CCNP ENARSI (300-410) : MPLS (MULTIPROTOCOL LABEL SWITC…Section 10

1. MPLS Fundamentals

In this section, we are going to talk about the MPLS fundamentals. Let's start with the MPLS Overview multiprotocol Label Switching NPLs is a packet forwarding method that makes forwarding decisions based on labels instead of the layer three destination of the packet. MPLS was designed to support many different layer three protocols, but in this section we focus on Ip only before we dive into MPLS. Let's be clear that with today's routers, MPLS is not much faster than traditional IP routing guys. So why would you even consider MPLS? Well, MPLS decreases the forwarding overhead of core routers, making them more efficient. In addition, MPLS can forward three other protocols besides IPV four. And NPLs support multiple services such as unicast routing, multicast routing, VPNs, traffic engineering, QoS, and something more. Therefore, guys, MPLS is really efficient and really flexible. Let's take a look at the MPLSLib and MPLS lfip on the screen.

Notice that the control plane of the MPLS-enabled router is responsible for exchanging labels with other MPLS-enabled routers and using a label distribution protocol in addition to exchanging routing information. Using routing protocols to populate the IP routing table, which is rep once labels have been exchanged, the label information is used to populate the lip, and then the best label information can be used to populate the forwarding information base, which is fit. So unlabeled packets can be labelled and the LFE so labeled packets can be forwarded, or labels can be removed when packets need to be forwarded by the fit. So yeah, it might look a little bit confusing. And let me tell you more about this one. Let's say an unlabeled IP packet arrives with a destination of 100 zero five. Because it is unlabeled, the Fib is used to make a forwarding position. If the Fib indicates that the outgoing interface is an MPLS enabled interface, a label is added to packets and the labelled packet is forwarded labelled out the MPLS interface. Let's say a labelled packet arrives on an MPLS enabled interface because it is labeled.

The lfip is used to make a forwarding decision. This time, if the lfip indicates that the outgoinginterface is an MPLS enabled interface, the label is removed, a new label is added, and the labelled packet is forwarded at the MPLS interface label. If the Lfib indicates that the outgoing interface is not an NPLs enabled interface, this time the label is removed and the unlabeled packet is forwarded unlabeled. After dusing the information in the fifth, let's go ahead with the label switching routers, LSRs. So there is only one true router. Five are part of the MPLS domain in the screen you can see here. So they are known as "Label Switching." routers because they support MPLS, understand MPLS labels, and can receive and transmit labelled packets on their interfaces. In this case, men prefer one and women prefer five. These two guys in here, or rather Five, are known as the H, LSRs, and Router Two. Routers three and four are named as intermediate LSRs. An edge LSR sits at the edge of the MPLS domain and adds labels to packets that are entering the MPLS domain, removes labels from packets that will be leaving the MPLS domain, and even forwards packets as needed based on labels or the lack of L label. An intermediate LSR sits within the MPLS domain and primarily forwards packets using label information.

And let's go ahead with the labelled switched path term. The label switched paths. An LSP is a cumulative labelled path that a labelled packet takes through the MPLS domain. It is the unidirectional path and you can see it on the screen Therefore, in a complex network with multiple potential paths between source and destination, it is possible that the LSP from source to destination could be different from the LSP that is used for the return traffic. However, typically the same path in reverse is used for return traffic because of the underlying dynamic routing protocols such as OSPF and EIGRP that are used to build the symmetrical network and its forwarding paths. In this case, the LSP from Rotter One to 100 00:24uses labels like 80, 711, 65, and 23 along the path. Each router examines the label to make a forwarding decision, removes the label, adds a new label if required, and then forwards the packet.

For MPLS to work, a label needs to be added to the packet. As I explained to you previously, the label is added between the layer two frame header and the layer three packet header. The label is four bytes, which is 32 bits in size and contains four different fields. You can see on the screen the fields and the first 20 bits of the label define the label number. The next three bits, EXP, are used for quality of service. The one-bit S field is used to define whether the label is the last label in the stack when more than one label is used in the packet.

For example, with MPLS VPNs, the final eight bits of TTL are just used like IPS TTL so that MPLSframes are discarded if they have not reached the destination by the time the TTL reaches zero. So MPLS-enabled routers automatically assign labels to every network that they know about. Guys, how does a router know about a network? It can be locally configured by configuring an IP address on a router interface and issuing the no shutdown comment. As you know, and it may also be known through the propagation of routing information with dynamic routing protocols such as OSPF and EIGRP. For example, on the screen you can see router five. This router here gave a label of 23 for the network 100 00:24, but Rogers labelled the same network with 65, rather than three labelled with eleven, rather two labelled with eight to seven, and rather one labelled with 19.

What should you take away from this is the labels' local significance. Each router, regardless of whether it is locally connected to network 1024 like router Five or not locally connected like the other routers, generates a local label for the network it knows about, regardless of how it learned about it about. So let's go ahead with label switching. Based on the information found in the IP routing table and the lip, the routers on the screen have populated their Elfib and Fib as shams. Okay, you can see the populated Elfibs and the populated Fibs.

When router one receives a packet with the label 19, it examines the LFB and notices that it must forward it with the label 87 to the right. However, in this case, if a packet is coming in the interface that is connected to the 192 network, it doesn't have a label as there are no neighbors imposing labels on packets out of that interface. Therefore, when the packet arrives on the interface connected to 192.168.1.1 with a destination IP address in the packet for 100 00:24, the Fib indicates that label 87 has to be added and the packet needs to be forwarded with two as the next hop address. When router two receives the packet with label 87, it examines the Lzip and notices that it must forward it with label Eleven and R Three as the next top addresses, as you can see here and on the RTD wire. Then what happens? Radher three is receiving the packet. Then when Radio Three receives the packet with label Eleven, it also checks the LF and labels it with 65 via Radar Four, and rather than four, it labels it with 23 and Four. When a packet comes to the rudder five, it examines the Lfiband and notices that there is no outgoing label here, which means it is the end of the LSP. Therefore, the label must be removed and normal routing has to occur with the IP routing table, which is Fib.

2. MPLS Layer-3 VPN

In this section, we will talk about the MPLS layer three VPNs. MPLS layer three VPN provides peer-to-peer connectivity between private customers across a shared network. On the screen, you can see a provider with an MPLS domain with Customer A and Customer B, both using the same MPLS domain to connect their own private sites together. The MPLS domain is referred to as the P network, and the customer sites are referred to as the C network. Here we have the P network, which is the MPLS domain, and those are the C networks.

And for example, for Customer A, we have SiteOne and SiteTwo, and these sites are connected to each other by using MPLS. So in an MPLS layer three VPN architecture, three customer routers are known as the "Ce Guys." That means the customer edge and they do not run MPLS. In fact, they have no knowledge of MPLS labels or even VRF instances, which makes it easier for the customer to take advantage of the benefits provided by a provider's MPLS domain. The customer edge routers connect to the PEprovider edge routers of the MPLS domain. So you can see here, here is the Customer Edgeand here is the PE Provider Edge router, and these are the PE routers, and we will talk about this one. Also, please note that the PE routers, such as PER One and PER Five, are the ingress and egress LSRs for the MPLS domain, and the Proter provider routers, such as PR Two, PR Three, and PR Four, are the intermediate LSRs of the MPLS domain.

The goal is to have Customer A, Site One, and Customer A, Site Two exchange their local routing information over the MPLS domain and then forward traffic as needed from Site One and Site Two over the MPLS domain. The same would be true for customer site one and customer site two. Due to the nature of the MPLSlayer three VPNs, overlapping address spaces between customers is of no concern. Therefore, customer A and customer B can use the same private IP address space. In order to support multiple customers, the PErouters need to use VRF instances to isolate customer information and traffic from other customers. A different VRF instance needs to be created for each customer, and the interface that connects to the customer's customer edge router needs to be associated with the VRF. The CE router and the PE router exchange IPV4 routes using a routing protocol such as Rip, EIGRP, OSPF, or BGP, and routes are placed in the customer-specific VRF table on the PE router. Therefore, from the customer's perspective, the PE router is simply another router in the customer's network, but it is under the control of the provider. However, note that all P routers are hidden from the customer. Once the PE routers learn routes from the CE routers, the PE routers redistribute the routes into multiportocol PGP so they can be exchanged with other PE routers. When another PE router receives the routes, they are redistributed into an IGP and placed in the correct customer VRF instance so they can be exchanged with the CE router.

An important point to consider is that the PE routers are not participating in BGP. Only DPE routers are forming a multiprotocol IBGP neighbourhood with each other and exchanging the routes using the underlying network that is built with an IGP. So the Pearls and the P routers are using a dynamic routing protocol to learn about all destinations in the P network, and only the PE routers are using MultiPro protocol IBGP on top of that to exchange the customer routes. Now let's go back to overlapping IP address spaces. If all customer routes are being redistributed into multiportocol BGP, how does the BGP handle identical network prefixes that belong to different customers? It uses a rap distinguisher Rd to expand the customer's IP prefix so that it includes a unique value that distinguishes it from the other identical prefixes. The Rd is generated and used by the PErouters on a per customer VR interface basis. And to keep things simple, the Rd is used regardless of whether there are overlapping address spaces, so the rod distinguisher is used all the time. The unique 64 bit Rd is propelled by the 32 bit customer prefix to create a 96 bit unique prefix called Vpnv for address. The multiprotocol IDGP neighbour exchanges this VPN v for address. So check the example on the screen.

The customer's A VRF instance is using an Rdof 100 I'm sorry, one column is 100, right? And the customer B vs RF instance is using an Rd, with a single column. When these rod distinguishers are prepended to the IPVfour prefixes, the results are a Vpnv four route of one column each of 100 addresses, and one column each of 10 column this IP address.Now, let's say Customer A also chooses to use the 10000:24 network and advertises it over to its other site. This is where the Ratdistinguishers keep everything unique. Customer A would have a VPN v four route of one column 100, column one, I'm sorry, 1024, and Customer B would have a route of one column, one 10 column, 00:24, and the following steps occur in this case. In Step one, the Ce router and the PErouter exchange routes using a dynamic routing protocol such as OSBF or EIGRP. In the second step, the PE router places customer-specific routes in the customer-specific VRF table. In step three, the routes in the customer's VRF table are redistributed in the multiprotocol BGPS Vpnv for routes. In step four, the PE routers exchangeVpnv for routes over their multiparticle IBGP. Peering on step five, the PE router redistributes theVpnv for routes such as OSPF, EIGRP, and so on into the customer-specific VRF table. And in step six, the PE router and CE router exchange routes using a dynamic routing protocol such as OSBF or EIGRP. So let's go ahead with the MPLS layer.

Three-VPN Label Stack A label stack is required for the MPLS domain to forward traffic. Specifically, two labels are required for traffic to be successfully forwarded through the MPLS domain. The first label that is attached to the packet is a VPN label, and the second label that is attached is the LDP label. When the IP packet arrives at the ingressPE router, the PE router attaches bot labels. The Egress router uses the VPN label to determine customer specifics about the packet and what should be done with it. The LDP label is used for label switching from PE to PE in the MPLS domain. The VPN labels are learned from the PE routers over multiprotocol IBGP peering, and the LDP labels are learned using the methods we explored in earlier sections. Let's say that when PE r five, this guy learns at ten 00:24 from CERB. It places it in the customer's Bvrf instance. It then redistributes it into multiporticle BGP and thus creates a VPN V4 four route of one columnone 10 and two columns of 10 200:24. This VPN V4 route needs a VPN label created for it so that the forwarding will be successful. In this case, PE R Five assigns it the label 35. As you can see here, this label is shared with PE R One over the MP IBGP peering they have. You can see here that it also has the 35. Now, anytime Pier One receives an IP packet that is destined for 100 200:24, it needs to attach the label 35 so the packet can be forwarded. However, this label is known only to the PE routers. Therefore, if PE R One forwards this VPN packet to Per Two, it will be dropped.

And as it has no idea what the VPN label35 means, the LDP label is needed to forward the packet from PE R One and Per Five. So the figure on the screen shows how LDP is used to exchange labels that have been generated by the P E Ratter's and the P rotors. Per Five instructs PR Four to pop the label, PR Four instructs PR Three to use label 52, and PR Two instructs PR One to use label 99. Okay, so once the complete LSP is now ready to label, switch the VPN packet from PE R One to Per Five. Now let's take a look at the figure on the screen. So now, when an IP packet destined to 100 200:24arrives at PE R One from Cera, PE R One determines that the packet needs a VPN label of 35. So PE R Five will know what to do with the VPN packet and an LDP label of 99 so that the VPN packet can be labelled switched to the MPLS domain. Once the label stack is complete, PE R One sends the label stack packet to PR Two. When PR Two receives it, it only examines the LDP label. Based on the LFP, it states that the label 99 needs to be swapped to ten and forwarded to PR Three. So it does so. When P R three receives it, it only examines the LDPlabel based on the LF. It states that the label needs to be swept to 52 and forwarded to PRR four. So it does so. And when PR Four receives it,it only exercises the LDP label. And based on LF, it states that the label 52 needs to be popped and forwarded to PR 50. So it makes this happen. Now, P-E-R five only needs to read the VPN label, which is 35, and based on the scenario, the label is removed and the VR instance for customer B is used to forward the IP packet to the CERB router.

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