Traffic leaking to Global Route Table (GRT) for the 7750 SR and 7950 XRS allows service providers to offer VPRN and Internet services to their customers over a single VRF interface. This supports IPv4 and, for the 7750 SR, requires the customer VPRN interfaces to terminate on a minimum of IOM3-XP and IMM hardware.
Packets entering a local VRF interface can have route processing results derived from the VPRN forwarding table or the GRT. The leaking and preferred lookup results are configured on a per VPRN basis. Configuration options can be general (for example, any lookup miss in the VPRN forwarding table can be resolved in the GRT), or specific (for example, specific route(s) should only be looked up in the GRT and ignored in the VPRN). In order to provide operational simplicity and improve streamlining, the CLI configuration is all contained within the context of the VPRN service.
This feature is enabled within the VPRN service context under config>service>vprn>grt-lookup. This is an administrative context and provides the container under which all specific commands can be entered, except for policy definition. Policy definitions remain unchanged but are referenced from this context.
The enable-grt command establishes the basic functionality. When it is configured, any lookup miss in the VRF table will be resolved in the GRT, if available. By itself, this only provides part of the solution. Packet forwarding within GRT must understand how to route packets back to the proper node and to the specific VPRN from which the destination exists. Destination prefixes must be leaked from the VPRN to the GRT through the use of policy. Policies are created under the config>router>policy-options hierarchy. By default, the number of prefixes leaked from the VPRN to the GRT is limited to five. The export-limit command under the grt-lookup hierarchy allows the service provider to override the default, or remove the limit.
When a VPRN forwarding table consists of a default route or an aggregate route, the customer may require the service provider to poke holes in those, or provide more specific route resolution in the GRT. In this case, the service provider may configure a static-route-entry and specify the GRT as the nexthop type.
The lookup result will prefer any successful lookup in the GRT that is equal to or more specific than the static route, bypassing any successful lookup in the local VPRN.
This feature and Unicast Reverse Path Forwarding (uRPF) are mutually exclusive. When a VPRN service is configured with either of these functions, the other cannot be enabled. Also, prefixes leaked from any VPRN should never conflict with prefixes leaked from any other VPRN or existing prefixes in the GRT. Prefixes should be globally unique with the service provider network and if these are propagated outside of a single providers network, they must be from the public IP space and globally unique. Network Address Translation (NAT) is not supported as part of this feature. The following type of routes will not be leaked from VPRN into the Global Routing Table (GRT):
Aggregate routes
Blackhole routes
BGP VPN extranet routes