OSPF Route Type Attribute
The route type attribute carries the OSPF area ID and LSA type, as indicated in Table
66 on page 481:
Table 66: Route Types and Route Origins
Origin of RouteType of Route
Type 1 LSA1 – intra-area route
Type 2 LSA2 – intra-area route
Type 3 LSA3 – interarea summary route
Type 5 LSA5 – external route (area ID = 0)
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:
â– Originates in an OSPF domain that is different from that of the destination PE
router.
â– 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.
OSPF and BGP/MPLS VPNs â– 481
Chapter 5: Configuring BGP-MPLS Applications
To Next Page
Loading...
Need help?
General
