MPLS

[shivkantbhurse]

what is Route Target and route distinguisher?

I would like to know the detailed information about the above two thing I tried a lot to understand these things but not able to understand ca anybody help me out?

[blau grana]

Hi,

there are several good explanations out there.

f.e.: http://packetlife.net/blog/2013/jun/10/route-distinguishers-and-route-targets/

in general:

- route target: extended community pinned to prefixes during export operation, RT is used to select specific prefixes which should be imported to specified VRFs

- route distinguisher: does exactly what name implies, distinguish prefixes in SP backbone; if two customers uses same address space f.e. 10.0.0.0/24, unique RD will make them unique per each VRF

let us know if you have problem to understand something specific.

[Julio E. Moisa]

Hi

The route-distinguisher or RD: It is used to create an identifier for a set of routes into a specific VRF, in this way we can now identify that the routes belong to that VRF and not to other one. In few words each VRF created on a router has different RD value.  RD value is locally significant. 

R5(config)#IP VRF CUSTOMER_A
R5(config-vrf)#RD 1:1
R5(config-vrf)#EXIT
R5(config)#IP VRF CUSTOMER_B
R5(config-vrf)#RD 1:1
% RD 1:1 already in use by VRF CUSTOMER_A

Now about Route-Target: is used to exchange the prefixes between 2 or more routers. This information is sent using the extended community through the BGP updates to the neighbors.

The route-target can be import or export and they are associated to a identifier, they can use the same identifier example: router-target export 1:1 and route-target import 1:1. This identifier will be used to know which prefixes will be received or advertised from a specific VRF.

The route-target is built with 64 bits same than Route distinguisher.

We can see a example, Router A and B are using the same VRF:

If the Router B wants to receive the routes advertised by Router A and vice versa, they must configure the following information:

Router A 

ip vrf CUSTOMER_A
rd 1:1
route-target export 1:1       <--- Exporting the routes using 1:1
route-target import 2:1       <--- Importing the routes using 2:1

Router B

ip vrf CUSTOMER_A
rd 1:1
route-target export 2:1     <--- Exporting the routes using 2:1
route-target import 1:1     <--- Importing the routes using 1:1

With that we will provide the prefixes at each other. 

In few words, the route target is used export or import routes and other routers must configure their VRFs if they want to receive or send the prefixes.