Unanswered Question
Jul 20th, 2007


I am confuse between two different terms of MPLS VPN RD and RT. What is the difference between both and where they are used?

will someone please clear this with some good example?

I have this problem too.
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mohammedmahmoud Fri, 07/20/2007 - 06:15


RD (Route distinguisher) is used to make the customer prefixes unique on a single PE router (providing address space overlapping), by simply being added to the customer IPv4 address to make a unique VPNv4 address.

RT (Route Target):

export RT -> attached to the routes when exported from the VRF (VPN identifier)

import RT -> Used to select which routes to be imported into the VRF from the routes received via MP-BGP (Import route filter)

RT is the mean of controlling routes propagation between PE routers, and simply acts as the VPN identifier in order for all the customer sites on different PEs to be able to communicate.


Mohammed Mahmoud.

dangal.43 Fri, 07/20/2007 - 06:17

once we attached RD to routes now why we need RT?

still i am not clear with RT will you please explain me that in details?

mohammedmahmoud Fri, 07/20/2007 - 06:19


RD makes the route unique on a single PE, while RT will be attached to the route in order to be tagged with the VPN membership, and accordingly when the route reaches the other PE router it is inserted into the appropriate VRF table.


Mohammed Mahmoud.

dangal.43 Fri, 07/20/2007 - 06:22

so RT is the VPN identifier right!!!

and do we have VRF table for every customer or for every VPN? i think its per every customer. so here we are identify the customer routes by RD then why we need RT?

mohammedmahmoud Fri, 07/20/2007 - 06:26


The RD is only significant on the local router attaching it - it is transmitted to other PEs over MP-BGP but ignored as soon as these PEs receive the route.

RT need will be visible in complex VPNs, where customers' sites needs to be a member in more than one VPN -> RD can't be used for this purpose it can only identify for which customer does this address belongs to.

The VRF is per each customer, but the VRF can be a member in multiple VPNs using RT.


Mohammed Mahmoud.

dangal.43 Fri, 07/20/2007 - 06:29


will you please explain me how RT can be used if you have multiple vpn to single VRF?

Like in which scenario and how RT will help out?

mohammedmahmoud Fri, 07/20/2007 - 06:37


Ok, what you are asking about is called complex MPLS VPNs, and it has a lot of usage, ex: Overlapping VPN, Central services VPN, and Managed CE routers service.

Simply what is done is that we can attach an export RT to a route (each export RT is a separate VPN identifier), and then by controlling the import RT we can control which routes to be imported to which VRF routing tables according to the case.

Now i'll ask you to review the thread from the start in order to organize this puzzle in your head, and please don't hesitate to ask more questions and i hope that i've been informative.


Mohammed Mahmoud.


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