Information Technology Reference
In-Depth Information
Table 10-7
Comparison and Contrast of Four Inter-AS Options (Continued)
Inter-AS
Label
Inter-AS
Filtering
Option
Complexity
Scalability
Non-VPN
transit AS
IPv4 labels
IPv4 addresses and
labels should be
filtered for inbound
and outbound.
Similar to the complexity
of multihop eBGP for
VPNv4, but there is the
added complexity of
coordinating among
three autonomous
systems.
High
VPNv4 routes are
exchanged between RRs.
ASBRs are not involved
in VPNv4 information
exchange.
Carrier Supporting Carrier VPN
So far, the MPLS VPN discussion has assumed that VPN customers are end customers—
that is, they are not service providers or carriers. In a case where VPN customers are
themselves carriers, the resource use on PEs can increase significantly.
Consider the scenario shown earlier in Figure 10-14. If the two customer carriers are ISPs
that carry full Internet routes between them, PE devices must hold these routes in a VRF. If
more than one VPN customer is an ISP, the resources (memory and CPU) on PE devices
become a severely limiting factor. This VPN model obviously is not scalable.
The CSC VPN model is developed for this purpose. Two scenarios are discussed in this
section:
•
Customer carriers exchange full Internet routes using a common VPN via the back-
bone carrier
•
Customer carriers provide VPN services themselves via a common VPN from the
backbone carrier, or hierarchical VPN
CSC for Full Internet Routes
Consider Figure 10-26, where AS 200 is the backbone VPN carrier that provides two-site
connectivity for AS 100. AS 100 Site 2 receives full Internet routes from an upstream
ISP, AS 400. An enterprise customer, AS 300, receives full Internet routes from its provider,
AS 100, via Site 1. Within AS 100 (both sites), LDP is enabled on links among all the
routers, and an IGP is used to advertise reachability for links and loopback addresses.
