MicroNugget: What is Multi-Protocol Label Switching (MPLS)?

multi-protocol label switching and nothing but multi-protocol label switching let's begin there are some absolutely amazing applications that work on top of an MPLS infrastructure like layer 3 VPN s and pseudo wires but I'd like to clear those away for this nugget so you and I can take a look at just MPLS by itself what exactly is it what does it do and how do we verify it's working MPLS is nothing more than a packet forwarding technology that instead of using for example IP addresses and layer 3 information to make forwarding decisions it uses something called a label that is attached to every single packet and that label forwarding based on a label instead of the IP address took a lot less resources back in the old days and that's why we used it back in the old days was because it was faster now today we don't get that much of a speed improvement by forwarding based on a label versus the actual IP packet header information however there are some amazing additional applications that we can use because of the MPLS like layer 3 VPN and pseudo wire let's take a look at the rules for what each router has to follow to make this happen to do label switching instead of IP forwarding on a packet by packet basis so for this discussion I want you to pretend that you and I are this router right here we're p5 the first rule is we need to assign labels that means we need to take all the routes in our routing table and assign a number to them and we can start at 16 and we can go into the tens of thousands so take any number you want this not used in sign a label that's the first goal let's focus on just one network as opposed to all the routes in the routing table and let's assume that P 4 P 5 P 6 and PE 2 have all assigned local labels to this one specific network now at this point they're just following the rules and that is to assign a label a number to each and every route in the routing table and for demonstration purposes P 4 chose 400 P 5 chose 500 P 6 chose 600 and PE 2 chose the number 200 regarding this specific network the second rule that we're going to follow is we're going to share those labels using something called label distribution protocol and when MPLS is enabled on the interfaces and in globe config' the label distribution protocol happens automatically and it goes something like this p4 is so proud that he has this label associated with the network one-one-one-one that he advertised that information so p5 that's us we hear this advertisement and we say ok thanks mr. P for we get it for this Network one-one-one-one your local label is 400 and we'll store that as a remote binding remembering that p4 local label for this network is 400 that same process happens all the way across the network and we're gonna advertise it in both directions like we can't help ourselves so p4 is gonna tell them this way p5 is gonna advertise it both ways p6 is gonna advertise it eventually everybody is going to know their neighbors labels for all of the routes in their routing table the next rule is to build a label based forwarding table and we have a problem and this is us you and I our p5 our challenge is this we've got p4 who's advertised regarding this network right here it's just the one one one one network he's advertising a label of 400 that he can reach it we also have p6 who's advertising the label of 600 that p6 can reach it who do we believe which one of those are we gonna put in a label forwarding table regarding this network and the answer is simple I would consult our unicast routing table for IP version 4 we take a look at the one-one-one network it's gonna be in our routing table hopefully and we're gonna identify what the next hop is and then we're gonna say to both of our neighbors dear mr. p4 timorous is p6 I know you all both our advertising labels but I'd looked at my unicast routing table and the next hop from a unicast routing perspective is owned by p4 that means that p4 is the best decision so for that reason in my forwarding database based on labels I'm gonna know that to get to the 1 1 1 Network I'm gonna go ahead and forward it directly to p4 and I'm also gonna put a label on that packet and I'm going to include an MPLS label off 400 because that way when p4 receives it p4 is one who told me that that was his label for the one-one-one Network and it can continue to be forwarded so once MPLS has been implemented on top of an IP version 4 working network every router is gonna know the label forwarding information p2 is going to think oh I need to afford it up to p6 and they need to impose label 600 on it when p6 gets that label 600 it already knows that it needs to forward it to p5 with label 500 when p5 gets that labeled incoming packet with MPLS label 500 it swaps it out and puts 400 and that's the label swapping process that each of these devices are going to use the very first the entire process of calculating which of my neighbors label advertisements is the best for a given network happens very very quickly and it happens before the first transit packet ever shows up the final rule with MPLS is use it use the label forwarding whenever you can so if there's a ping packet for example destined to 1 1 1 1 cents into p2 he's gonna look at his forwarding information and see that there's an advertised label of 600 that p6 has shown to him for that Network he's going to impose label 600 and then p6 will swap it with 500 and he'll swapped it out with 400 as this packet his label switched across the entire network to verify the labels we can use a simple trace route on an iOS router and trace all the way from p2 over to one dot one dot one dot one it'll actually show us the actual labels that are being imposed on the network so this one right here is being imposed by PE too and that's because that's the label that p6 advertise to them then p6 swaps out the 600 with a 500 as before it sends it to p5 p5 swaps out the 500 with a 400 as it sends it to p4 because that's the label that p4 advertised and then for the last one it does something really cool called a penultimate hot pop which you can come see me in the CBT nugget series and I'll tell you all about that and much much more if we were to do packet analysis of that packet right after it left the PE 2 device before it hit p6 we would see there's the layer 2 there's the IP header at layer 3 and this is the MPLS and this is like what I call a or 2.5 this is the extra information that's added to a packet is 32 bits this much right here and here's the label that PE 2 is imposing on this packet for the benefit of being received by P 6 P 6 will receive that know exactly how to forward it it'll swap the label with 504 P 5 and forward it on its way in micro- taken a look at MPLS which is a packet forwarding technology which makes forwarding decisions based on labels inside the MPLS headers instead of the layer 3 IP headers I hope this information has been informative for you and I'd like to thank you for viewing

Channel: CBT Nuggets

Views: 147,988

Keywords: cbt nuggets, what is mpls, computer based training, computer based training ppt, computer based training software, computer based training examples, computer based training programs, computer based training software free, how to make computer based training, computer based training method, powerpoint computer based training, multiprotocol label switching

Id: huKkCK8AJ7I

Length: 6min 58sec (418 seconds)

Published: Tue Jan 22 2013