Juniper JUNOSE, Configuration Manual

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can provide finer-grained control of route distribution by associating any combination
of export, import, global export, and global import maps with VRFs. As shown in
Figure 91 on page 421, a route is distributed (leaked) between RIBs and its attributes
are changed as specified in the route map when the map returns an accept message.
If the map returns a deny message, then the route is not distributed.
Figure 91: Import and Export Maps
Both IPv4 and IPv6 VPNs are supported. You can specify that only IPv4 or only IPv6
routes are imported or exported. By default, the import or export map applies to
both kinds of routes. You can configure some maps to apply to IPv4 routes and
different maps to apply to IPv6 routes.
When the name or the contents of a route map change, BGP automatically waits for
a nonconfigurable hold-down interval of 30 seconds and then re-imports or re-exports
the appropriate routes using the modified route map.
Even when suppressed by an aggregate or auto-summary route, the more specific
routes are distributed. Aggregation and auto-summarization take place in each VRF
independently. For example, a route that is imported into a VRF is only aggregated
in that VRF if an aggregate address has been configured in the context of the BGP
address family for that VRF.
Routes maintain their type when exported. Private prefixes are exported without
being converted into public prefixes. Consequently the prefix of an exported route
is the same as the original route. Global export maps are therefore not useful when
NAT is enabled.
Characteristics of Import and Global Import Maps
Import maps and global import maps can import both labeled and unlabeled routes.
If you want to import only one or the other, you can use a
match mpls-label
command in the global import route map. Furthermore, if BGP imports labeled routes
from the global BGP non-VPN RIB into a VRF RIB and then advertises them further
upstream as labeled routes, the MPLS cross-connects are correctly created and MPLS
Configuring BGP VPN Services
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Chapter 5: Configuring BGP-MPLS Applications