What is the real significance of Route distinguisher?

[karthivedhu]

Please refer the attached exhibit and let me know what is the real significance of Route distinguisher in a MPLS core.
Router sitting in middle is a vpnv4 RR and all the PE's are client to the Vpnv4 RR
Whats the significance of RD in PE4.
How the prefix 10.1.0.0/24 vpnv4 route be present in the vpnv4 table of PE4 and which entry will qualify into the VRF D routing table of PE4.

Since import rt 1:1 & 1:2 given at PE4, which of the below received vpnv4 prefix would be qualified as winner in to the VRF D routing table of PE4.

< rd:ipv4-prefix:rt> < rd:ipv4-prefix:rt>
<1:1:10.1.0.0/24:1:1> & <1:2:10.1.0.0./24:1:2>

The reason why i ask is, RD is said to be local to the router(as per mpls book terms). But when i tried the above scenario @ PE4 on giving the command
sh ip bgp vpnv4 all
I could see
Route Distinguisher 1:1 (default vrf for VRF D)
*>10.1.0.0/24
Route Distinguisher 1:2 <--- @PE4 why an entry like this is created under vpnv4 table(How has RD of PE3 influenced this)
*>10.1.0.0/24

Many thanks in advance.

[singhkulbir29881]

Hi karthivedhu,

The Route Distinguisher is used to keep all prefixes in the BGP table unique. Let says Router A has two vrf's and same subnet is running behind both vrf's. When Router A sends an advertisement to it peer (Lets say router B). Then it will send an advertisement which includes the RD as tag with both the routes so that router B comes to know that both the routes are unique not the same advertisement sent by router A twice.

[karthivedhu]

Thanks for the quick response my friend.

But my question was 

10.1.0.0/24 -->vrf A rd 1:1 , imp&exp (rt 1:1) (PE1)        }

}  ---> (VPNV4-RR) ---> (PE3) vrf C rd 1:1                                       imp rt 1:1 imp rt   2:1

10.1.0.0/24 -->vrf B rd 2:1, imp&exp  (rt 2:1) (PE2)        }

So now in the above set up, both vpnv4 prefix

(a.)1:1: 10.1.0.0/24, 

(b) 2:1:10.1.0.0/24 

will be received at PE3, which route it will install in vrf C table?

[singhkulbir29881]

It depends on RT import you configured for vrf C. 

In vrf D you configured RT import both 1:1 &1:2 and for vrf A and vrf B you configured RD & RT values same so PE4 has nothing to distinguish between between routes advertising by both PE1 & PE2. It will assume that both advertisements are same and select the best path between two and for PE3 it has RD different (1:2) so it keeps that routes under RD 1:2

Go through the following link to understand the these terms more clearly:-

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

If this is helpful please rate thin or if you got you answer then marked it as answered. 

[karthivedhu]

Both PE 1 & PE 2 have same subnet 10.1.0.0/24, 

at PE 1 -Vrf A has rd = 1:1 & both imp & exp rt = 1:1

&

at PE 2 -Vrf B has rd = 2:1 & both imp & exp rt = 2:1

So both these PE's will advertise the route to Vpnv4 RR, and in return VPNv4 RR advertises these routes to PE 3, as PE 3 has both import rt 1:1 & 2:1 , 

what route will be in PE3 vpnv4 table and which route will be imported into its vrf C table.

Ultimately, what I'm trying to ask is, what if same  LAN subnet are assigned for 2 branches of a same customer

[singhkulbir29881]

So vrf A (PE1)

RD 1:1

imp & exp rt 1:1

subnet 10.1.0.0/24

vrf B (PE2)

RD 2:1

imp & exp rt 2:1

subnet 10.1.0.0/24

vrf D (PE4)

RD 1:1

imp & exp rt 1:1 & 2:1

subnet 10.1.0.0/24

vrf C (PE3)

RD 2:1

imp rt 1:1 & 2:1

subnet 10.1.0.0/2

Assume that PE1 & PE 2 routes are best routes for vpnv4RR.

So in this case, as PE3 has same RD as PE2 so it will discard the PE2 routes and keep his own route under RD 2:1 and keep PE1's route under 1:1 but in this case BGP also plays a major role. Vpnv4 RR will only reflects the routes to PE3 that are best routes for it. So, Verify the BGP best routes too.

If this was helpful please rate.

[ROHANTARDEKAR]

yes you will see both prefix in PE3 VPNV4 table

use command show ip bgp vpnv4 vrf C 10.1.0.0 (you will see two routes one with RD 1:1 and other one with RD 2:1)

But in IPV4 table you will get only one route installed depending upon BGP route selection criteria

if nothing is changed then most probably route selection will go on IGP cost to next-hop