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MPLS ( RD and VRF concept )

January 26, 2011

MPLS  (RD and VRF concept)

Today we are going to talk about one of the basic concept in the MPLS environment. Well when we are talking about the service providers domain, the first thing came into mind is MPLS- Multi Protocol labelled switching

So MPLS is the technology used by service providers to connect the customer sites across the globe. To understand the MPLS we should know the basic routing, the concept of CEF, what is VRF, how data plane and control plane and also why we are using the various protocol like LDP/TDP, MP-iBGP protocol and what is label switching, what is PUSH/POP operations
Lets start with the basic concept of MPLS, in short we can say that MPLS where routing at the edge and label switching in the core operates. The Edge devices in the MPLS network is PE router, which connected to CE ( Customer Edge router ) via routing protocol, it can be BGP or static routing that totally depends upon the number of the subnets routed into the domain of service provider.

We have two kind of MPLS services taken by customer, it can be L2MPLS and L3MPLS, now question what is L2MPLS and L3MPLS and when and where they can be opt for the operation.

What is the difference between L2MPLS and L3MPLS

L2MPLS is taken by the customer where customer don't want to share the L3 routing information with the service provider, it means the instead of pushing the packets from CE routers, we are pushing the frames from L2 device ( Frame relay switch or ATM Switch ) to the customer. The L2MPLS tunnel created end to end via the L2 media switches.

L3MPLS is taken by the customer where customer ready to share the L3 routing information with the customer and want to connect to the remote site. So the packets are being transferred from CE router to PE routers via routing protocol ( Static route or BGP or any other, depends upon the network design which customer actually requires )

So hope you understand the basic difference between L2MPLS and L3MPLS features and approach.

For L3MPLS, a single service provider PE router is connected to the various CE routers of different customer having the same IP subnet at local network. The subnets can be segregated  via RD ( Route distinguisher ) with VRF. Let me take an example

MPLS basic operations
Fig 1.1- MPLS basic operations

  • Customer -1 : Subnet 10.10.10.0/24
  • Customer -2 : Subnet 10.10.10.0/24
Both the customers have the same subnet in their local domain and send the subnets to PE routers at service provider end, now it can only be distinguished via VRF+RD at the PE end. Now the situation is like
  • Customer -1 : Network-1 ( subnet 10.10.10.0/24 ) + VRF-1 + RD100---- VPN 1 now
  • Customer -2 : Network-2 ( Subnet 10.10.10.0/24 ) + VRF-2 + RD200---- VPN 2 now
So now the network is different for both the customers. I hope you guys understand the concept of RD and VRF here.

RD : Route Distinguisher 
RD is used to distinguished the same subnet getting from two different customer

VRF: Virtual route Forwarding 
Its the Logical name to distinguish the network in the MPLS domain.