- Bronze, 100 points or more
1. What is a neighbor on a local LAN segment in OSPF?
a. A neighbor is a router in the same area
b. A neighbor is a router in the same classful network
c. A neighbor is a router on a multiaccess link, with an adjacency with a DR
d. A neighbor is another router with the same network address
8. In learning a new route, what will an internal OSPF router do if a received LSA is not found in the topological database?
a. The LSA is flooded immediately out of all the OSPF interfaces, except the interface from which the LSA was received.
b. The LSA is dropped and a message is sent to the transmitting router.
c. The LSA is placed in the topological database and an acknowledgment is sent to the transmitting router
d. The sequence numbers are checked, and if the LSA is valid, it is entered into the topology database.
These are the only two questions I got wrong on the chapter quiz, except I still kinda stand by my answers. I'd like to see how others weigh in on this, to see if I'm really misunderstanding something or if the book is wrong (which these BSCI prep books so often are...)
The books do need editing. However, your last post refers to a different OSPF procedure than the 2nd question posted initially. An "unrecognized/unknown LSA" is different from an LSA not already installed in the link-state database. An "unrecognized/unknown LSA" is an LSA with an LS-Type not supported by the OSPF implementation of a particular router (e.g. not one of the LS types 1-5). (Unless I misunderstood something, and you are referring to a question in the book you did not post in this thread.) Anyway, the phrase "unknown LSA" can be misleading.
For question 1, the answers a, b are obviously wrong, so we are left with c,d. The defence I can think of for the book answer (that is answer d) is the following: c is wrong because the router that achieved adjacency with a DR, is not necessarily our neighbor, unless our interface is up/up and we follow proper Hello procedures, etc. Questions involving 'neighbors' are often a mess because no single router is a neighbor on its own. You need 2 routers to define what a neighbor is. Answer d is better in this respect. To be honest, when I first read answer d I thought it was referring to a router having the exact IP address as we do, so I thought of it as being very wrong! Only after the process of elimination did I understand that it refers to the "same subnet" criterion for neighbors. Of course, other criteria have to be met as well as others have said.
The second question needs more work. b is obviously wrong. d is also wrong because sequence numbers are irrelevant in this case (LSA is not in the database, we are not trying to see if it is more recent than some other in the database), and an LS sequence number check is not a validity check to solely determine installation of the LSA. So, we are left with a and c.
Answer c is not completely wrong, but the phrase "transmitting router" suggests a "direct acknowledgement" procedure, which is used in special cases (sending ack immediately to the neighbor's IP address when a duplicate LSA is received) and not always, while also acks can be sent as multicasts. See RFC 2328 (Section 13.5. Sending Link State Acknowledgment packets).
RFC 2328 (Section 13. The Flooding Procedure) says: "(5) Otherwise, find the instance of this LSA that is currently contained in the router's link state database. If there is no database copy, or the received LSA is more recent than the database copy...the following steps must be performed:
(a) If there is already a database copy ...
(b) Otherwise immediately flood the new LSA out some subset of the router's interfaces (see Section 13.3). In some cases (...) the LSA will be flooded back out the receiving interface. This occurrence should be noted for later use by the acknowledgment process (Section 13.5)...
(e) Possibly acknowledge the receipt of the LSA by sending a Link State Acknowledgment packet back out the receiving interface."
Note point (e) above, especially the words "possibly" and "back out receiving interface", which does not necessarily mean "to the transmitting router". I am not saying answer a is always true (see point b above), but it is more generally true, which possibly makes it a better answer than the others.