VRF in MPLS Networks: Virtual Routing and Forwarding Explained Simply

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

we'll be simply explaining virtual rooting and forwarding in the context of MLS if you haven't already done so don't

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there's much more to come so this way you can stay tuned now as I mentioned before MLS is a technology that is

rarely implemented alone it is deployed in combination with various other Technologies and protocols to deliver

its innovative and flexible capabilities one of those features that is closely related to MLS and is often deployed

with it are vrfs now vrf stands for virtual rooting and forwarding a vrf instance can be

thought of as a separate rooting domain within the same rooting Hardware Ware each vrf that is configured within a

router has its own rooting table and individual interfaces on such a router are assigned to a particular vrf you can

think of vrfs as a virtual slicing up of a router resulting in multiple individual virtual routers within your

physical Hardware now what do vrfs look like well this diagram here will help us to further visualize vrfs and how they

work here we have a router with four interfaces and we have two vrfs configured on this router we can name

our vrfs whatever we want but I'll call mine red and blue you'll find that colors are often used to name vrfs but

you can use whatever you want now gigabit Ethernet 01 and 02 belong to the vrf called Blue while 03 and 04 belong

to the vrf called red and each vrf creates its own separate rot table now here we can see the rooting table of vrf

blue using this command on The Cisco iOS router that is configured with vrfs the pipe begin Gateway operator that I've

added here simply makes the output begin at the word Gateway so what this actually does is it omits all of the

text with the various codes that appear before this phrase of Gateway of Last Resort is not set in any case the

rooting t here only contains interfaces in vrf blue and the IP addresses of those interfaces it knows nothing of

these other addresses or interfaces as they belong to another vrf similarly we can take a look at the rooting table of

vrf red the IP addresses and interface in this vrf are shown while those in the blue vrf are completely unknown each vrf

is a distinct virtual rooting domain and by default packets cannot be rooted from one vrf to another for example if a

packet comes in on gigabit Ethernet 03 with a destination IP of 1010101 which is the IP address of this

interface here the destination address will be looked up in this rooting table of the red vrf and since there is no

match it will simply be dropped but what happens if we have another interface here that is not a assigned to any vrf

how is that handled interfaces and networks that do not have an explicitly assigned vrf are part of the global

rooting table by default the global rooting table and the interfaces and networks that belong to it can be

thought of collectively as the default vrf meaning any traffic that is not associated with a specific vrf will be

rooted to This Global rooting table now interface gigabit 00 here for example is not explicitly assigned to any vrf so

this interface along with its network will appear in the global rooting table now if we take a look at that Global

rooting table we'll see something like this here you can see the interface gigabit Ethernet 00 and the IP address

that belongs to that interface now you can think of vrfs as virtual distinct and separate routers running within the

physical router in a similar sense as to how vlans virtually segment a switch into multiple virtual switches now these

vrfs are so independent are so separate from each other that you can actually assign the same IP address to two

different interfaces on a router like so as long as those interfaces are in different vrfs in any case that's the

gist of vrfs and how they work within a single router now what bearing does all of this have on an MLs Network well isps

that offer Network Services to their customers want to be able to leverage the same network for hundreds or even

thousands of customers so vrfs are ideal for that separation of traffic in an MLs Network such as this one each customer

site is assigned a vrf and this vrf is configured only within the PE routers so for this particular customer

site with C1 here the interface here on pe1 is configured on a vrf called Green while this one here is configured on a

vrf called Blue now there's an indication here of the default Rd or root distinguisher this is an integral

component of this whole process but we won't talk about it now we'll see it in action in an upcoming video similarly if

we look at pe2 here we have the same vrf green configured on this interface now these vrfs help to differentiate iate

the multiple customers that may exist on each PE router for example another vrf called orange is configured here that

way each customer can use whatever IP addresses they choose even if their IP address spaces actually overlap because

the vrfs are completely independent of each other now here comes the final piece of

the puzzle remember vrfs are only configured on pees pters have nothing nothing to do with vrfs pters in the

context of mLs are just responsible for using label switching to get from one PE router to the other so the big question

is for communication from ce1 to ce2 to take place in such a way so that it is segmented from all other customers how

must pe1 here send traffic so that it will be directed appropriately via the pters to pe2 and to C2 well MLS as we

have said works with a lot of other protocols pe1 and pe2 must share information about the vrfs that they

serve and this is done using multiprotocol bgp now this needs a bit of

explaining remember in a previous video we saw how the MLS network was set up so that the PE routers became ibgp peers in

the examples We examined we used those ibgp peerings to share IP rootes between those PE routers but in this case today

these router are actually sharing information about vrfs now to share in this information about vrfs we use an

additional capability of bgp which is called multiprotocol bgp or MP bgp now this is just a fancy term that's saying

that bgp here can advertise things other than simple IP addresses in this scenario here bgp enables the PE routers

to inform each other of the vrfs that they have so that they can use the appropriate labels to ensure that the

traffic is directed correctly over the MLS topology but in order for MLS to do this

it will require the use of a couple more Technologies and protocols specifically it needs to employ root distinguishers

or RDS which we saw briefly in the output of the roters here but we also need things like root targets or RT

as well as VPN V4 Roots but we won't talk about those here we'll talk about these aspects of mls in the very next

video for now I hope you found this video useful and if you have please click that Thumbs Up Button if you have

any feedback or questions feel free to put those in the comments below I'm always happy to respond to those and

please subscribe to get updates to newly published videos I'm Lazarus at Telecom Tech I hope this video has been helpful

for you and I'd like to thank you for watching