How MPLS Layer 3 VPN Works

The previous lesson ended on an open problem.
Two customers share the same provider network, and nobody coordinates their private address ranges.
That is what you are going to solve here.
One Routing Table Is Not Enough
Look at the four customer sites once more.
Figure 1 – Two customers share the same provider network
Three things to notice on this figure:
Customer 1 uses 10.1.1.0/24 and 10.2.1.0/24
Customer 2 uses 10.1.2.0/24 and 10.2.2.0/24
Both companies land on the same two PE routers
Your PE router has a single routing table.
If both customers use the same private subnet, the routing table has no way to keep both routes distinct.
The two companies end up mixed together, and a provider selling private connectivity can never allow that.

Answer the question below

Two customers advertise the same subnet. Can one single routing table hold both routes?

One VRF per Customer
To solve this, each PE router keeps one VRF per customer: a separate routing table for every customer it connects.
Figure 2 – Each PE keeps one VRF per customer
PE-1 and PE-2 each hold two independent tables, VRF C1 and VRF C2, one for each customer they serve.
A VRF (Virtual Routing and Forwarding) instance is a routing table that sits beside the global routing table.
The CE exchanges routing information with the PE, and the PE stores those routes in the VRF dedicated to that customer.
Since each VRF is its own routing table, overlapping addresses are no longer a problem: every customer can advertise its private routes and use whatever address space it wants.

Answer the question below

A PE keeps one _______ per customer
Your PE keeps the two customers apart locally.
But a table on PE-1 is useless to a branch office behind PE-2, at the other end of the core.
MP-BGP Between PE Routers
The IGP already runs on every router of the P-network and reaches all of them. Why not let it carry the customer routes too?
Two reasons stand in the way:
the IGP cannot tell which route belongs to which customer, so identical subnets collide again
every P router would start carrying customer prefixes
A provider can serve hundreds of customers over the same core, so the core IGP carries no customer routes at all and stays small enough to converge quickly.

Answer the question below

Do P routers hold customer routes?

The provider needs a protocol built to carry huge numbers of routes, with extra information attached to each one.
There is one obvious candidate for that job.
PE-1's VRF C1 and PE-2's VRF C1 have had no connection between them until now.
P1, P2 and P3 stay exactly as they are below — the new piece is that missing connection, at the edges.
Figure 3 – PE-1 and PE-2 open an MP-BGP session
In the figure above, PE-1 and PE-2 open an MP-BGP (Multiprotocol BGP) session that runs from one edge of the domain to the other, while P1, P2 and P3 take no part in it.
What Each PE Does
Each PE feeds the routes in its VRFs into that session.
The PE at the far end takes them back out and installs them in the matching VRF.
P routers in the middle keep doing exactly what they did in the previous lessons: swap labels, forward packets.

Answer the question below

Only the _______ routers run MP-BGP