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How are external routes and internal routes generated in ospf?


Asked by Aylin Craig on Dec 11, 2021 FAQ



External routes are generated by an ASBR, while internal routes can be generated by any OSPF router. Note the peculiar case of type 4 LSAs. These LSAs are injected into the backbone by the ABR of an area which contains an ASBR. This is to ensure all other routers in the OSPF domain can reach the ASBR.
Accordingly,
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 Routes: They fall under two categories, External Type-1 and External Type-2.
Furthermore, Routes which 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. Type 6: Group membership link entry generated by multicast OSPF routers.
Besides,
This LSA is circulated throughout the OSPF domain except for Stub, Totally Stubby and Not-so-stubby areas. External routes are propagated through an OSPF area as a type 5 from an ASBR, or type 7 LSA from an ASBR in an NSSA. These routes from outside the OSPF domain are can either be E1 or E2 routes and they are treated in slightly different ways.
Subsequently,
OSPF external routes in a normal area would appear as either E1 routes or E2 routes. By default when we configure the redistribution of external routes in an OSPF area the route type for the external network is E2 which appear as OE2 in the routing table.