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.
T/F: ASBRs advertise type 1 LSAs so that the DROTHER routers can build their LSDBs. DROTHER routers only receive LSAs.
Every OSPF router advertises a type 1 LSA.
Type 1 LSAs are the essential building blocks within the LSDB. A type 1 LSA entry exists for each OSPF-enabled link (that is, every interface and its attached networks). Figure 9-5 shows that in this example, the type 1 LSAs are not advertised outside Area 1234, which means the underlying topology in an area is invisible to other areas.
NOTE: Type 1 LSAs for an area are shown with the command show ip ospf database router.
Examine the diagram. How many DR segments are there in Area 1234?
Area 1234 has only one DR segment that connects R1, R2, and R3 because R3 has not formed an OSPF adjacency on the 10.3.3.0/24 network segment. On the 10.123.1.0/24 network segment, R3 is elected as the DR, and R2 is elected as the BDR because of the order of the RIDs.
NOTE: Detailed type 2 LSA information is shown with the command show ip ospf database network.
Now that we have the type 2 LSA for Area 1234, all the network links are connected. Figure 9-8 provides a visualization of the type 1 and type 2 LSAs, which correspond with Area 1234 perfectly.
NOTE: When the DR changes for a network segment, a new type 2 LSA is created, causing SPF to run again within the OSPF area.
What are Type 3 LSAs and what is their role?
Type 3 LSAs represent networks from other areas. The role of the ABRs is to participate in multiple OSPF areas and ensure that the networks associated with type 1 LSAs are reachable in the non-originating OSPF areas.
As explained earlier, ABRs do not forward type 1 or type 2 LSAs into other areas. When an ABR receives a type 1 LSA, it creates a type 3 LSA referencing the network in the original type 1 LSA; the type 2 LSA is used to determine the network mask of the multi-access network. The ABR then advertises the type 3 LSA into other areas. If an ABR receives a type 3 LSA from Area 0 (the backbone), it regenerates a new type 3 LSA for the nonbackbone area and lists itself as the advertising router, with the additional cost metric.
T/F: The advertising router for type 3 LSAs is the first ABR that advertises the prefix.
False.
The advertising router for type 3 LSAs is the last ABR that advertises the prefix.
The metric within the type 3 LSA uses the following logic:
- If the type 3 LSA is created from a type 1 LSA, it is the total path metric to reach the originating router in the type 1 LSA.
- If the type 3 LSA is created from a type 3 LSA from Area 0, it is the total path metric to the ABR plus the metric in the original type 3 LSA.
For example, from Figure 9-9, as R2 advertises the 10.123.1.0/24 network, the following happens:
- R4 receives R2’s type 1 LSA and creates a new type 3 LSA by using the metric 65: The cost of 1 for R2’s LAN interface and 64 for the serial link between R2 and R4.
- R4 advertises the type 3 LSA with the metric 65 into Area 0.
- R5 receives the type 3 LSA and creates a new type 3 LSA for Area 56, using the metric 66: The cost of 1 for the link between R4 and R5 plus the original type 3 LSA metric 65.
- R6 receives the type 3 LSA. Part of R6’s calculation is the metric to reach the ABR (R5), which is 1 plus the metric in the type 3 LSA (66). R6 therefore calculates the met- ric 67 to reach 10.123.1.0/24.
The type 3 LSA contains which of the following:
- the link-state ID (network number)
- the subnet mask
- the IP address of the advertising ABR
- external routes that have been redistributed into the ABR
- the metric for the network prefix.
The type 3 LSA contains:
- the link-state ID (network number)
- the subnet mask
- the IP address of the advertising ABR
- the metric for the network prefix.
External routes are injected into an OSPF area by the ASBR and are included in type 5 LSA Summary.
Figure 9-10 provides R4’s perspective of the type 3 LSA created by ABR (R5) for the 10.56.1.0/24 network. R4 does not know if the 10.56.1.0/24 network is directly attached to the ABR (R5) or multiple hops away. R4 knows that its metric to the ABR (R5) is 1 and that the type 3 LSA already has a metric of 1, so its total path metric to reach the 10.56.1.0/24 network is 2.
What is the metric from R3’s perspective to reach the network 10.56.1.0/24?
Figure 9-11 provides R3’s perspective of the type 3 LSA created by ABR (R4) for the 10.56.1.0/24 network. R3 does not know if the 10.56.1.0/24 network is directly attached to the ABR (R4) or multiple hops away. R3 knows that its metric to the ABR (R4) is 65 and that the type 3 LSA already has a metric of 2, so its total path metric to reach the 10.56.1.0/24 network is 67.
See attached diargram. What will the metric be for the route to network 10.34.1.0/24 learned by R1? How does the route reach R1, starting at the origin at R4.
There is no route for 10.34.1.0/24 on R1.
At first glance, it looks like routes in the routing tables on R2 and R3 in Figure 9-13 are being advertised across area 23. The 10.34.1.0/24 network was advertised into OSPF by R3 and R4 as a type 1 LSA. R3 is an ABR and converts Area 34’s 10.34.1.0/24 type 1 LSA into a type 3 LSA in Area 0. R3 uses the type 3 LSA from Area 0 to generate the type 3 LSA for Area 23. R2 is able to install the type 3 LSA from Area 23 into its routing table.
Most people would assume that the 10.34.1.0/24 route learned by Area 23 would then advertise into R2’s Area 0 and then propagate to Area 12. However, they would be wrong. There are three fundamental rules ABRs use the for creating type 3 LSAs:
- Type 1 LSAs received from an area create type 3 LSAs into the backbone area and nonbackbone areas.
- Type 3 LSAs received from Area 0 are created for the nonbackbone area.
- Type 3 LSAs received from a nonbackbone area only insert into the LSDB for the source area. ABRs do not create a type 3 LSA for the other areas (including a segmented Area 0).
See attached diagram. Which route will R1 choose to reach 10.4.4.0/24?
In Figure 9-14, R1 is computing the route to 10.4.4.0/24.
Instead of taking the faster Ethernet connection (R1–R2–R4), R1 takes the path across the slower serial link (R1–R3–R4) to R4 because that is the intra-area path. Intra-area paths are always preferred over interarea paths.
Example 9-6 shows R1’s routing table entry for the 10.4.4.0/24 network. Notice that the metric is 111 and that the intra-area path was selected over the interarea path with the lower total path metric.
See attached diagram. What path will be selected by R1 to reach R6?
The next priority after intra-area routes for selecting a path to a network is selection of the path with the lowest total path metric to the destination. If there is a tie in metric, both routes install into the OSPF RIB. All interarea paths for a route must go through Area 0 to be considered.
In Figure 9-15, R1 is computing the path to R6. R1 uses the path R1–R3–R5–R6 because its total path metric is 35 versus the R1–R2–R4–R6 path, with a metric of 40.
What is Interarea Summarization? What does it reduce specifically?
Interarea summarization reduces the number of type 3 LSAs that an ABR advertises into an area (typically the backbone area) when it receives type 1 LSAs. The network summarization range is associated with a specific source area for type 1 LSAs.
When a type 1 LSA within the summarization range reaches the ABR from the source area, the ABR creates a type 3 LSA for the summarized network range. The ABR suppresses the more specific type 3 LSAs, thereby preventing the generation of the subordinate route’s type 3 LSAs. Interarea summarization does not impact the type 1 LSAs in the source area.
Figure 9-19 shows 15 type 1 LSAs (172.16.1.0/24 through 172.16.15.0/24) being summarized into one type 3 LSA (the 172.16.0.0/20 network).
T/F: The default metric for a Type 3 LSA is the highest of the two metrics from the Type 1&2 LSAs used to compose the Type 3 LSA.
False.
The default metric for the summary LSA is the smallest metric associated with an LSA, ; however, it can be set as part of the configuration.
In Figure 9-20, R1 summarizes three prefixes with various path costs. The 172.16.3.0/24 prefix has the lowest metric, so that metric is used for the summarized route.
What is the command to add a static cost of 45 to a summary route?
area 12 range 172.16.0.0 255.255.0.0 cost 45
A static cost of 45 could be added to the summary route to reduce CPU load if any of the summarized networks flap, for example.
How does an ABR prevent routing loops where portions of the summarized network range do not have a more specific route in the RIB?
The ABR performing interarea summarization installs a discard route—that is, a route to the Null0 interface that matches the summarized network range.
Discard routes prevent routing loops where portions of the summarized network range do not have a more specific route in the RIB. The AD for the OSPF summary discard route for internal networks is 110, and it is 254 for external networks.
Example 9-10 shows the discard route on R2 for the 172.16.0.0/16 prefix.
See attached diagram, where R1 is advertising the 172.16.1.0/24, 172.16.2.0/24, and 172.16.3.0/24 networks. What is the command to filter out the 172.16.2.0/24 network?
area 12 range 172.16.2.0 255.255.255.0 not-advertise
This command prevents creation of any type 3 LSAs for the network 172.16.2.0/24, thus making the subordinate routes visible only within the area where the route originates.
See attached diagram for context.
Examine the attached diagram, Fig 9-22. The 172.16.1.0/24 network needs to be present in Area 0 but removed in Area 34. How can you use route filtering via summarization to remove that route?
The short answer is that you cannot do it with summarization filtering.
Although filtering via summarization is very easy, it is limited in its ability. For example, in Figure 9-22, if the 172.16.1.0/24 network needs to be present in Area 0 but removed in Area 34, it is not possible to filter the route using summarization.
T/F: ABRs can filter routes as they advertise out of an area or into an area.
True.
ABRs can filter routes as they advertise out of an area or into an area. This is called area filtering.
What is Local OSPF Filtering?
In some scenarios, routes need to be removed only on specific routers in an area. OSPF is a link-state protocol that requires all routers in the same area to maintain an identical copy of the LSDB for that area. A route can exist in the OSPF LSDB, but it could be prevented from being installed in the local RIB. This is accomplished by using a Distribute List. Figure 9-25 illustrates this concept.
A distribute list on an ABR does not prevent type 1 LSAs from becoming type 3 LSAs in a different area because the type 3 LSA generation occurs before the distribute list is processed.
However, a distribute list on an ABR prevents type 3 LSAs coming from the backbone from being regenerated into nonbackbone areas because this regeneration process happens after the distribute list is processed. A distribute list should not be used for filtering of prefixes between areas; the following section identifies more preferred techniques.
What is the command to filter routes with OSPF using a distribute list?
A distribute list is configured under the OSPF process with the command
- distribute-list {acl-number | acl-name | prefix prefix-list-name | route-map route-map-name} in.
To demonstrate this concept, the topology from Figure 9-24 is used again. Say that R1 is advertising the 172.16.1.0/24, 172.16.2.0/24, and 172.16.3.0/24 network prefixes. R2 filters the 172.16.3.0/24 network from entering its RIB. The configuration is provided in Example 9-15.
