Information Technology Reference
In-Depth Information
Figure 10-26 Full Internet Routes via VPN
iBGP IPv4
AS 100
VPNa
Site1
AS 100
VPNa
Site2
ASBR2
ASBR1
IGP + Label
IGP + Label
CE1
CE2
iBGP VPNv4
PE1
PE2
AS 200
Backbone Carrier
eBGP IPv4
eBGP IPv4
AS 300
Enterprise
Customer
AS 400
Upstream
ISP
ASBR3
ASBR4
It is important to note here that the backbone carrier needs to carry two types of routes:
Full Internet routes that are carried between ASBR1 and ASBR2
IGP routes that are used to provide reachability within AS 100
The CSC solution is to extend the label switching from the PE devices' VRF interfaces to
CE devices. Both CE devices advertise internal IGP routes to PE devices using an IGP or
BGP, which redistribute them into multiprotocol iBGP to be advertised to the remote PE
devices. Additionally, PEs and CEs exchange labels for these routes using LDP/TDP or
BGP. Thus, an LSP is formed from CE1 to CE2, which completes the end-to-end LSP from
ASBR1 to ASBR2.
An iBGP session is used between ASBR1 and ASBR2 to exchange the full Internet routes.
Because of the end-to-end LSP, these routes do not need to be carried in any other
routers. As a result, PE devices need to carry iBGP next-hop reachability information
only for AS 100, not the full Internet routes.
If AS 100 is not running MPLS, all routers in AS 100 must be BGP speakers and peer with
each other in iBGP full mesh. Alternatively, RRs can be used to increase scalability.
NOTE
Figure 10-27 shows prefix and label distribution. The BGP prefix 172.16.0.0/16 (simulating
an Internet route) is received by ASBR2, which advertises, with the next hop reset to itself
(192.168.100.4), to ASBR1. For ASBR1 to advertise the prefix to ASBR3 in AS 300, it
needs to know how to reach ASBR2, the BGP next hop.
 
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