Figure 513 Backbone Carrier Supporting an ISP
m ■ £*usrior rt)u!0f inlwmation (IPv4 NLflJs} I6GP-* j iGP routes fpr ISP silas i and 2
iGP routes fpr ISP silas i and 2
In order to solve the scalability problem, the backbone carrier is configured so that it allows only internal routes of the customer carrier (IGP routes) to be exchanged between the CE routers of the customer carrier and the PE routers of the backbone carrier. MPLS is enabled on the interface between the CE router of the customer carrier and the PE router of the backbone carrier. Internal routes go to any of the routers within the ISP, and external routes go to the Internet. The number of internal routes is much smaller than the number of external routes. Restricting the routes between the CE routers of the customer carrier and the PE routers of the backbone carrier significantly reduces the number of routes that the PE router needs to maintain.
Because the PE routers do not have to carry external routes in the VRF routing table, they can use the incoming label in the packet to forward the customer carrier Internet traffic. Adding MPLS to the routers provides a consistent method of transporting packets from the customer carrier to the backbone carrier. MPLS allows the exchange of an MPLS label between the PE and the CE routers for every internal customer carrier route. The routers in the customer carrier have all the external routes either through IBGP or route redistribution to provide Internet connectivity.
Continue reading here: MPLS Redundancy Using HSRP
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