Table 93: Route Types and Route Origins (continued)
Origin of RouteType of Route
Type 7 LSA7 – external route (area ID = 0)
MP-BGP uses the route type conveyed by this extended community attribute to determine
the best OSPF route when it installs the routes in the VRF forwarding table on the
destination PE router.
Distributing OSPF Routes from PE Router to CE Router
At the remote PE site, MP-BGP converts the OSPF routes to BGP VPN-IPv4 routes and
sends them across the BGP/MPLS VPN backbone. At the destination PE router, MP-BGP
must redistribute the BGP VPN-IPv4 routes back into OSPF IPv4 routes. The PE OSPF
router becomes the originator of the routes, which are either type 5 external routes or
type 3 internal routes. The PE router can announce the OSPF routes to the appropriate
CE router through its directly connected PE-CE OSPF link.
If the route has a route type of inter or intra, it is redistributed as a type 3 summary interarea
route and the destination PE router generates a type 3 LSA for it.
A route is redistributed as an external route if the route:
•
Originates in an OSPF domain that is different from that of the destination PE router.
•
Has a route type of 5 or 7, both of which indicate an external route.
In the first case, the PE router advertises the route as an external type 2 route. In the
second case, the PE router advertises the route as an external type 2 route if the
least-significant bit is set in the option byte in the route type extended community
attribute; otherwise the PE router advertises the route as external type 1 route.
Preventing Routing Loops
PE routes disregard OSPF routes received from a CE router if the routes are advertised
by:
•
A type 3 LSA with the most-significant bit set in the LSA options field.
•
A type 5 LSA that has a tag value equal to the VPN route tag associated with the OSPF
VRF on that PE router.
When the destination PE router originates a type 3 LSA learned from BGP to a CE router,
the PE router sets the most-significant bit in the LSA options field to identify the LSA as
being generated from a PE router. Doing this prevents the LSA from being passed back
to the BGP/MPLS VPN through a different PE router.
When the destination PE router originates a type 5 LSA learned from BGP to a CE router,
the PE router replaces the external route tag in the LSA with the VPN route tag. You
configure the VPN route tag for the OPSF VRF on the PE router with the domain-tag
command. The value of a VPN route tag must be unique within an OSPF domain, so that
481Copyright © 2010, Juniper Networks, Inc.
Chapter 6: Configuring BGP-MPLS Applications
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