Hi,
In a MPLS L3VPN many customer sites and many customers are connected to the same SP. The SPs routers use BGP (MP-BGP with VPNv4) to distribute the customer routes amongst each other. In simple terms:
The problem to be solved: which route should be seen where? How does one router know, if an update belongs to customer A or customer B (or both)?
This is the purpose of the route-targets.
If an SP router (PE) inserts a customer route into BGP your "route-target export" statement will add the specified route-targets. As described in the previous post, they are BGP extended communities and will be sent in the BGP updates.
Think of them as tags indicating a certain route property, like VPN membership, "belongs to customer A" and such.
If a PE receives an update from other PE routers it will check to which customer(s) (precisely: which VRF(s)) this update belongs.
The "route-target import" statements mean: "In case an update has the route-target X:Y then it belongs to this customer (i.e. VRF)".
In brief:
- the PE taking the route into BGP marks with RTs (route-target export)
- the other PEs look for that RT (route-target import) to know who should get which route.
Hope this helps! Please rate all posts.
Regards, Martin
