RE: MPLS L3 VPN Questions

It's confusing because they use the same format, but in reality they are two completely unrelated attributes. The simplest explanation is that the Route Distinguisher (RD) makes the route unique, and the Route Target (RT) controls the VPN membership.

The reason you need the RD is that multiple customers can have the same IPv4 route, but should not have the same VPNv4 route. For example if two separate customers both advertise the IPv4 prefix 10.0.0.0/24 to the Service Provider, the SP needs to tell the difference between them in the BGP table. To accomplish this, the prefixes get separate RD values appended onto them, making unique VPNv4 routes. E.g. if the RDs are 1:1 and 2:2, the VPNv4 routes are 1:1:10.0.0.0/24 and 2:2:10.0.0.0/24, which means they are treated as unrelated routes in the BGP bestpath selection process.

The reason you need the RT is that the VRF membership assignment is only locally significant. The RT is basically a tag value that tells the remote PEs what VRFs a route is supposed to belong to. Additionally decoupling the RT from the RD allows the SP to form complex topologies, such as hub-and-spoke or central services VPNs in addition to full mesh. The "export" route target controls the tag as the route leaves the VRF and goes into BGP. The "import" route target controls what routes enter the VRF from BGP on the other side.

HTH,

Brian McGahan, CCIE #8593 (R&S/SP/Security)
bmcgahan_at_INE.com

Internetwork Expert, Inc.
http://www.INE.com
________________________________________
From: nobody_at_groupstudy.com [nobody_at_groupstudy.com] On Behalf Of Elliott Reyes [fontananetworkengineer_at_gmail.com]
Sent: Friday, June 03, 2011 12:01 PM
To: Cisco certification
Subject: MPLS L3 VPN Questions

Hey Guys,

       I'm just trying to get a better understanding of how to build these
MPLS VPN's properly and I guess the documentation and INE workbook just
confuses me a little bit. So if anyone can assist me that would be great.

Like Denzel in the movie Philadelphia. Explain it to me like I'm a 6 year
old.

1. When defining the VRF's between two PE routers.

Example

R1

Step 1 - Create VRF locally on R1

ip vrf VPN_A

 rd 100:1 <---- ROUTE DISTINGUISHER (This is locally significant to R1 and
add's a 8 byte value to IPV4 prefix to create a VPNIPV4 prefix. )

Step 2 - Assign Route-Target command
 route-target both 100:1 <----- allows routing information from to traverse
from R1 VPN_A to R2 VPN_A thru their connected interfaces and import/export
routing information.

R1 <----------> R2

int fa0/1 (R1)
 ip vrf forwarding VPN_A
 ip add 10.1.1.1 255.255.255.0

R2

Step 1 - Create VRF locally on R2

ip vrf VPN_A

 rd 100:1 <---- ROUTE DISTINGUISHER (This is locally significant to R2 and
add's a 8 byte value to IPV4 prefix to create a VPNIPV4 prefix. )

Step 2 - Assign Route-Target command
 route-target both 100:1 <----- allows routing information from to traverse
from R2 VPN_A to R1 VPN_A thru their connected interfaces and import/export
routing information.

Question - Does the route-target have to match the route distinguisher on
both ends. if the route-target is say 100:2 and the route distinguisher is
100:1 on VPN_A this technically shouldn't work
across the VPN correct.

R2 <----------> R1

int fa0/1 (R2)
 ip vrf forwarding VPN_A
 ip add 10.1.1.2 255.255.255.0

thanks

Blogs and organic groups at http://www.ccie.net
Received on Fri Jun 03 2011 - 16:47:18 ART