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Cisco 10000 Series Router Software Configuration Guide
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Chapter 4 Configuring Multiprotocol Label Switching
Half-Duplex VRF
The HDVRF feature addresses the limitations previously imposed on hub and spoke topologies by
removing the requirement of one VRF per spoke and ensuring that subscriber traffic always traverses the
central link between the wholesale service provider and the ISP, whether the subscriber traffic is being
routed to a remote network by way of the upstream ISP or to another locally or remotely connected
subscriber.
Figure 4-3
shows a sample hub and spoke topology for HDVRF.
Figure 4-3
Hub and Spoke Topology for Half-Duplex VRF
The Half-Duplex VRF feature is described in the following topics:
•
Upstream and Downstream VRFs, page 4-22
•
Reverse Path Forwarding Check Support, page 4-23
•
Feature History for Half-Duplex VRF, page 4-23
•
Restrictions for Half-Duplex VRF, page 4-23
•
Prerequisites for Half-Duplex VRF, page 4-23
•
Configuration Tasks for Half-Duplex VRF, page 4-24
•
Configuration Examples for Half-Duplex VRF, page 4-26
•
Monitoring and Maintaining Half-Duplex VRF, page 4-29
Upstream and Downstream VRFs
HDVRF uses two unidirectional VRFs, called upstream VRF and downstream VRF, to forward IP traffic
between the spokes and the hub PE router.
The upstream VRF is used to forward the IP traffic from the spokes toward the MPLS VPN backbone.
This VRF typically contains only a default route; but, depending on the configuration, it might also
contain such information as summary routes and multiple default routes. The default route points to the
interface on the hub PE router that connects to the upstream ISP. The Cisco 10000 series router
dynamically learns about the default route from the routing updates that the hub PE router or home
gateway sends. The upstream VRF also contains the virtual access interfaces that connect the spokes,
but it contains no other local interfaces.
The downstream VRF is used to forward the traffic from the MPLS core back to the spokes. This VRF
contains PPP peer routes for the spokes and per-user static routes imported from the authentication,
authorization, and accounting (AAA) server. It also contains the routes imported from the hub PE router.
These routes are the dynamically allocated virtual access interfaces of the subscribers associated with a
particular service.
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CE1
CE2
ISP
MPLS Core
Spoke PE
Router
P Router
Hub PE
Router
Hub CE
Router
Spokes
