4. Dense Mode Configuration
Now in this video we’ll see the configuration of IP pin dense mode. Now, before we go ahead, let’s try to quickly revise some of the dense mode concepts. Now, dense mode uses something called flood and prune behavior where all the routers initially flood out the multicast traffic or the multiclass packets out of all the multicast interfaces. Now, if the downstream router does not have any user who needs a multicast traffic, it’s going to send something called prune message. Like in this example here. If it is example, the server is going to send out the multicast traffic out of all the interfaces and if this particular router do not have any receiver, it’s going to simply send a special message called prune that it doesn’t want to receive the multicast traffic.
Now, based on that it is going to stop the multicast traffic on this particular interface. Now, if you have a receiver, probably it’s going to flood out of it’s going to receive the multicast traffic on that particular interface. Now, the upstream router will exclude the interface from the output list. Normally once you receive the prune message, what this router will do is this router is going to remove this particular interface from the outgoing interface so that it’s not going to send the multicast traffic on the interface connecting between these two routers. Now, Pim is going to work fine in the small networks if you have a very few users, but it has some scalability issues. Like every prune interface is going to expire in every three minutes.
Like generally when it is sending a prune message it’s going to expire for three minutes by default and the flooding on this interface will will resume again. And if you want to stop again, it has to send the prune message again and again. Now, this is something periodically flooding and pruning behavior makes the dense mode not really scalable. Probably in this section we’ll practically verify how it’s going to work. Now, the consideration is going to be straightforward on all the routers. Now, this is the topology which I’m going to use for verifying on the router 1234 we are going to enable IP multicast routing. That’s something what we are going to do first. And then we need to go to each and every interface which is connected like on the router one.
I want to ensure that it should send multicast traffic on these two interfaces as well as I have some users in the land who are receiving the multicast traffic. So I’m going to enable on all the land and the van interfaces. Now to enable we just need to add this command IPP dense mode and the same thing on s one by zero as well as s one by one. Now, once we configure this command, you will see the neighbor ship between the router one and router two. Like we can verify with show IP Pim neighbors, they will use some special multicast address to listen their hello messages. It’s going to send the hello message on 2240 00:13 and there is a periodical hello messages for every 30 seconds and the default default timer, a dead timer will be 90 seconds.
And we can verify this Pim neighbors by using Shyp peam neighbors command. So that’s something what we are going to verify here. So let’s get into the command line to verify the same. So I got my routers pre configured according to the Topology here. Now I can see the Topology here, I got my routers router file 12345. Now on all these routers I have configured igp already and I’m using ospf protocol as an igp protocol. Now, igp is mandatory to configure because it’s not going to use igp to advertise the multicast routing information, but it is going to do the loop prevention mechanism here in multicast. That is a reverse path forwarding check. I will be discussing that in the next videos. You need to have an igp table in order to figure out the source as well.
So the first thing, I already have the igp configured here. If you go and verify on the router two, I got show Iposphere neighbor, router Two is forming the neighborship with router One and router Three. And similarly if I go and check on the router four, if I give Show IP osp of neighbors on the router four, it is forming the neighborship with router One, router Three and router Five. The next thing, if you verify the routing table, I have adudized all the connected interfaces in my diagram. So the next thing I’m going to configure the tense mode on this routers. So the first thing I’m going to say IP multicast routing and then we need to go to the interfaces and I’m going to enable Ipm tense Mode. We can also use pass mode.
Right now we’ll go with tense mode and then interface s one by zero Ipm dense mode. So I got some debug commands enabled already. So interface S one by let me check the commands. What I configure on the F zero by zero, s one by zero. I need to enable on S one by one as well. IP PM ten. Okay, done. So let me copy paste the same commands on the remaining routers. So IP multicast routing on the router Two. On the router two. Also I’m using IP multicast routing and then interface s zero by 00:10 mode. And what is other interface s one by one? ipvm tense mode. Now, once you configure on both these routers, you can see Show IP Pim neighbors. You can see the router Two and router one form the neighbor relationship.
And there is one more command we can use show IP pim interfaces. It’s going to list out what are the interfaces enabled with Pim. The version two Pim and enable with which mode dance mode here. And the default query interval is 30 seconds by default. Now similarly, let’s go to the remaining routers and quickly finish the configuration here I got the same interfaces on all the routers so it’s going to make the things easy for us to copy paste the configurations. Now on the router four also I got the same interfaces, I got two van interfaces and one LAN interface. Now on the router four I got one more interface s one by two here you can see so I need to enable which is connecting to the router file and then finally on the router file on the router file we just got only few interfaces.
On the router file we got s one by zero there is no s one by one so we don’t need on this on the router five there is only s one by zero. Now the first thing we’ll try to verify the pim neighbor shape. So I’ll go to router four and I’ll say show IP pim neighbors. Now on the router four I should see three pim neighbors. Now one is router one, router three and router five. So I can see there are three pim neighbors on this interfaces and you can see the uptime here. Now there aren’t pimperation two by default and the mode we are using s mode here. Now the next thing we’ll come to the Dr priority value as well on the broadcast networks we’ll talk about that later on. So when I say show Ipm interfaces, show Ipm interface is a very useful command to confirm whether you have enabled Ipm densport or not.
Now on the router four I got four interfaces, one LAN and three van interfaces and you can see all the interfaces are listed here. Now the next thing now this is something we are going to configure on all the routers here. Now the next thing what we are going to do is we are going to simulate a client on the router file which is supposed to get a multicast traffic. Now the real scenario is we have a computer which is running a multicast application which will automatically send out a query message. Now that is what igmp report. Message to the router informing that I want to receive a multicast traffic. And then the router is going to figure out where is the source exactly for that multicast group. Probably in our routers, in this lab, even in your cci lab exams.
Also probably we are not running a real multicast application. Instead we are going to this interface at zero by zero and we’ll configure a command called IP. igmp join group and you can use any of the multicast rotor theorem. Now what this command will do is this command is going to simulate a host which wants to join the multicast 224 five and wants to see the multicast traffic from this particular group address. Now iOS provide this command is going to designate a group membership on that particular interface and this command makes the router to behave just like a member of a specific group. But here we don’t have a real host which are connected to who wants to dismal traffic.
It’s just a simulation of host who wants to receive the multicast traffic from that particular group address. Now, once we configure this command, it’s going to simulate the host or the clients. And then we can simulate the server from any one of these routers. We can take any one of these router. Let’s say I have a server here and here we don’t have a real server again here. And we try to ping from the router one to this multicache group at just 224 five. And if you’re able to get a reply, then it’s going to confirm that from the router one you are able to reach, provide them, send out the multicast traffic to this particular host. It’s going to work with something called flood in prune.
It’s going to flood out of all the interfaces and then these routers will automatically prune because they don’t have any receiver who wants to receive the multicast traffic. Now that’s how we are going to simulate here. And then we’ll be using some commands like Show IP, embroidered command to verify the entries. So let’s go to the command line on the router file where I’m simulating the host or the clients. And it can be any interface. I’m going to use a zero by zero interface. Just ensure that that interface is enabled with dense mode or sparse mode. Right now we are using dense mode and I’m going to say IP. igmp JoinGroup command 224 five. That’s a group address. Now, once we give this command, if I verify show Ipm route.
Now, in my routing table, you can see this entry here added here. But right now we don’t have any specific entries. Like you can see incoming interface is null and I’m not receiving any multicast traffic for this particular group. An outgoing interface defines oil, that is outgoing interface list, which defines what are the outgoing interfaces from where this multicast has to be sent. Now, what we are going to do is we are going to simulate a server on the router and we are going to generate a ping request to this multicast proved 224 five. And then we’ll verify the table again. Now, from the router one, if I try to ping 224 five, I should get a reply here. So you can see the reply is coming. If you want to repeat something like five times, six times, you can just repeat the multip here.
You can see the request reply is coming here. The reply is coming from firefighter five two. And this is your firefighter five two host. That is router file here. If you further go and verify the same on the router file, on the router filipe, you show Ipm root. Now, here you will see an entry will be added here. Four two. It’s saying that the router of I users if they want to receive any multicast traffic for this group, this is what we call as F kama G entries. And this is your group address. And it’s coming from source. That is router one. And the router one IP address is four four two because this is the shortest path here. In fact, router one is having multiple addresses over there, but it’s using four two because this is the shortest path to reach that particular server.
Now if you have any clients who want to receive the multicast traffic for this particular group it will be sent to four two because we are simulating a server over there but we don’t have a real server over there. Now, similarly, if you have a real server, it’s going to send out a multicast traffic on this router and then it is going to flood out. So, like that, every router will come to know above that particular source. Now, here, in order to do that flooding, we are generating a traffic from the router one to that particular multicast address. Now, similarly, if you go and check on the remaining routers, you will find show ipm route, let’s say, on the router four. And if you want to be specific, you can use show ipm route 224 five.
If you verify I got some entries here now in this entries if you observe here the router four is receiving the multicast traffic from one one and four dot four two and one and four dot four two are the interfaces of the router one and it is receiving on s one by one interface that’s the incoming interface. So it’s receiving on this interface the multicast traffic and it is going to send out on the remaining two interfaces. It’s going to simply send out of all the remaining two interfaces here. That’s what it says. And similarly, it is also receiving from s one by one that’s the incoming interface s one by one. In fact, it is the same thing. Same thing. It’s distinct from interface, this interface, and it is sending out of both the interfaces here.
Now, again the shortest path is going to decide it’s going to build the shortest path three again and then on the router. Three. Also, if you try to verify Show Ipm route and also you’ll see the same thing here. So let me be specific. 224 five that’s going to build as kama G entries. Where the source address? Is one one and four two the outer one addresses of both the interfaces and it is receiving on s one by zero and it is sending out of s one by one. So it’s receiving on this interface and is sending out of this interface now based on the loop prevention mechanism it is also receiving from here. But it is not using this because of the reverse path forwarding check. And we’ll talk about this in our next section more in detail.
Now here you can see once I generate the multicast traffic from here, by sending a ping request to 224 five four five, it’s going to generate, it’s going to flood the eskoma genres to all the routers, saying that I’m the one who is sending the multicast traffic four to 24 five. Now, all the routers are going to maintain those entries in their own multicast routing tables. And if they have any receiver, probably here, if they have any receiver who wants to receive the multicast traffic, they are simply going to send out or send out to this router. Now here there is no more rp. Initially, it is going flood the traffic and if they don’t have any receivers, probably they are going to send out a prune messages saying that I don’t have a receiver here, probably it’s going to prune.
5. RPF Check – Loop Prevention
Now in this section we’ll try to understand the loop prevention mechanism in the multicasting. We call it as a reverse path forwarding check. Now reverse path forwarding check is used to eliminate the loops which may occur when your router floods the multicast traffic. Now, the prem protocol we use here, like here in this scenario, normally the prem protocol is used to advertise the multicast information. Now there is a possibility that the video server here is sending out the multicast traffic to the receiver here. Now, there is a possibility that you are receiving the multicast traffic from multiple sites. Now, how the router is going to figure out from where it has to this is the multicast traffic where it should not receive the multicast traffic and that is something decided based on the rp of check.
Now, whenever a router receives a multicast packet, it is going to look at the source IP of the packet. Now in this scenario, let’s say the router one is receiving the multicast traffic from video server ten one one, that is a source address and then it’s going to see the outgoing interface. Now here it is receiving the multicast traffic from both s zero by zero as well as it is receiving from s zero by one. So it’s going to see the incoming receiving interface from where it is receiving and then it is going to check the normal routing table. It’s going to see the routing table show IP route. It’s going to see that to reach the ten dollar network, what is the best route, the shortest path to that particular best route? Let’s say in my scenario this is the shortest path.
Now, in your routing table, you’ll see that it goes by s zero by zero is the best path. So probably what it is going to do is it’s going to receive the multicast traffic only on this interface, whereas rpf check passes here because the source to reach that multicast server, it’s going to check the reverse path and it is matching the entry here. And when it is receiving from the other side, it’s not going to match here because according to this, this is the best route to reach that server. And that’s what you see in the routing table. It’s going to receive the multicast traffic only on this interface, whereas on the other interface it’s going to discard the particular packets.
So that’s how the rpf is going to the multicast routers are going to figure out the loops and it will only receive on the interface a multicast packet which passes the rpf check. Now, to practically verify this behavior, we’ll go to our lab, which we are done in the previous scenario. In the previous we configured something IGP here and then we have enabled Ipm dens mode on this interface. So what I’m going to do is we have already generated a traffic from the R one on the router file. We have configured a join group command 224 five. So let me quickly show you the basic setup or the configuration, what we did already here. Now on all the routers, if you verify Show Iposp of neighbor I have IGP configured that’s the first step.
And then I have also enabled with IP pim dense mode. So if you verify show IP ppm neighbors, you’ll see the neighborship is up on all the routers. Dense mode is also pre configured. So that is something I did in the previous lab. If you go through with my previous videos, you will find this configuration information. Now the third thing on the router phi interface, f zero by zero, we have configured a join group command IP IGMP join group command where we are simulating the host who wants to see the multicast traffic. So showrun interface. Now the command is IP IGMP join group command. Now we are simulating the server on the router one. And we’ll try to generate a traffic from router one.
We try to ping to that multicaster address. You should see the reply if everything is okay. And now we’ll try to verify the rpf check. Let’s go to router file here. If I give show I-P-M root, show I-P-M root here. Now M route for 224 dot five four dot five. That is the multicast group address we are using here. And you can see here I’m missing the multicast traffic from four four two. That’s a source and it is receiving on s one by zero interface. Right, it’s missing s one by zero interface. And the router is going to check the routing table, show IP route four dot network or four two. It should match the exit interface on s one by zero, s one by zero incoming interface. So once it matches, it’s going to receive the multicast traffic.
Now that’s okay because you just have only one interface here. But what if we go on the router four? Now on the router four, if I give show Ipm root to verify the rpf table 224 phi. Now here you can see I’m receiving the multicast traffic from four door network. In fact, it’s coming from four door network as well as it’s coming from here from one door network as well. Now here the incoming interface from four dot network is s one by one. This is s one by one here. And it’s going to verify this incoming interface has to match show IP route. So if I say show IP ospf, it should match that particular photo network. In fact, photo network is writing, it’s writing added on S one by one. I think here. Photo network is on s one by one.
So which means it’s going to match the routing table and this one. So the interface, it is receiving the multicast traffic and to reach that particular server it’s matching. So it’s going to receive the multicast traffic on this interface. But whereas if it’s still receiving the multicast traffic from this particular source, it’s receiving from this site as well. But it’s not installed in the multicast routing table because this is the shortest path here. So this is how it’s going to prevent the loop in general. Now to further verify, what I’m going to do is to further verify I’ll go to this router one and router four interface. I’ll remove the spark dense mode here. Now this interface, I’ll remove the dense mode here. Now that will automatically make your rpf to fail. Let’s go to router one.
That’s what I did in the lab workbook as well. If you just go back here, I just disable this interface here or you can try trace. Also I’ll go to router four interfaces one by one. I’ll send no IP pim dense mode. So I’m going to remove the dense mode on router router four interface. Now, if you go to router four, if I give show Ipm root so if you go and verify show ipm root for 224 five, you should see that 224 five no, you don’t find any intrigue here. You don’t find any here. You can see incoming interface now because it is not receiving the it is not actually, let me generate some traffic here. So if I try to ping now, I should not see the reply here. If I just repeat it multiple times, I can see I’m not able to get a reply from 224 five or five.
Now, the reason here is the router four is receiving the multicast traffic from one one. It’s an incoming interface is null now the incoming interface is null and the reason is, if you verify show IP route ospf. Now the onedown network is received on one one. Now here you can see if you just check here, it’s receiving the multicast traffic from one one. It’s receiving from this side but not from here because this interface is not enabled with dense mode. So which means this interface is not multicast enabled. So we are receiving the multicast traffic from this side, one or one. And it’s going to check it’s going to do some rp of check. And in that rp of check, the interface where it is receiving it is receiving on s one by zero.
That’s a receiving interface. It has to match the routing table to reach one dot network. Now, to reach one dot network, what is the path here? It’s s one by one. Now, according to the routing table, to reach this one dot network, the outgoing interface is s one by zero but it is receiving the multicast traffic from s one by one so it’s not matching. So if it does not match your rpf, check will fail. So it’s not going to receive the multicast traffic on that point. It simply drops the multicast traffic even though it receives from here. Now, to fix this either you need to ensure that your IGP path used by used by the routers, it should be the best path and that best path should have a multicast enable.
That is one possible solution. That’s something you need to keep in mind. Or we need to change the IGP cost to go via this route. This should be the best route. That is one more way to figure out this. Or we can manually configure the multi static route M route. We can say to fix this one. There are multiple ways to do this. Either you can shut down this interface if you shut down this interface automatically, it matches. Let me try this. So you can see if I shut down this interface, if I give a shut down command now, this interface will be down. So to reach this one dot network, this will be the best path and you are also receiving the multicast traffic from this interface. So it’s going to match. So then you can see the reply.
So that is one possible solution you can go with. Or you can manually configure M route command because in some scenarios you may not running multicasting on this interface, but this will be the shortest part to reach the multicast source. So in those kind of scenarios we can configure M route commands. There is also one possible solution. You can go ahead with that one, or the other way we can even change the IGP cost also. So in general we can go with any one of the solution if you come across this scenario. But when you’re doing some troubleshooting, let’s say from the route row one, you are not able to ping to this multicast group address on the client side.
Probably you need to check the rpf check. So you need to ensure that your rpf check should work fine. Now, if your rpf check fails, probably what you will see is if you show Ipm route you’ll find the incoming interface null normally. So you’ll see something like incoming interface is null. So whenever you generate some traffic now you don’t see the entries. Let me generate the traffic first and then if I give you Ipm route I can see incoming interface in null. So it’s receiving the multicast traffic but it’s ignoring on this interface because of the rp of failure.
