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Chapter 2. Understanding Quality of Serv... > CUCM CAC and GK: Hub-and-Spoke/Fully...

CUCM CAC and GK: Hub-and-Spoke/Fully Meshed MPLS

When the underlying WAN is Multiprotocol Label Switching (MPLS) based, it is important to consider some important differences in how CAC should be provisioned over an MPLS WAN compared to how it is provisioned over more traditional (Layer 2) WANs.

First, although it is possible for service providers to configure a true hub-and-spoke customer routing topology over an MPLS Layer 3 VPN, most service providers instead provision an any-to-any routing and connectivity model over which customer traffic flows. The fact that all sites have direct connectivity can present a few challenges as far as CAC is concerned.

Customer edge (CE) routers in an MPLS Layer 3 VPN peer only with provider edge (PE) routers, not with each other. Also, PE routers use MP-BGP to advertise customer routes to each other (irrespective of any PE-CE routing protocol). So, the service provider MPLS network is invisible as far as customer routing is concerned. Although customer traffic is IP routed between CE and PE devices, it is label switched across the service provider’s MPLS backbone; so for all intents and purposes, the service provider MPLS backbone is invisible to customer routing and traffic (assuming careful configuration by the service provider). However, knowledge of the underlying service provider network topology is not required to provision CAC.


  

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