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DES-7200 Configuration Guide Chapter 2 Configuring BGP IP VPN
2-42
Internal route
The internal routes refer to the routes inside the network of Second Carrier, namely the intra-AS
routes. The internal routes are used to guarantee the intercommunication of Second Carrier's
own network. Such routes must be jointly maintained by the First Carrier PE and the Second
Carrier.
External route
Since the Second Carrier is a service provider, its network may be connected to multiple
third-party networks. The route between Second Carrier and the third-party network is called
external route. If the Second Carrier provides traditional IP service for users, then the external
routes will include routes of user network; if the Second Carrier is connected to Internet, then the
external routes will include Internet routes; if the Second Carrier provides MPLS VPN service for
users, then the external routes will include user's VPN routes.
Generally, there are tremendous external routes. To maintain good scalability, the First Carrier
will not maintain external routes, which will be maintained independently by the Second Carrier.
VPN tunnel
VPN tunnel is the LSP tunnel established between VPN devices. In the CSC model, the LSP
tunnel between Second Carrier devices is the VPN tunnel.
2.3.3.2 Working principle
PE-CE route and label distribution
To achieve good scalability, the number of routes to be maintained by the First Carrier must be
reduced. Therefore, the CSC model hands over external route maintenance to the Second
Carrier, while the external traffic must use the VPN tunnel to cross First Carrier. To support CSC
model, the First Carrier PE must support VPN tunnel.
To support VPN tunnel, the First Carrier PE (CSC-PE) and the Second Carrier CE (CSC-CE)
must distribute the label binding information to each other. Depending on whether the CSC-PE
and CSC-CE are in the same autonomous system, the following routing protocols may be used
to exchange and distribute internal routes:
z If CSC-PE and CSC-CE are in the same autonomous system, IGP is generally used to
exchange internal routes, and LDP is used to exchange label binding information.