Cisco CCIE Service Provider Written Exam: Tier 1 Service Provider Backbone Network Design

Tier 1 Service Provider Backbone Network Design


A Tier 1 service provider is implementing a backbone network to support connectivity for multiple Tier 2 service providers, by using a Carrier Supporting Carrier design.

Each Tier 2 service provider requires the ability to implement MPLS through the Tier 1 backbone.

What occurs with respect to traffic for the Tier 2 providers?



Click on the arrows to vote for the correct answer

A. B. C. D.


In a Carrier Supporting Carrier (CSC) design, a Tier 1 service provider connects multiple Tier 2 service providers and provides them with connectivity and transit services. Each Tier 2 service provider can use the Tier 1 backbone network to transport traffic for its customers, and may also require the ability to implement MPLS in order to provide virtual private network ( VPN) services.

Answer A suggests that the Tier 2 providers would use MPLS over GRE tunnels to create an LSP (Label Switched Path) through the Tier 1 backbone. This would involve encapsulating the MPLS packets in GRE (Generic Routing Encapsulation) packets, which would then be transported across the backbone. While this approach is technically possible, it may not be the most efficient or scalable way to implement MPLS in a CSC design, as it would require the creation and management of multiple tunnels for each customer VRF (Virtual Routing and Forwarding) instance.

Answer B suggests that connectivity between Tier 2 providers would be established via a full mesh of MPLS L2 VPN (Layer 2 Virtual Private Network) circuits. This approach would involve configuring each Tier 2 provider to create a full mesh of L2 VPN circuits with all other Tier 2 providers in the CSC, allowing for direct communication between any two customer sites across the backbone. While this approach would provide optimal connectivity and flexibility, it may also be complex to configure and maintain, especially as the number of Tier 2 providers and customers grows.

Answer C suggests that Tier 2 traffic across the backbone network would have an additional label pushed onto the existing MPLS label stack. This is a more likely scenario, as it would allow for MPLS labels to be stacked and swapped at each hop across the backbone, while also providing the necessary VPN functionality for the Tier 2 providers. Each Tier 2 provider would configure its routers to use a unique VPN label for its customers, which would be exchanged with the Tier 1 provider using MP-BGP (Multiprotocol Border Gateway Protocol) and added to the existing MPLS label stack as the packets traverse the backbone.

Answer D suggests that the Tier 1 provider would implement each Tier 2 provider into the backbone via BGP (Border Gateway Protocol) into the default global routing table. While this approach is possible, it may not provide the necessary isolation and security for the Tier 2 providers, as their routes would be visible to all other providers and customers in the backbone. It is more likely that each Tier 2 provider would be implemented into the backbone using a separate VRF, which would provide a private routing and forwarding table for each provider's customers.