1. OSPF Route Types

  • Intra-Area — In a multiarea OSPF network, routes that originate within an area are known by the routers in the same area as Intra-Area routes. These routes are flagged as O in the show ip route command output.
  • Inter-Area — When a route crosses an OSPF Area Border Router (ABR), the route is known as an OSPF Inter-Area route. These routes are flagged as O IA in the show ip route command output.
      • Both Intra and Inter-Area routes are also called OSPF Internal routes, as they are generated by OSPF itself when an interface is covered with the OSPF network  command.
  • External Type-2 or External Type-1 — Routes that were redistributed into OSPF, such as Connected, Static, or other Routing Protocol, are known as External Type-2 or External Type-1. These routes are flagged as O E2 or O E1 in the show ip route command output.
  • NSSA External Type-2 or NSSA External Type 1 — When an area is configured as a Not-So-Stub Area (NSSA), and routes are redistributed into OSPF, the routes are known as NSSA External Type-2 or NSSA External Type-1. These routes are flagged as O N2 or O N1 in the show ip route command output.
 

2. Redistribution of Only OSPF Internal (Intra and Inter-Area) Routes into BGP

If you configure the redistribution of OSPF into BGP without keywords, only OSPF intra-area and inter-area routes are redistributed into BGP, by default. Y
 

3. Redistribution of Only OSPF External (Type 1 and 2) Routes into BGP

Use the external  keyword along with the redistribute  command under router bgp  to redistribute OSPF external routes into BGP. With the external  keyword, you have three choices:
  • Redistribute both External Type-1 and Type-2 (Default)
  • Redistribute Type-1
  • Redistribute Type-2
Enter the commands in the configuration mode as described here: