MPLS VPNs - Route Distinguisher vs Route Target

MPLS VPN – Route Distinguisher vs Route Target is a short video comprising important concepts and frequently asked questions about RDs and RTs in MPLS interviews.

I have tried to simplify them in terms of what RDs and RTs are, how do we configure them and why do we need them. Hope it will help network professionals in nailing service provider job interviews 🙂

MPLS VPN – Route Distinguisher vs Route Target

VRF (Virtual Router and Forwarding):

Assuming that you have basic understanding of MPLS and its related terminologies, I am going to start discussing VRFs that allow multiple routing and forwarding tables to exist on a device. VRFs act as integral part in MPLS L3 VPNs because it makes a single PE router appear as different routers to CE devices. Customers having same subnets can be terminated on the same PE router as they end up reaching in different VRF tables.

In summary, VRFs make routing information from one customer completely separate from other customers and the routes are tunneled over the service provider MPLS network.​

Note that “VRF” is a Cisco term and VRFs are called Routing Instances in Juniper and VPN Instances in Huawei. They are also known as Contexts by some other vendors.

VRF Example Scenario: Let us take an instance wherein both customer A and customer B have got the same subnet, say for their branch offices and terminate at the provider side on PE-01, they can be easily differentiated at the provider end on the basis of their respective VRFs.

However, it is important to note that VRFs are local to the router. So, we need to have an additional attribute that may distinguish the IP information in the MPLS core. So, this introduces the Route Distinguisher (RD) that differentiates a set of routes that are part of one VRF from another.

RD (Route Distinguisher):

An RD is a unique 64-bit number that gets appended to the conventional 32-bit IP address, thus making it a 96-bit VPNv4 address. The format used to represent RD is ASN:NN where ASN is the service provider’s AS number and NN is a number that identifies the site of the customer.​

The function of RD is to make routes belonging to different VRFs unique in the MPLS core. To do this, we need to advertise VPNv4 routes and we accomplish this task using MP-BGP (Multi-protocol BGP).​

RT (Route Target):

Now to get proper and correct routing across an MPLS VPN, we need to discuss Route Targets (RT). RTs define VPN membership as they allow the router to control the import and export of routes among different VRFs.

So, let us say if customer A located at branch X wants to have connectivity with customer A located at branch Y, the RTs will have to be imported and exported between respective VRFs.

MP-BGP (Multi-protocol BGP):

It is an extension of the BGP protocol that enables BGP to carry routing information for multiple network layers and address families. MP-BGP supports IPv4 unicast/multicast, IPv6 unicast/multicast and VPNv4 routes.

In order to exchange VPNv4 routes, MP-BGP usesNLRI (Network Layer Reachability Information) that comprises following attributes:

  • RD (Route Distinguisher)
  • IPv4 prefix
  • Next Hop
  • VPN Label

E2E Connectivity for L3 VPNs:

​Now that you are familiar with technologies used in MPLS L3 VPNs, below steps would help you in understanding end-to-end flow for an L3 VPN:

  • ​The CE connects to the PE using static or dynamic routing. Dynamic routing includes either an IGP or BGP. BGP is the preferred protocol as service providers already run BGP in their core.
  • Once the CE-PE connectivity is established, the routes then enter customer’s VRF configured at the PE router. All customer routes learnt for this customer will be present in this VRF.
  • The PE router will then redistribute everything into BGP. However, if we use BGP as a routing protocol between CE-PE routing, no redistribution will be required in that case.
  • And last but not the least, in order to import and export routes, RTs serve the purpose. ​

It is important to note that service providers do also run LDP or any other tag distribution protocol over the IGP.

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Haider Khalid

IP MPLS & Enterprise Core Network Engineer, CCIE# 52939
Haider Khalid is an IP MPLS & Enterprise Core Network Engineer (CCIE# 52939) who has worked with several ISPs & Telecom operators in Pakistan, Middle East and the UK. He is always keen to learn new technologies and likes to share them with his peers and other people. In case of any questions or feedback, please feel free to drop a comment below or connect with him on LinkedIn.

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