Solutions to Scenario 19-4, Part C-Verification and Questions

15 Mar

Solutions to Scenario 19-4, Part C—Verification and Questions
The answers to the questions for Scenario 19-4, Part C, are as follows:

1. Which command do you use to configure a totally stubby area?
The command used to create a totally stubby area is as follows:

2. What do the letters ASBR stand for, and what does this device do?

The letters ASBR stand for autonomous system boundary router. This is an OSPF router that connects the OSPF domain to another routing domain. This is when redistribution is required. The ASBR does not have to be situated in Area 0, although that is recommended.

3. Where is summarization performed in OSPF?

Summarization is configured on the ABR or the ASBR.

Summarization at the ABR creates an advertisement that represents many networks within the area. This summary advertisement is propagated into Area 0 and, from there, into the other areas.

Summarization at the ASBR creates an advertisement that represents many networks within the autonomous system. Redistributing this summary advertisement into another routing protocol propagates it to the outside world.

4. Give the command to configure the ASBR to summarize the networks 131.99.224.128, 131.99.224.224, and 131.99.224.0 through 131.99.230.0 for redistribution into the RIP process, using a 20-bit subnet mask.

The command to summarize the networks 131.99.224.128, 131.99.224.224, and 131.99.224.0
through 131.99.240.0 for redistribution into the RIP process is as follows:

Because RIPv1 is using the same classful network address, it is important to note that RIP would need to use the same mask because it does not support VLSM.

5. Explain the difference between prefix routing and subnetting.

The difference between prefix routing and subnetting is that prefix routing creates supernets for the Internet. That summarizes class addresses, for example, creating one network from 16 Class C addresses. The new mask would be 255.255.240.0 or a prefix of /20.

Subnetting is the means by which the classful address is broken down to create many smaller networks addressing fewer hosts, instead of one network with the capability to address many hosts. VLSM is the capability to subnet the class address provided by the Internet, to create subnets with different subnet masks, so that the number of hosts can vary according to the need of the segment.

Prefix routing moves the mask to the left, whereas subnetting moves the mask to the right.

6. State one consideration when configuring multiarea OSPF across a nonbroadcast multiaccess (NBMA) network.

Considerations when configuring multiarea OSPF across a nonbroadcast multiaccess (NBMA) network include these:

— The NBMA network can be created as Area 0. The reasoning is that if the NBMA is used to connect all remote sites, all traffic will have to traverse this network. If the remote sites are satellite areas, all traffic would have to traverse the NBMA, so it makes sense to make Area O the backbone area. This works well in a full-mesh
environment, although it results in a large number of LSAs being flooded into the WAN and puts extra demands on the routers connecting to the NBMA network.

— In a hub-and-spoke NBMA network, it makes sense to assign the hub network as Area 0 with the other remote sites and the NBMA network as other areas. This is a good
design if the satellite areas are stub areas, because it means that the routing information, and thus network overhead, is kept to a minimum over the NBMA cloud. Depending on the design, the rest of the network might constitute one other area or multiple areas. This will depend on the size and growth expectations of the OSPF domain.

7. What command do you use to turn on OSPF?

The command to turn on OSPF is as follows:

The process-id is subtly different from the autonomous system number used in IGRP and EIGRP. The process-id identifies the OSPF routing process on the router and has no significance outside the router. This allows more than one process to be configured on a router. Although it is unusual, there are instances in which this configuration is appropriate.

8. Explain why OSPF supports VLSM.

OSPF supports VLSM because it carries the subnet mask in the routing updates. Therefore, each router can reference the appropriate mask for each network.

9. Explain why all areas must connect through the backbone Area 0.

Area 0 forms a common path for all areas to connect. Therefore, it ensures that all areas are aware of all networks within the OSPF domain.

10. Explain the purpose of the network command in OSPF.

The network command is used to assign an interface or a group of interfaces to an area. When the interfaces are identified, they will participate in the OSPF routing process for the area to which they belong.

11. Which command do you use to show a router’s internal OSPF routing table?

The command show ip ospf border-routers displays the internal routing table of the OSPF internal router.

12. Which command shows the use of VLSM on the OSPF network?

The show ip route command shows not only all the available routes, but also the masks used. If different masks are used, this command states that the subnet is variably subnetted, with the number of subnets and masks.

13. Which command verifies the establishment of adjacencies with other routers on the same network?

The command show ip ospf neighbor shows the neighbors, the neighbor ID, the connecting interface address of the neighboring router, the outgoing interface on the router to connect to the neighbor, the length of time since the last communication from the neighbor, the connection state, and whether the neighbor is a designated router (DR) or backup designated router (BDR).

14. How would you determine whether there is more than one IP routing protocol running on a router? If more than one IP routing protocol is running on a router, how would you know how redistribution is configured?

There are a couple of ways to ascertain whether more than one routing protocol is running on a router. The first way is by issuing this command:

Because this command shows the configuration that is currently running, it is easy enough to see if more than one protocol is running and, if redistribution is running, how it is configured. The other and more straightforward method is to issue the following command:

This command shows all the IP protocols configured on the router, as well as the details of that configuration, including the redistribution.

15. When troubleshooting an OSPF configuration over an NBMA network, which command shows the network type that has been configured? The command show running will display this information, but greater analysis of the timers and costs is required.

The command show ip ospf interface indicates the network type, the delays set for the timers, and the number of neighbors and adjacencies.

16. Give the appropriate mask to use on a point-to-point serial interface, where IP unnumbered is not an option.

The mask to be used in a point-to-point link is 255.255.255.252. This mask allows the allocation of two addresses and is a typical mask for point-to-point networks.

17. What command would you use to identify that an adjacency could not be formed because one router was configured as a stub, while another was not?

The command debug ip ospf adjacency will show that there is a problem in the establishment of an adjacency. The error message would point to a mismatched stub/transit area option bit. You could also do a show ip ospf interface to see that the interface has no neighbors. At this point, it would be wise to check the interface configuration.

18. Which command do you use to ensure that the virtual link is active?

The command show ip ospf virtual-links verifies the virtual link configuration.

19. What concern should you have when using the debug command?

The debug command is a command that should be used with caution because it can use an enormous amount of system resources. Because it is given priority 1 as a process, it can eventually bring your router to a standstill. It is important to run the utility for a limited time to capture the required output. It is also best not to use the debug command from the console unless logging to the log file only. If the console screen freezes while using debug , there is no recovery other than to reboot the system.

20. What is a floating static route, and when would you use one?

You would use a floating static route when a router has a link that is used as a backup using a medium such as a dialup line. The intention is to have no routing protocol running across the link that would keep the link active at a high cost.

The first task is to create a static route so that the routing protocol is not required. Unfortunately, the administrative distance that is used to select the best routes offered by different routing protocols states that a static route is the best route and will use it for all traffic. Thus, despite your best efforts, the backup link becomes a low-bandwidth, expensive primary link. To change the link to take backup status, manually configure the administrative distance to have a higher value than the dynamic routing protocol.

Scenario 19-5
Part A of Scenario 19-5 begins with some guidelines that include planning the transition of the network to run EIGRP as the routing protocol. To transition smoothly, the intention is to slowly integrate EIGRP; this requires redistribution between EIGRP and OSPF and the filtering of updates between the protocols. After you complete Part A, Part B of the scenario asks you to configure the three routers to implement the planned design and a few other features. Finally, Part C asks you to

examine router command output to discover details about the current operation. Part C also lists some questions related to the user interface and protocol specifications.

Scenario 19-5, Part A—Planning
Your job is to deploy a new network with three sites, as shown in Figure 19-7.

The OSPF network has limitations, and the decision has been made to change the routing protocol to EIGRP. Use the addressing scheme presented in the solutions for Part A of Scenario 19-4. For Part A of this scenario, perform the following tasks:

1. The planning committee has stated that the transition to EIGRP should start at the edges of the network and move inward in a controlled manner. Give a brief explanation of how you would implement a transition plan. Is summarization possible?

2. Redistribution is necessary. State the routers that would be responsible for redistribution and the nature of the redistribution.

3. To prevent routing loops, the company has decided to implement distribution filters. Plan the content, location, and implementation of the filters.

Solutions to Scenario 19-5, Part A—Planning
Keeping the design as simple as possible, yet not making it so simple that the network cannot evolve, is a good practice. In these suggested answers, remember that many solutions are available. The reasoning behind this planning is to allow the optimum solution while maintaining the strength of the existing network. As in the Scenario 19-4 solution, the addressing scheme allows summarization at the core level of the network. The first few bits in the third octet indicate the summarization bits.

Many organizations decide to transition the network by starting at the outer peripheries of the network to minimize the impact of these changes. The problems that might occur when a change is made to an organization can be catastrophic. Inevitably, something is overlooked, or you have the dubious honor of discovering a new bug. Obviously, if the change is made to a stub network, the problem can be isolated and fixed speedily. Centralized routers with problems tend to share them
with the rest of the network.

1. The planning committee has stated that the transition to EIGRP should start at the edges of the network and move inward in a controlled manner. Give a brief explanation of how you would implement a transition plan. Is summarization possible?

The transition plan is straightforward because the network in this scenario is very small. However, it reflects the strategy required by any size network.

Figure 19-7 Scenario 19-5 Network Diagram

The implementation of EIGRP should start in Area 1. The reason for this is that there is currently no redistribution in the area, and it has an addressing scheme that is already set up forsummarization.

Summarization is definitely possible. Although EIGRP has no restrictions on summarization, OSPF only permits it at the ABR or ASBR. In EIGRP, summarization is configured at the interface level, which allows for great flexibility, as well as confusion.

There are two ways to consider summarization in this network. The first is simply to implement the summarization put in place by OSPF. The addressing scheme supports it, and it makes good design sense. The other is necessary during the transition phase. This summarization occurs at the point of redistribution, to the external routes being passed into the new routing protocol.

2. Redistribution is necessary. State the routers that would be responsible for redistribution and the nature of the redistribution.

Router A would be responsible for redistribution at the first level of transition, which would redistribute the routes from A1. If this transition goes smoothly, A2 would be configured for EIGRP. Then Area 2 could be changed to run EIGRP, one router at a time. The most complex redistribution would be in Area 3, which is already redistributing RIP into OSPF. Because RIP is here to stay in Building 2 of Area 3, you must take care in the transition to EIGRP.

Building C1 would be an easy transition with redistribution on Router C. Router C2 would need to cut over to EIGRP and turn on redistribution immediately.

At this point, the only routers running OSPF are the core Routers A, B, and C. The choice now is whether to transition one router at a time or to cut over in one fell swoop. If the transition is done at night, cutting over everything is the safer method. Only three routers are involved, and the transition of the areas has now proven successful. There is a backup to restore everything to the status quo, in case problems are encountered.

Although this appears rather dangerous, it is much safer and cleaner than configuring further redistribution on the three core routers.

3. To prevent routing loops, the company has decided to implement distribution filters. Plan the content, location, and implementation of the filters.

The routers must prevent feedback of routing information during redistribution. This can be done to avoid routing loops, which can result in the death of your network.

The purpose of the distribute lists is to prevent the routing processes from telling each other what they already know. You can impose split horizons between the routing protocols manually.

Simply, when Router A is configured to redistribute the EIGRP routes from Router A1 into OSPF, OSPF will have a filter permitting only those routes to be redistributed. In this way, the routes that OSPF sends into EIGRP will not be propagated back into OSPF. Likewise, these filters will be set on Routers B and C.

NOTE This example is given to discuss the role of distribute lists and filters. In reality, there is little chance for a routing loop between OSPF and EIGRP, because both protocols mark redistributed routes as external, using administrative distance to prefer internal routes.

Scenario 19-5, Part B—Configuration
The next step in your job is to deploy the network designed in Scenario 19-5, Part A. Use the solutions for Part A of Scenario 19-5 to direct you in configuring the addressing and summarization at the ABRs. For Scenario 19-5, Part B, perform the following tasks:

1. Configure basic EIGRP for Routers A, B, and C based on the design in Scenario 19-5, Part A.
2. Configure summarization on Routers A, B, and C.
3. Configure redistribution between OSPF and EIGRP on Routers A, B, and C.
4. Configure distribution filters on Routers A, B, and C.

Solutions to Scenario 19-5, Part B—Configuration
Example 19-5, Example 19-6, and Example 19-7 show the configurations for Scenario 19-5, Part B, given the preceding criteria.

Example 19-5 Scenario 19-5 Router A Configuration

Example 19-5 Scenario 19-5 Router A Configuration (Continued)

Example 19-6 Scenario 19-5 Router B Configuration (Continued)

Example 19-7 Scenario 19-5 Router C Configuration (Continued)

Scenario 19-5, Part C—Verification and Questions
Answer the questions following Example 19-8. Use Example 19-8 as a reference when the question refers directly to this scenario. Although not all of these questions are directly tied to the previous scenario, they all probe foundational knowledge required by the technology examined in this scenario.

NOTE In the network from which these commands were captured, several administrative settings not mentioned in the scenario were configured. For instance, the enable password was configured. Any show running-config commands in the examples in this chapter might have other unrelated configuration.

Example 19-8 The show and debug Output Screens for Scenario 19-5, Router A

Example 19-8 The show and debug Output Screens for Scenario 19-5, Router A (Continued)

Answer the following questions:

1. Which command is used to see whether a neighbor adjacency has been created?
2. What is the meaning of the acronym SIA, and where would you see it?
3. What is the difference between the neighbor table and the topology table?
4. How are default routes identified and advertised in EIGRP?
5. What command shows the different IP routing protocols running on a system?
6. How would you detect that an EIGRP neighbor has become unavailable?
7. In redistributing an IP routing protocol, how would you prevent routes from being propagated back into the originating protocol?
8. Which routing protocol supports multiple Layer 3 protocols?
9. When redistributing OSPF routes into EIGRP, how would you state the metric to be used in the new routing protocol?
10. Where would you see the successors for a route?
11. How would you turn on EIGRP?
12. How would you configure router summarization in EIGRP?
13. What command shows the route summarization configured in EIGRP?
14. What is the purpose of the bandwidth percentage configuration in EIGRP?
15. Where would you see the administrative distance for a route?
16. Which debug command is used to identify that there is a problem in creating an adjacency in EIGRP?
17. What is the purpose of the subnet parameter in the EIGRP and OSPF redistribution command?
18. What routes can be redistributed into another routing protocol?
19. What is the difference between the in and the out parameters when set on a distribute list?
20. What should you consider in configuring EIGRP across an NBMA cloud?

Solutions to Scenario 19-5, Part C—Verification and Questions
The answers to the questions for Scenario 19-5, Part C are as follows:
1. Which command is used to see whether a neighbor adjacency has been created?
The command that shows whether an adjacency has been formed is show ip eigrp topology .

The codes, which are part of this command’s output, show the adjacency. Codes tell the state of the topology table entry. Passive and active refer to the EIGRP state, with respect to this destination. Update, query, and reply refer to the type of packet that is being sent. The codes are as follows:

• P—passive: Indicates no EIGRP computations are being performed for this destination.
• A—active: Indicates EIGRP is actively trying to find an alternative path for this destination. If the neighbors have to be polled for a good neighbor, this consumes resources on the network.

• U—update: Indicates that an update packet was sent to this destination.
• Q—query: Indicates that a query packet was sent to this destination.
• R—reply: Indicates that a reply packet was sent to this destination.
• r—reply status: A flag that is set after the software has sent a query and is waiting for a reply.

• successors: Gives the number of successors. This number corresponds to the number of next hops in the IP routing table.
• FD—feasible distance: Used in the feasibility condition check. If the neighbor’s reported distance (the metric after the slash) is less than the feasible distance, the feasibility condition is met and that path is a feasible successor. When the software determines it has a feasible successor, it fails over to that FS instead of resolving a
path using queries.
• replies: Gives the number of replies that are still outstanding (that have not been received) with respect to this destination. This information appears only when the
destination is in active state.
• state: Gives the exact EIGRP state of this destination. It can be the number 0, 1, 2, or

3. This information appears only when the destination is active.
• via: Gives the IP address of the peer that told the software about this destination. The topology table lists the destination route and the number of successors or paths to that route. The via field lists these successors, and the remaining entries on the list are feasible successors.

2. What is the meaning of the acronym SIA, and where would you see it?

The acronym SIA stands for Stuck in Active, which means that an EIGRP neighbor has not replied to a query that was sent out. If the neighbor does not reply within a limited time, it is presumed dead and is flushed from the tables. This is to prevent a route from being permanently active as an alternative path is sought from unresponsive neighbors.

An error message will be generated to the screen, but it is possible to identify the problem by looking at the log files or issuing commands. In the show ip eigrp topology command, any neighbors that show an R have not yet replied (the active timer shows how long the route has been active) and might be Stuck in Active. It is advisable to run this command several times; you begin to see which neighbors are not responding to queries (or which interfaces seem to have many unanswered queries). You should also examine this neighbor to see if it is consistently waiting for replies from any of its neighbors. Repeat this process until you find the router that is consistently not answering queries.

The problems are often on the link to this neighbor, or with memory or CPU utilization with this neighbor.

It is often better to reduce the query range instead of increasing the SIA timer.

3. What is the difference between the neighbor table and the topology table?

The neighbor table holds information about EIGRP neighbors, whereas the topology table lists all the routes known to have feasible successors.

4. How are default routes identified and advertised in EIGRP?

Default routes are shown with the address 0.0.0.0 and are advertised as an external address. This affects the administrative distance.

5. What command shows the different IP routing protocols running on a system?

The command that shows all the different IP routing protocols running on a system is show ip protocols . This shows the protocols, redistribution, and many other details.

6. How would you detect that an EIGRP neighbor has become unavailable?

The command show ip eigrp topology would show that there had not been a reply to packets sent to the neighbor. The command show ip eigrp neighbors shows the neighbors, how long they have been in the table, and the last time they were heard from.

7. In redistributing an IP routing protocol, how would you prevent routes from being propagated back into the originating protocol?

This would be done by creating a distribute list that permits only those routes that did not originate from the protocol into which the updates are being redistributed.

8. Which routing protocol supports multiple Layer 3 protocols?

EIGRP supports IP, AppleTalk, and IPX.

9. When redistributing OSPF routes into EIGRP, how would you state the metric to be used in the new routing protocol?

There are two ways of stating the metric to be used by the redistributed networks. The first is to include the metric on the redistribution command. This will give the stated metric to all routes sent to EIGRP from the routing process OSPF 200:

The second command assigns the same metric to all routes distributed into EIGRP from any source:

10. Where would you see the successors for a route?

The successors to a route are held in the routing table, because it is the current next hop that is being used to forward traffic to the remote destination. The command that shows these successors is as follows:

11. How would you turn on EIGRP?
The command to turn on EIGRP is as follows. This starts the routing process:

The following command identifies the interfaces that will receive, send, and advertise updates for that process:

12. How would you configure router summarization in EIGRP?

Route summarization can be configured in EIGRP. This is achieved at the interface level and allows great flexibility in configuration. The command is as follows:

13. What command shows the route summarization configured in EIGRP?

The command show ip route shows the summarization.

14. What is the purpose of the bandwidth percentage configuration in EIGRP?

The purpose of the bandwidth percentage configuration in EIGRP is to limit the amount of bandwidth that can be taken by EIGRP routing traffic. By default, this is limited to 50 percent of the link. In NBMA clouds—in particular, Frame Relay—it might be advisable to tune this parameter, depending on the Committed Information Rate (CIR) of the links.

15. Where would you see the administrative distance for a route?

You can see the administrative distance of a route in the routing table. It is the number in brackets placed after the destination network. The first number is the administrative distance; the second is the metric.

16. Which debug command is used to identify that there is a problem in creating an adjacency in EIGRP?

This command:

adds a filter to this command

and displays only IP packets for the stated process and address.

17. What is the purpose of the subnets parameter in the OSPF redistribution command?

The subnets parameter in the OSPF redistribution commands allows the subnets of major networks that are not directly connected to be redistributed into the protocol OSPF. Without this, only major networks will be distributed.

18. What routes can be redistributed into another routing protocol?

Static routes, directly connected routes, or routes learned dynamically from another routing protocol can be redistributed.

19. What is the difference between the in and the out parameters when set on a distribute list?

The in parameter determines routes entering a routing protocol from another routing protocol or from another router. The out parameter defines the routes that can be redistributed into another routing protocol either to another routing process or from an interface. For example, in the following commands, routes are redistributed from RIP into OSPF. Any outbound update sourced from RIP that is in the network 10.0.0.0 will be redistributed into OSPF:

The in and out parameters can also be set against an interface or globally against all updates entering the router. However, the out parameter cannot be used at the interface level for linkstate routing protocols, as the routing table is not sent in updates. For example,

20. What should you consider in configuring EIGRP across an NBMA cloud?

When configuring EIGRP over NBMA clouds, you should consider the nature of the link. Is it point-to-point or multipoint? Also consider overhead traffic associated with EIGRP and bandwidth utilization:

— Over a point-to-point interface, set the bandwidth to reflect the CIR of the permanent virtual circuit (PVC).
— Over multipoint Frame Relay, ATM, Switched Multimegabit Data Services (SMDS), and ISDN PRI, the bandwidth is divided equally among the links. The configuration should reflect the percentage of the available bandwidth.

If the PVCs have different CIRs, either convert the links to point-to-point or configure the bandwidth to be a multiple of the lowest available CIR by the number of PVCs.

It is important in point-to-multipoint configurations to turn off split horizons in order to ensure that the routing updates are propagated to the other links.

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