In this lesson we will talk about the Tunnels used for VPN Services that is set up via MPLS (Multiprotocol Label Switching). Here there are some key points that you must know to understand VPN Service structure well.
Think about that. Physically we have one port. In services structure, there are Transport Tunnels that is set up via LDP or RSVP-TE. As you know LDP and RSVP-TE are the two protocols that are used in MPLS as label switching protocols. Shortly, Transport Tunnels are the tunnels in which the label switching is done with Transport Labels. This is the Outher Label of the VPN packet. LSPs (Label Swithed Paths) are set up with these protocols.
As you know VPN packets has two labels. One is the classical label, here it is mentioned as Transport Label. And the other is for customer specific VPN label. This is mentioned as Service Label in here.
Service Labels (VPN labels) are set up with MP-BGP or TLDP (Targeted LDP). This label is also called as Inner Label. The packet transport across the service provider network is done with switching the Outher Label (Transport Label). But when the packet leaves the provider core, at the service provider edge, packet is routed acording to the Inner Label (Service Label or VPN label). Service Labels identifies the destination customer site which the VPN Service will arrive.
Here, it is important to define something about TLDP. TLDP sessions are the the LDP sessions that is between non directly connected peers.
VPLS and VPWSare the services that uses TLDP for signalling. But VPRN uses MP-BGP for signalling.
We can sum up the network signaling below. Think about the service provider core. There are PE routers at the edges and P routers in the middle. Between the PE routers the below steps must be done for all the routers.
An IGP is used between all the routers. This is for the reachability.
LDP or RSVP-TE is used to signal the Transport Labels. After the convergence of the LDP of RSVP-TE ans LSP is created.
TLDP or MP-BGP is used to signal the Service Labels. By doing this, it identifies each VPN Service.