Chapter 8


"Do I Know This Already?" Quiz






b, d










a, b, c


a, c, d






a, c



In a totally stubby area, which routes are not propagated into the area?


There will be no summary or external routes propagated by the ABR into the area. Thus, there will be no Type 3, Type 4, or Type 5 LSAs sent into the other area.


Can a virtual link contain a stub area?


No, a virtual link cannot contain a stub area. A stub area cannot accept external LSAs, and by definition, the virtual link is traversing a different area.


An ABR must be resident in which area?


An ABR must be resident in Area 0, as well as in the area that is connecting to the backbone area. It has two topological databases, one for each area in which it is resident, so that it knows how to forward traffic.


What LSAs will the ABR forward?


ABR forwards summary LSAs. It forwards both Type 3 LSAs and Type 4 LSAs. Type 3 LSAs are forwarded to the other ABRs, and Type 4 LSAs are forwarded to the ASBRs. ABR also forwards Type 3 LSAs from other areas into its own area. If the ABR has multiple links in the same area, it also forwards Type 1 and Type 2 LSAs in its capacity as an internal router.


State two advantages in creating areas in OSPF.


The advantages in creating areas in OSPF include the following:

- It is easier to manage and administrate smaller areas, where hopefully many of the design considerations and even configuration are standardized.

- It uses a smaller topology table, which reduces the CPU, memory, and network bandwidth consumption.

- Fewer SPF calculations are involved because the topology table is smaller and there is less likelihood of change in the network.

- It uses a smaller routing table if summarization is in operation.


What is an external route, and on which type of router will this route be introduced?


An external route is a route that did not originate in the OSPF domain. It has been redistributed from another routing protocol or static routing. An external route is introduced into the OSPF domain by an ASBR.


Why is the use of summarization important in the design of OSPF?


Summarization is important in the design of OSPF because it supports a hierarchical design and allows for the summarization of IP subnets between areas, which reduces the size of the routing tables, which in turn reduces the CPU and memory requirements.


How many routers does Cisco suggest is the limit to have in a single area?


Cisco suggests that there should not be more than 50 routers in any single area.


What are the restrictions to be considered in the creation of a stub area or a totally stubby area?


Some restrictions govern creating a stub or totally stubby area. Because no external routes are allowed in these areas, the following restrictions are in place:

- No external routes are allowed.

- No virtual links are allowed.

- No redistribution is allowed.

- No ASBR routers are allowed.

- The area is not the backbone area.

- All the routers are configured to be stub routers.


A virtual link in OSPF is used to solve what problem?


The virtual link provides a disconnected area with a logical path to the backbone. The virtual link must be established between two ABRs that have a common area, with one ABR connected to the backbone. It can also be used to connect two area 0s together. This might be necessary when two companies merge, each with its own area 0, or if, due to the loss of a link, the area 0 becomes bisected.


State one disadvantage for making an NBMA cloud Area 0.


Creating the NBMA cloud as one OSPF area, preferably Area 0, causes summary LSAs to be flooded throughout the NBMA network. This results in a large number of routers recalculating whenever there is a change that requires the topology table to be updated, and the Frame Relay cloud can become saturated . If the Frame Relay cloud has a problem, then the entire network might suffer.


State one advantage in making the centralized routers and network resources dwell in Area 0 while the Frame Relay cloud and the stub remote LANs reside in satellite stub areas.


One advantage of this design is that any flooding of external LSAs is prevented from entering the Frame Relay network, because it is a stub network. This reduces the network overhead.


How does creating a number of areas in OSPF reduce the number of SPF calculations?


The number of SPF calculations is reduced because the size of the topology table is reduced. This lessens the likelihood of a change in the network and, thus, SPF calculations.


How does a stub area differ from the backbone area?


A stub area differs from the backbone area in that it does not propagate external routes into its area. The backbone is obliged to forward these LSAs to ensure connectivity throughout the network.


How does a totally stubby area differ from a stub area?


A totally stubby area differs from a stub area in that it propagates neither external routes nor summary routes from other areas. This is a Cisco solution to minimize the amount of CPU and memory required of the routers within the area. Connectivity is achieved by the use of default routes, which are advertised to the internal routers.


State the different LSA types.


The different LSA update types are as follows :

- Router link Sent by the router, stating the links directly connected. These are flooded through the area. This update is identified by the type code Type 1.

- Network link Sent by the DR, stating the links for the LAN for which it is the DR. These LSAs are flooded throughout the area. This update is identified by the type code Type 2.

- Summary link Sent by the ABR into the backbone. It states the IP subnets within the area that are to be advertised into other areas. This is where summarization would be configured. This update is identified by the type code Type 3.

- Summary link (to an ASBR) Sent from an ABR to a router that connects to the outside world (ASBR). It contains the metric cost from the ABR to the ASBR. This update is identified by the type code Type 4.

- External link Sent to the ASBRs to which the organization is directly connected. This update is identified by the type code Type 5.

- The NSSA External LSA These LSAs are created by the ASBR residing in an NSSA. This LSA is similar to an autonomous system external LSA, except that this LSA is contained within the NSSA area and is not propagated into other areas.


Where does the backbone router reside, and what is its function?


OSPF has special restrictions when multiple areas are involved. If more than one area is configured, one of these areas must be Area 0. This is called the backbone. When designing networks, it is good practice to start with Area 0 and then expand into other areas later.

The backbone must be at the center of all other areasthat is, all areas must be physically connected to the backbone. The reasoning behind this is that OSPF expects all areas to inject routing information into the backbone; in turn, the backbone will disseminate that information into other areas.


There are two types of summarization. What are they?


The two types of summarization are as follows:

- Interarea route summarization These routes are sent between areas. The ABR will summarize routes if the network within the area was designed using contiguous addresses, conforming to both a physical and a logical hierarchy.

- External route summarization These are routes sent into OSPF from another routing protocol. This summarization also demands a hierarchical design using contiguous addresses. This is employed at the ASBR.


For how many LANS does Cisco suggest a router should serve as a DR or a BDR?


Cisco suggests that a router should be a DR or a BDR for only one LAN.


Which router type creates LSA Types 3 and 4?


The ABR creates the LSA Types 3 and 4.

CCNP BSCI Exam Certification Guide
CCNP BSCI Exam Certification Guide (CCNP Self-Study, 642-801) (3rd Edition)
ISBN: 1587200856
EAN: 2147483647
Year: 2002
Pages: 194
Authors: Clare Gough © 2008-2017.
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