350-501 SPCOR Cisco CCNP Service Provider – MPLS Label Distribution Protocol part 3

6. MPLS Label Forwarding

MPLS label forwarding. Now in this section we’ll continue with our LDP configurations. Like if you remember, we have seen how to configure LDP inside the service powerful network initially by using something called MPLS IP command. Now in this section will verify how LDP is going to work or how it’s going to do the label forwarding. So the entire process goes in five steps. The first step will be IGP is built based on the routing table based on your Fib table. Now inside the service for network if you remember we have configured IGP which is responsible for building your Fib table forwarding information based table by using Cisco Express forwarding method.

And the second method it’s going to assign every label switch router is going to assign the local label for each route learn let’s take an example. I got a 40 row network here. Now 41 network is going to use a diet ticket network. So router four is going to assign one label for 40 dot network. Similar way the router three when it receives 40 dot network, it is going to assign one label same thing, same way router three, router two also assign one label and router one is also going to assign one label for 40 dot network. So similarly each and every network will be assigned with one local label. So we’ll be verifying this practically what label are assigned in our scenario, the practical lab scenario which we are using here.

On the third step, once the label is assigned by the router, now they are going to share that label information with the neighbors. Let’s say router four is going to assign the label of 40 it’s going to share with router three and it’s going to say that for your network it is going to add a label of 40. So now router 3 may be assigning the label of 30 for the same network and then it’s going to add to dice to router two and router four as well. Now all the routers they will exchange the labels whatever they are using for those respective networks by using the label distribution protocol LDP with the neighbors. And then the fourth step will be based on that information propagated.

Now LDP just now it has propagated all the information. Then it is going to build from FRB it is going to build something called Label Forwarding Information Base and this label forwarding information base is the lookup which it is going to do if any packet enters. By seeing this label forwarding information base, the routers are inside the service network or forward forward which is your outgoing label and outgoing interface. So the final step will be forwarding the packet based on the label lookup. Let’s try to verify the same thing practically. So for our lab, if you remember, we have configured LDP already inside these four routers in our previous session.

If I just go back and verify on the routers on the router one. If I use Show MPLS LDP neighbors on the router one, I have one neighbor and then if I go back to router three, I have two neighbors on the router three, router two and router four. So LDP is something three concrete in my scenario. So what I’m going to do is I’m going to verify one by one. So for verification, I had to take one specific network here for understanding. In fact, the same process happens for all the networks. So the first thing what I’m going to do is I’m going to verify what is the label assigned for 40 dot network.

So we’ll try to verify what router four is going to assign the label and then what router three is going to assign the label router. Router two is going to assign 40 40 dot network and router one what label it is going to assign for 40. Dot network as per our scenario will practically verify with the respective show commands. And then finally we’ll also see how router one is going to communicate with the same photo network based on label forwarding. So how the label forwarding is going to work in this scenario. So there are two things we are going to learn here. The first thing, we are going to start with the advertising, the label information, the label information, how the label information is propagated practically we verify that.

And the next thing how the label LEP is going to forward based on the labels. So two things, so let’s first start with the first one, how label information. And the example I’m going to take is 40. Net one. If I just get back to router four and I verify Show MPLS. So there are two commands which I’m going to use in this scenario. The first command will be Show MPLS LDB bindings. This command is going to show you what is the local binding and what is the remote binding like. If remember I have discussed it’s going to build its own LSR is going to share the label information with the respective neighbors. Now, that information will be stored in your label binding information and then there’s one more command which I’ll be using to verify the Lfid.

The other command will be show MPLS forwarding table. So MPLS forwarding table is something we’ll be using later on. The first thing I’m going to use show MPLS LDP bindings. Now if I want I can view the complete command here, but if I use complete command it is going to give you the entry information of each and every network it has learned, what is the local label, what is the remote label, all the labels information which is received from its neighbors. I’m going to specifically point to 40 dot network and I think I have to give the subnet mask or eight. Okay, so now I’m going to specifically focus on one network that is 40 dot network. Now in this scenario for 40 dot network. What is a local label? The local binding is implicit null.

Now when I say implicit null now always remember, this is what we call as local label for each and every directly contained interface. The default label will be implicit null. So whenever you see this implicit null you need to understand it’s a directly connected interfaces. And this label comes under the category of resort labels. We call it a label three. Now router four as this 40 dot network is directly connected interface. So it is going to add a label called implicit null. So I’m going to write down here just for understanding. So and then remote binding. Remote binding will see that is something in our next step will verify. Let’s go and verify on the router three show implicit LDP bindings and for 40 or network submit mask is eight.

I’m going to see what is the local label here. The local label is 30 two. So which means router three is going to assign a label of 30 two for 40 row network. So we are going to focus on one network, 40 row network. So this the same thing applies for each and every network. So we are going to specifically focus on one network. If I go back to router two and I give the same command, what is that command? Show MPLS LDP findings. And then what is the network I’m using? What is our network? It is a slash value of eight. What’s the local label here? 20 one. So I’m going to write down here so that we can easily understand how the labels which is going to work. So the label is 20 one for 40 dot network.

And then finally if I go to router one and if I give the same command show embrace LDP bindings for network 40 dot network and what is the local binding? 10 three. So if you remember, we have given ranges. Now that ranges is going to help us to easily differentiate the labels coming from router one, router two, router three or router four. So it’s 10 three is the label assigned for 40 dot network. Now here from this outputs whatever we have seen here, the router four is going to assign with the label of impressive null. And the router three assigns 30 two label for 40 dot network and router two assigns a local label of 20 one for 40 dot network. And the router one is going to assign with a local label of 10 three for 40 dot network.

Now we have just seen the second step here. Labels which are outer. Every outer inside the service for network is going to assign a local label for each raft learn. So that’s what we have been verifying here. If you just come back to my diagram here. The router four assigns the label of implicit null and then router three with 30 two. Router two with 20 one and router one with 103103. Okay, so now moving on to the third step, the labels feature is going to share that label information with the neighbors using LDP protocol. Now what happens here is now using LDP protocol, the routers are going to share that label information. Router four is going to advertise that. Okay, so for your network I’m using the label of Mditnal.

Now what router three is going to do is router going to route three is going to write that my local label is 30 two for your network and I’m receiving for the same network I’m receiving the remote binding that’s for remote binding with a label of implicit null. Implicit null is actually a behavior called PHP penalty made hopping. I’ll be getting into that in more in detail in my next section. Okay, so similarly what router two is going to do is router two is going to receive the same information again here. Router two receives saying that from router three it is going to label of 30 two. So router two is going to write my local label is 20 one and my remote binding towards router three.

What is the remote binding here? 30 two, which means, it says that the router three is going to use 30 two for that label and then remote binding again towards router one is going to use with a label of 10 three. So each and every router is going to maintain your local label and also it is going to maintain the remote binding label so that it can decide what should be the outgoing label. So let’s try to verify the same thing here. If I just come back to my command line, let’s start with the router for so I’m going to verify the same thing here. So if you see the output here, show implicit LDP bindings, my local binding is implicit now. Now here you’ll see tag information base and TSR. Generally you’ll find local binding TSR.

Here you can see TSR tags which router, it’s the same thing like labels which router. So even though Cisco change using to LDP, but still most of the terminology you’ll find with tag distribution protocol. So which means when you say TSR, you need to understand it’s an LSR labels with router. So when you see TIB, it’s nothing but label information based, the same thing tag information based or labels information based, it doesn’t make difference. So router four is going to assign a label of implicit null because it’s a directly connected and then it is going to advertise that information back to router three saying that my label is implicit null. Okay, so now similar way. Now, Router Three here is going to receive it’s going to add a local label of 30 two.

And then from the Router Four it has received saying that what is a remote label the Router Four is using the label of implicit null. And Router Two is using the label of 20 One because Router Two is advertising to Router Three and Router Four is advertising that label information to Router Three as well. Similarly, if I just go back to router two here, router Two is going to assign the label of 20 one. That’s what we’ll learn. And then it’s going to do the remote binding. So if it has to send to reach this network, if it has to go via router one, it goes via R 10 three. If it has to go to router three, it goes via outgoing label of 30 two. Similar way if I go back to router One, router One will have a local label of 10 three. That’s what we wrote here.

And then it’s going to write the remote binding what router two is using. Now, based on this information, we can really figure out how the label distribution label information is being shared between the multiple labels. Now based on this information, it is going to build a table called Lfib Label Forwarding Information Base. Because normally what happens is now to reach from Router One to Router four here I just got only one route. So that’s the reason you will find only one remote binding. So sometimes you may come across a scenario where you may have multiple routes to reach this destination. Just like we have an OSP of database table. Now maybe in that or OSP of EHRP Topology table, you may have multiple routes to reach the same destination.

But out of these multiple routes, any one route will be used. That is the best route will be installed in the routing table. Right now based on this routing table, you have something called Forwarding Information Bases built by the SF. And based on this, again, it’s going to build Label Forwarding Information Base. And if you want to verify this label forwarding Information Base. That is Lfib. The next step we need to give a command called Show MPLS Forwarding Table. So let’s go to command line and verify the forwarding information based table after Router One. Show MPLS forwarding. Table now here you can see to reach router for it or network, there is an incoming label, there is outgoing label.

So what we’ll do is we’ll try to take one example, we’ll try to understand how exactly the communication from router One to reach for a network happens. So just how we verified the first step. The first step is how the label information is advertised and how it is shared between the multiple LDB neighbors and where it is maintained. By end we can verify that by using Show MPL bindings. And the second step we are going to verify how LDP is going to forward the labels by using a command called Show MPL is Forwarding Label. So I’m going to take an example how Router One is going to communicate with 40 dot network based on Label forwarding information Based. So my requirement is here.

This ten one one which is my source. It’s going to communicate with 40 111 which is a normal pure IP packet. Take an example, you have a customer coming from here which is a normal IP packet. It goes to router one and then router two, router three, router four before it comes back as a normal IP packet. So inside the service for network here it is label switch. So we’ll be verifying how it’s going to work in this scenario. So first I’ll start with the router one. So on the router one the packet enters as a normal IP packet. Okay? So normal IP packet and what router one is going to do. So it is going to simply see the source is ten one one and the destination is 41 one. So you want to communicate between ten one one and 41 one.

Now if you just try to verify there is a communication here. If I just try to ping 41 one which is on the route of four and I can say the source is mylan interface, I can see the communication even if I trace, I can see the packet goes very fine by using the labels the label forwarded. Now I want to just verify how exactly the routing lookup or the label lookup is happening. Just like routing lookup, we are going to verify them. So router one I’m going to say show MPLS forwarding table. What’s the network I’m going to use 40 dot network with eight submit mass. So whenever any user on the router one enters and he says I want to go to 40 dot network, it is going to simply send with outgoing label of 20 one.

That’s what you’ll see, outgoing label and it is simply switched out of s one by zero interface. Now if any user enters the router one and he says I want to go to 41 one, the packet will be switched with a label of outgoing label of 20 one. That is nothing but the label used by the router two to reach the same number because of the it’s going to use the remote label. So the local label is 10 three and the outgoing label will be 20 one and then the packet reaches router two here. Now once it reaches router two, what happens? Now router two again I’m going to use the same command show in pillars forwarding table to reach 40 dot network and then with a slash eight submit mask. Now router two is going to use a local label of 20 one.

That’s what we have learned here. And then when it is sending, the outgoing label is 30 two. Now any user on the router two, if it has to send any information to party or network, it will simply send out of 30 out of s one by one interface with the label of outgoing level of 20302. That is something used by router three. Similar way if go back to router three again show MPLS forwarding table 40 dot network to reach. And on the Router three it is going to use a local label of 30 two and when it is sending, it is sending with a pop tag. Now, the pop tag means it is going to remove the label and it is not exactly sending with the IP packet. It is going to remove the label with a pop tag saying that.

Now normally what happens there is something called PHP penultimate Hopping. Now before today is the destination. So whenever a router four is advertising its own label with Implicit Null, the Router three understands that based on this Implicit Null information, the Router three understands that okay, it’s a directly connected to the next router. Instead of sending with the Implicit Null label, or instead of sending it a label, it simply remove the label, it will pop the label and then send to the router four. And the router four is going to do the routing look up and it will see where is that 40 door network is on f zero by zero interface and it will simply forward the packet to the LAN interface. From there it reaches to the specific host on the land.

Now, this is how your label forwarding exactly works. Now in this scenario, if you remember just before this, we have seen the penultimate hopping PHP. Now let’s try to understand what is that PHP exactly? The PHP is used in the label switch path to improve the efficiency inside your MPLS service core network. Now, PHP is a feature which is going to reduce the routing look up on the one hop before router that is penultimate hopping. So probably about this PHP, I’ll be discussing in my next section much more in detail with some examples where if there is a PHP, how it works, how it behaves, and if there is no PHP, how it is going to behave.