Ch 9: Advanced OSPF Flashcards
T/F: A router with an interface associated with Area 1 and Area 2 will be able to inject routes learned from one area into another area.
False.
A router needs to have an interface in Area 0 so that it can be an ABR.
T/F: A member router contains a complete copy of the LSDBs for every area in the routing domain.
False.
An OSPF router only contains copies of the LSDBs for the areas it participates in.
How many OSPF link-state announcement (LSA) types are used for routing traditional IPv4 packets?
a. Two
b. Three
c. Five
d. Six
e. Seven
D.
OSPF uses six OSPF LSA types for routing IPv4 packets (Types 1, 2, 3, 4, 5, and 7). Additional LSAs exist for IPv6 and MPLS.
What is the LSA age field in the LSDB used for?
- For version control—to ensure that the most recent LSA is present
- To age out old LSAs by removing an LSA when its age reaches zero
- For troubleshooting—to identify exactly when the LSA was advertised
- To age out old LSAs by removing an LSA when it reaches 3600 seconds
4.
LSAs are deemed invalid when they reach 3600 seconds and are purged from the LSDB.
Which LSA type exists in all OSPF areas?
- Type 1, Router LSA
- Type 2, Network LSA
- Type 3, Summary LSA
- AS external
Type 1.
A router LSA (type 1) is associated with each OSPF-enabled interface.
T/F: When an ABR receives a network LSA, the ABR forwards the network LSA to the other connected areas.
False.
Network LSAs (type 2) are not advertised outside the originating area. They are used with router LSAs (type 1) to build the summary LSA (type 3).
When a type 3 LSA is received in a nonbackbone area, what does the ABR do?
- Discards the type 3 LSA and does not process it
- Installs the type 3 LSA for only the area where it was received
- Advertises the type 3 LSA to the backbone area and displays an error
- Advertises the type 3 LSA to the backbone area
2.
Type 3 LSAs received from a nonbackbone area only insert into the LSDB for the source area. ABRs do not create type 3 LSAs for the other areas.
T/F: OSPF uses the shortest total path metric to identify the best path for every internal OSPF route (intra-area and interarea).
False.
OSPF prefers intra-area routes over interarea routes as the first logic check. In the event that both paths use the same type, the total path metric is used.
T/F: Breaking a large OSPF topology into smaller OSPF areas can be considered a form of summarization.
True.
While the number of network prefixes might remain the same, the numbers of type 1 and type 2 LSAs are reduced.
How is the process of summarizing routes on an OSPF router accomplished?
- By using the interface configuration command summary-address network prefix-length
- By using the OSPF process configuration command summary-address network prefix-length
- By using the OSPF process configuration command area area-id range network subnet-mask
- By using the interface configuration command area area-id summary-address network subnet-mask
3.
OSPF summarization occurs at the area level and is configured under the OSPF process.
OSPF supports filtering of routes using which of the following techniques? (Choose two.)
- Summarization, using the no-advertise option
- LSA filtering, which prevents type 1 LSAs from being advertised through a member router
- Area filtering, which prevents type 1 LSAs from being generated into a type 3 LSA
- Injection of an OSPF discard route on the router that filtering should apply
1 and 3.
LSA filtering occurs on the ABR and can occur with summarization (using the no-advertise keyword) or with area filtering (preventing the Type 3 LSAs from entering into the new area).
T/F: An An OSPF interface can belong to more than one area.
False.
T/F: All routers within the same OSPF area maintain an identical copy of the link-state database (LSDB).
True.
T/F: The area ID is not included in the OSPF hello packet.
False. It is included with the Hello packet and required to form an adjacency.
What happens in OSPF when a topology change occurs (such as a link flap or an additional network being added) within an area?
All routers in the same OSPF area calculate the SPF tree again. This takes time and a lot of CPU.
Routers outside that area do not calculate the full SPF tree again but perform a partial SPF calculation if the metrics have changed or a prefix is removed.
T/F: Routers that are connected to more than one area will automatically inject routes between areas.
False.
Just because a router connects to multiple OSPF areas does not mean the routes from one area will be injected into another area. Figure 9-1 shows router R1 connected to Area 1 and Area 2. Routes from Area 1 will not advertise into Area 2 and vice versa.
What is Area 0?
- Area 0 is a special area called the backbone.
- By design, all areas must connect to Area 0 because OSPF expects all areas to inject routing information into the backbone.
- Area 0 advertises the routes into other areas.
- The backbone design is crucial to preventing routing loops.
What is an ABR?
Area border routers (ABRs) are OSPF routers connected to Area 0 and another OSPF area, per Cisco definition and according to RFC 3509.
ABRs are responsible for advertising routes from one area and injecting them into a different OSPF area. Every ABR needs to participate in Area 0; otherwise, routes will not advertise into another area. ABRs compute an SPF tree for every area that they participate in.
Figure 9-2 shows that R1 is connected to Area 0, Area 1, and Area 2. R1 is a proper ABR because it now participates in Area 0. The following occurs on R1:
- Routes from Area 1 advertise into Area 0.
- Routes from Area 2 advertise into Area 0.
- Routes from Area 0 advertise into Area 1 and 2. This includes the local Area 0 routes, in addition to the routes that were advertised into Area 0 from Area 1 and Area 2.
T/F: ABRs compute an SPF tree for every area that they participate in.
True.
Some questions on the Area ID…
- How many bits is the Area ID in OSPF?
- How is it formatted?
- Can the different definition formats form adjacencies?
- In what format is the Area ID advertised in Hello packets?
The area ID is a 32-bit field and can be formatted in simple decimal (0 through 4,294,967,295) or dotted decimal (0.0.0.0 through 255.255.255.255).
During router configuration, the area can use decimal format on one router and dotted-decimal format on a different router, and the routers can still form an adjacency. OSPF advertises the area ID in dotted-decimal format in the OSPF hello packet.
What is an ‘intra-area route’?
Network routes that are learned from other OSPF routers within the same area are known as intra-area routes.
In Figure 9-3, the network link between R2 and R4 (10.24.1.0/29) is an intra-area route to R1. The IP routing table displays OSPF intra-area routes with an O.
What is an interarea route?
Network routes that are learned from other OSPF routers from a different area using an ABR are known as interarea routes. In Figure 9-3, the network link between R4 and R5 (10.45.1.0/24) is an interarea route to R1. The IP routing table displays OSPF interarea routers with O IA.
Example 9-3 provides the routing table for R1 from Figure 9-3. Notice that R1’s OSPF routing table shows routes from within Area 1234 as intra-area (O routes) and routes from Area 0 and Area 56 as interarea (O IA routes).
T/F: Crossing a slow serial link will cause the route metric to increase dramatically.
This is true. Routes that must cross a slow serial link, which have an interface cost of 64, are must less desirable.
How are routes from outside the OSPF domain injected into the OSPF domain?
External routes are routes learned from outside the OSPF domain but injected into an OSPF domain through redistribution.
External OSPF routes can come from a different OSPF domain or from a different routing protocol.
External OSPF routes are beyond the scope of the CCNP and CCIE Enterprise Core ENCOR 350-401 exam and are not covered in this book.