Introduction to MPLS Route Target ( RT )

Route Target is one of the extended community of BGP used in MPLS networks. Before to understand the concept of the Route Target, you need to understand the basic concept like VRF, RD which is generally used in MPLS network. VRF ( Virtual route forwarding ) and RD ( Route distinguisher ) which we discussed earlier in the post, Now the other major and important term is RT ( Route Target ) Used with RD and VRF.

Now why RT is used in the MPLS Network, RT ( Route Target ) comes into the pictures when you have multiple remote sites which is connected to MPLS. So there are two concepts of Import and export in RT, so now the question is where these import and export are being used.

On any VRF, when you configure RT import, it imports all the prefixes that matches the configured RT value as one of the attribute in the BGP update. So in any-any VRF, it is common to see all PE configured with same RT value (You may see more RT depending on inter-VRF scenarios).

So RT is used to export the routes from one site and import at other side and vice versa. So by using import and export you can have the specific routes at the sites. It's the concept of the route filtering for specified site.

Fig 1.1- MPLS Route Target Concept

As i earlier said, RT is a extended community of 32 bit and the information regarding the RT will be sent by MP-iBGP protocol to all the PE's where customer sites are directly connected.

It is not mandatory to use ASN as part of RD or RT. You can use XXX:YYY format. The use of ASN:XXX is useful from operational point of view. 

We will come up with the next session where we can define the difference between RT and RD and the other features used in the MPLS like the explanation of MP-iBGP used instead of iBGP and also the feature for Traffic engineering like RSVP.