Chapter 7

"Do I Know This Already?"

7.A and E
13.A and D
14.A and D
22.A and C
23.A, B, D


1.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.
2.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.
3.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.
4.The 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.
5.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.

  • Convergence is faster with smaller areas.

6.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.
7.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. It also speeds convergence.
9.Some restrictions govern creating a stub or totally stubby area. 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.

10.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.
11.Having a Frame Relay NBMA cloud within one OSPF area 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, the entire network could suffer.
12.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.
13.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.
14.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.
15.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. Interarea connectivity is achieved through the use of default routes, which are advertised to the internal routers (intra-area connectivity is handled as usual).
16.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.

  • Network summary link— Sent by the ABR into the backbone, stating 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.

  • AS external ASBR summary link— 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.

  • 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.

17.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 areas—that 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.

18.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.

19.The ABR creates the LSA Types 3 and 4.
20.The command show ip ospf [process-id area-id] database network displays the network link-state information.
21.The command area area-id stub no-summary will create a totally stubby area. This is a subcommand to the router ospf process-id command. The command is necessary only on the ABR, but all the other routers in the area must be configured as stub routers.
22.A virtual link is a link that creates a tunnel through an area to the backbone (Area 0). This allows an area that cannot connect directly to the backbone to connect virtually. The command to create the link is area area-id virtual-link router-id. Note that the area-id that is supplied is that of the transit area, and the router-id is that of the router at the other end of the link. The command needs to be configured at both ends of the tunnel.
23.Summarization is done at area boundaries. The command to start summarization is the area range command, with the syntax area area-id range address mask. To summarize external routes, use the summary-address command on the ASBRs.
24.The command summary-address address mask is the command that you would use to summarize external routes before injecting them into the OSPF domain.

A virtual link is used when an area is not directly attached to the backbone area (Area 0). This may be due to poor design and a lack of understanding about the operation of OSPF, or it may be due to a link failure. The most common cause of an area separating from the backbone is link failure, which can also cause the backbone to be segmented. In these instances, the virtual link is used to join the two backbone areas together. Segmented backbone areas might also result from two companies merging.

25.The command to define the cost of a default route propagated into another area is area area-id default-cost cost.
26.It is appropriate to define a default cost for the default route when a stub area has more than one ABR. This command allows the ABR, or exit point for the area, to be determined by the network administrator. If this link or the ABR fails, the other ABR will become the exit point for the area.
27.The default cost for the default route is defined on the ABR. The ABR will then automatically generate and advertise the route cost along with the default route.
28.The command syntax to configure a stub area is area area-id stub. This command is configured on the ABR connecting to the area and on all the routers within the area. Once the configuration is completed, the hellos are generated with the E bit set to 0. All routers in the area will only form adjacencies with other routers that have the E bit set.
29.The area range command is configured on the ABR because it dictates the networks that will be advertised out of the area. It is used to consolidate and summarize the routes at an area boundary.
30.The commands are as follows:

network area 1

network area 0

31.The syntax is as follows:


32.The area range command is used to summarize networks between areas and is configured on the ABR. The summary-address command is used to summarize external networks on an ASBR into the OSPF routing domain.
33.The command area 1 stub no-summary creates a totally stubby area. The number after the word area indicates the area that is being defined as a totally stubby area. This is necessary because the router might be an ABR with connections to many areas. Once this command is issued, it prevents summarized and external routes from being propagated by the ABR into the area. To reach the networks and hosts outside the area, routers must use the default route advertised by the ABR into the area.
34.The reason to configure the router process to log adjacency changes to syslog as opposed to running debug is an issue of resources. It takes fewer router and administrator resources to report on a change of state as it happens than to have the debugger running constantly. The debug process has the highest priority and thus everything waits for it.
35.Many OSPF problems stem from adjacency problems that propagate throughout the network. Many problems are often traced back to neighbor discrepancies. If a router configured for OSPF routing is not seeing an OSPF neighbor on an attached network, do the following:
  • Make sure that both routers are configured with the same IP mask, MTU, InterfaceHello timer, OSPF Hello interval, and OSPF dead interval.

  • Make sure that both neighbors are the same area type and are part of the same area.

  • Use the debug and show commands to trace the problem.

36.The configuration is between the ABRs, where one of the ABRs resides in Area 0 and the other in the area that is disconnected from the backbone. Both of the ABRs are also members of the transit area. Having created the virtual link, both ABRs are now members of Area 0, the disconnected area, and the transit area.
37.The command area 1 default-cost 15 will assign a cost of 15 to the default route that is to be propagated into the stub area. This command is configured on the ABR attached to the stub area.
38.The parameter area-id is the area ID of the transit area. Thus, if the ABR in Area 0 is creating a virtual link with the ABR in Area 3 through Area 2, the area ID stated in the command is Area 2. The router ID is the router ID of the router with whom the link is to be formed and a neighbor relationship and adjacency established.