Which of the following is true of external LSAs in the Junos OS?
A.
External LSAs are of Type 4.
B.
Type 2 external LSAs do not consider internal reachability metrics.
C.
The route preferences for Type 1 and Type 2 External LSA is 150 and 155, respectively.
D.
Type 7 LSAs are never allowed in not-so-stubby areas.
Explanation:
Operation of OSPFAt a very high level, the operation of OSPF is easily explained:
1. OSPF-speaking routers send Hello packets out all OSPF-enabled interfaces. If two routers sharing a common data link agree on certain parameters specified in their respective Hello packets, they will become neighbors.
2. Adjacencies, which can be thought of as virtual point-to-point links, are formed between some neighbors. OSPF defines several network types and several router types. The establishment of an adjacency is determined by the types of routers exchanging Hellos and the type of network over which the Hellos are exchanged.
3. Each router sends link-state advertisements (LSAs) over all adjacencies. The LSAs describe all of the router’s links, or interfaces, the router’s neighbors, and the state of the links. These links might be to stub networks (networks with no other router attached), to other OSPF routers, to networks in other areas, or to external networks (networks learned from another routing process). Because of the varying types of link-state information, OSPF defines multiple LSA types.
4. Each router receiving an LSA from a neighbor records the LSA in its link-state database and sends a copy of the LSA to all of its other neighbors.
5. By flooding LSAs throughout an area, all routers will build identical link-state databases.
6. When the databases are complete, each router uses the SPF algorithm to calculate a loop-free graph describing the shortest (lowest cost) path to every known destination, with itself as the root. This graph is the SPF tree.
7. Each router builds its route table from its SPF tree