Flush timer after 240 seconds. These values ββcan be changed, but our CCNA course does not cover these issues, so we will use the default timer values. Similarly, our course does not cover outgoing and incoming filter list updates for all router interfaces.
Next here is Protocol Remapping - RIP, this option applies when the device uses multiple protocols, for example, it shows how RIP interacts with OSPF and how OSPF interacts with RIP. Redistribution is also outside the scope of your CCNA course.
The following shows that the protocol uses the auto-summarization of routes, which we talked about in the previous video, and that the administrative distance is 120, we also discussed this.
Let's take a look at the show ip route command in detail. You can see that the networks 192.168.1.0/24 and 192.168.2.0/24 are directly connected to the router, two more networks, 3.0 and 4.0, use the RIP routing protocol. Both of these networks are accessible through the GigabitEthernet0/1 interface and a device with an IP address of 192.168.2.2. The information in square brackets is important - the first number is the administrative distance, or administrative distance, the second is the number of hops. The number of hops is a metric of the RIP protocol. Other protocols, such as OSPF, have their own metrics, which we'll talk about in a related topic.
As we have already discussed, administrative distance means the degree of trust. A static route with an administrative distance of 1 has the highest degree of trust. Therefore, the smaller this value, the better.
Let's assume that the network 192.168.3.0/24 is accessible both through the g0/1 interface, which uses RIP, and through the g0/0 interface, which uses static routing. In this case, the router will direct all traffic along the static route through f0 / 0, because this route is more trustworthy. In this sense, the RIP protocol with an administrative distance of 120 is worse than the static routing protocol with a distance of 1.
Another important command for troubleshooting is the show ip interface g0/1 command. It displays all the information about the parameters and status of a particular router port on the screen.
For us, the line that says that split horizon is enabled is important, because you may have problems due to the fact that this mode is disabled. Therefore, if you experience problems, you should make sure that split-horizon mode is enabled for this interface. Note that this mode is enabled by default.
I believe that we have covered enough questions related to the RIP protocol that you should not have any difficulties with this topic when taking the exam.
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