How BGP Is Used in the MPLS Core: Edge Routing Explained

hello this is Lazarus at Telecom Tech where Telecom and networking Technologies are simply explained today

we'll be simply explaining how bgp is typically employed in the MLS core now don't forget to subscribe to enable

notifications and to like this video If you like this video and if you have questions or feedback please feel free

to write them in the comments below I'm always happy to read them and to respond now in a typical MLS ology like this one

we have the CE rter the PE rter and the pters CE stands for customer Edge and these routers are located at the edge of

each customer Network the C routers connect to the provider Edge routers or the PE rter and those rter are found all

along the perimeter or the edge of the MLS Network finally we have the provider routers or the P rooters which

constitute the very core of the MLS Network now to make all of this work in the way that we

demonstrated in previous videos we need to deploy various Technologies first we need a rooting

protocol between the CE and the PE this way the CE can advertise the networks found on the customer Network to the PE

router here we can use any rooting protocol we choose including ospf eigrp or even

bgp secondly we need to establish an underlay rooting protocol within the ISP infrastructure this is so that all of

the p and PE routers can find each other and communicate with each other remember this rooting protocol is not here to

advertise customer networks it's there to establish the control plane on the ISP infrastructure on top of which MLS

will be able to function typically for large scale ISP networks it's either ospf or Isis that are used

for this underlay rooting protocol third we need to deploy some protocol that will allow the PE routs to

share their customer networks with each other so that they know what labels to push onto the packets as they send them

through the MLS core and this is where bgp comes in now for our purposes we're going to

assume that the whole of the ISP network is contained within a single bgp autonomous system or as we'll use as

number 100 to Define it this as contains all of the PE and pters that belong to this MLS

topology now remember we already have an igp running which allows all of the PE and pters to reach each other since

we're configuring bgp within a single as we're going to be creating ibgp peerings now where are we going to configure ibgp

specifically we'll need to create a set of full mesh ibgp peerings between all of the PE roters however the pters will

not be participating in bgp at all now I know what some of you are thinking isn't it a prerequisite to have a full mesh

ibgp peering within an AS how can we leave the pters out of it well yes but that prerequisite is applied only to bgp

routers the pters are not bgp routers you're allowed to have a router within your topology that's not running bgp and

that is not participating in any bgp peerings it may even be relaying bgp messages between ibgp peers because of

the underlying igp but it still won't be violating the ibgp full mesh peering rule so only the PE rter participate in

bgp by becoming ibgp peers with all other PE routers in other words only the outer edge of the MLS Network

participates in ibgp not the inner core of pters now the reason for this is that

bgp in such a scenario is used to allow PE roters to share the customer networks with each other

so pe1 here will advertise to all other PE rter the IP addresses behind the C1 router here so pe2 here populates its

bgp table with this network but remember PE roters are also running ldp and are becoming ldp neighbors with pters so

this PE rter learns which label it should push on a packet to get those packets to the proper destination

now the realization that bgp runs on the perimeter of the MLS network will help us to further understand why MLS and

label switching is more efficient than IP rooting if we didn't use MLS all of these routs in the ISP infrastructure

must learn about all of the roots that exist in all of the customer networks that means that this pter here must have

a rooting table that includes all of the networks of all of these potential hundreds of customer routers but with

our MLS setup using bgp the pters are completely oblivious to all customer networks isn't that a huge load off so

when ce1 forwards a packet to pe1 that is destined for this customer Network here how does it get to pe2 and then to

C2 well from pe1 all through the MLS core only label switching takes place remember label switching that efficient

method of directing traffic the pters only use labels and nothing else they don't care about the source and

destination IPS in the packet they simply use the labels that the PE routers push onto the packets as they're

sent through the topology now in this video we just talked about bgp and its ability to

exchange the IP prefixes of the customer networks between the PE router however bgp is able to provide much more for MLS

in particular it's ideal to allow the use of what are known as layer 3 vpns within an MLs topology until now we've

been allowing customers to communicate with each other over the MLS infrastructure but we haven't isolated

those customers from each other and as you can understand that is definitely a prerequisite in order to put this into

any production Network layer 3 vpns in MLS allow this separation of customer networks on the MLS topology and it's

bgp that makes that possible we'll talk about that in a future video now i' would like to mention that the title of

this video is using bgp in the MLS core but strictly speaking there is no bgp in the core pters it's only running along

the perimeter between the PE routers but it was difficult for me to find a title that was concise and descriptive enough

enough to convey that meaning so I just thought I'd stick with that title in any case that covers the use of bgp on the

edge of the MLS core Network I hope you found this video useful and if you have please click that Thumbs Up Button if

you have any questions feel free to leave a comment below and please subscribe to get updates to newly

published videos I'm Lazarus at Telecom Tech I hope this has been helpful for you and I'd like to thank you for

watching