Information Technology Reference
In-Depth Information
repeated occurrences by adding an optional number after this command. This number helps
suppress routing information loops, because updates containing more occurrences of its AS
number are denied.
When the prefix is advertised from PE2 to CE2, the AS_PATH is 100 100 100 100. This is
because AS Override configured on PE2 replaces 65000 with 100.
VPNs Across AS Borders
The architecture of MPLS VPN presented so far requires that a single provider offer the
VPN service. PE devices use multiprotocol iBGP to exchange customer VPNv4 prefixes.
As the deployment of MPLS VPN expands, the single-provider requirement becomes a
restriction.
Consider the scenario shown in Figure 10-13. A customer (VPNa) has two sites, Site 1 and
Site 2, that need VPN connectivity. However, Site 1 is connected to AS 100, whereas Site 2 is
connected to AS 200. Both providers support MPLS VPN. For the MPLS VPN connectivity
between the two sites to work, the two providers need to exchange multiprotocol eBGP
information between the two AS border routers (ASBRs). This is called inter-AS VPN ,
where providers are in a peer-to-peer relationship.
Figure 10-13 MPLS VPN over Two Provider Backbones
AS 200
AS 100
PE1
ASBR 1
ASBR 2
PE2
VPNa
Site 2
VPNa
Site 1
CE2
CE1
As another example, consider Figure 10-14. The same VPN connectivity is required between
Site 1 and Site 2, as in Figure 10-13, but providers AS 100 and AS 200 do not have direct
connectivity between them. Instead, they both connect to another provider, AS 300, which
provides VPN services. Additionally, providers AS 100 and AS 200 intend to exchange full
Internet tables between them. This is called Carrier Supporting Carrier (CSC), or carrier's
carrier VPN, where providers are in a client/server relationship. Provider AS 300 is called
the backbone carrier, and AS 100 and AS 200 are called customer carriers.
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