The show isis database Command
The show isis database command shows the LSPs held in the local database. The LSP ID shows the system ID of the generating router and whether this LSP is from a router or a pseudonode. The last octet shows whether the LSP was too large to ﬁt into one PDU; a nonzero value in this ﬁeld indicates that this is a fragment of an LSP. The number states the fragment number.
Because IS-IS is a link-state protocol, the database should be identical on every router of the same level within the area.
The EXEC command has the following syntax:
show isiiss area-tag database [level-1] [level--2] [l1] [l2] [detail] [lspid]
Table 12-5 explains the syntax of this command.
Table 12-5 Explanation of the show isis database Command
Table 12-6 explains the meaning of the ﬁelds in the output. Note that both Level 1 and Level 2
databases are shown because the router is running the default conﬁguration of Level 1-2 routing. All
the LSP information is contained in these databases, including the LSPs generated by the router
itself. An asterisk marks these entries in the output.
The show isis database detail Command
The show isis database detail command shows the complete LSP and the values for the individual ﬁelds. The EXEC command has the same syntax structure as shown in the show isis database command in the preceding section.
Example 12-12 shows output for the show isis database detail command. The highlighted lines show that there are two databases: one for the Level 1 routing and the other for the Level 2 routing. The example shows, for each LSP, the area and the IP address of the transmitting interface and the metric cost to the IP routes it knows. The default metric is a cost of 10; therefore, a metric of 20 indicates a route that is two hops away.
Example 12-12 Output for the show isis database detail Command
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Example 12-12 Output for the show isis database detail Command (Continued)
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Table 12-7 explains the meaning of the ﬁelds in the output.
Table 12-7 Explanation of the show isis database detail Command
Troubleshooting the Integrated IS-IS Operation
Unfortunately, even after the most careful planning, conﬁgurations can fail to work, and the most scrutinized networks can break. The show commands are essential for hunting down problems. Careful documentation of the troubleshooting steps taken enable the administrator to build a solid understanding of the problem and systematically eliminate possible problem sources. Having this log is invaluable when, if all else fails, you have to elicit the help of the Technical Assistance Center (TAC) engineers at Cisco. The following commands are useful for troubleshooting Integrated IS-IS:
■ show isis spf-log
■ debug commands
The following sections describe these commands in greater detail.
The show isis spf-log Command
The show isis spf-log command explains a great deal about the SPF calculations on the router. It gives the events that triggered SPF for the last 20 occurrences.
To display how often and why the router has run a full SPF calculation, use the show isis spf-log EXEC command.
Example 12-13 shows output for the show isis spf-log command. The highlighted lines show the normal periodic SPF calculations. The example also shows that Router A sent an LSP because it had set the ATT bit, and later it generated a new LSP when new adjacencies came online. This output shows the healthy workings of an IS-IS network, but the command is very useful if the network appears unstable, with routes appearing and disappearing.
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Example 12-13 Output for the show isis spf-log Command
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Table 12-8 explains the meaning of the ﬁelds in the output screen.
Table 12-8 Explanation of the show isis spf-log Command
The debug Commands
The debug command is a helpful troubleshooting tool, but it does have certain disadvantages of which you must be aware. The debug command has the highest process priority and forces the router into process switching. It is capable of consuming all the resources on the router, thus becoming the problem instead of helping to solve the problem. It is important to turn on debug just for the speciﬁc task to be monitored and to turn it off as soon as the data has been gathered. The no form of this command disables debugging output. You should direct the output to a log ﬁle, because each character sent to the console forces processor interrupt; in this way, the data can be perused with care.
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The debug commands are EXEC commands. Table 12-9 lists the options available for monitoring Integrated IS-IS.
Table 12-9 The debug Command Options for Integrated IS-IS
The “Foundation Summary” section of each chapter lists the most important facts from the chapter. Although this section does not list every fact from the chapter that will be on your exam, a wellprepared candidate should, at a minimum, know all the details in each “Foundation Summary” before going to take the exam.
Table 12-10 summarizes the commands covered in this chapter.
Table 12-10 Summary of Commands
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Table 12-11 shows the debug command options discussed in this chapter.
Table 12-11 The debug Command Options for Integrated IS-IS
As mentioned in the introduction, “All About the CCNP, CCDP, and CCIP Certiﬁcations,” you have two choices for review questions. The questions that follow next give you a bigger challenge than the exam itself by using an open-ended question format. By reviewing now with this more difﬁcult question format, you can exercise your memory better and prove your conceptual and factual knowledge of this chapter. The answers to these questions are found in Appendix A.
For more practice with examlike question formats, including questions using a router simulator and multichoice questions, use the exam engine on the CD-ROM.
1. What command is used to conﬁgure the Integrated IS-IS router process?
2. What is the default routing level on a Cisco router?
3. What command is used to conﬁgure Integrated IS-IS routing on the interface?
4. How is the NET address conﬁgured on the router?
5. What command is used to show the state of adjacencies on the router?
6. What command identiﬁes the designated intermediate system router for your LAN?
7. Explain brieﬂy what show isis database reveals.
8. What command reveals the trigger for the last SPF calculation on the router?
9. For Frame Relay, when would you conﬁgure the map command with the broadcast parameter?
10. Which command is used to display all update packets that are both received and sent by a router?
11. State two reasons why a router may not be able to ﬁnd a neighbor.
12. Which command shows the LSPs in detail?
13. How would you ensure that an adjacency has been established?
14. What are the steps required for a basic conﬁguration in IS-IS?
15. Give the commands required to summarize the networks 10.10.0.0 through to 10.10.255.0 into another area of IS-IS.
16. Which command veriﬁes the circuit type and the metric?
17. Which network topology defaults in the Broadcast mode?
18. For which WAN topology is a point-to-point conﬁguration recommended?
19. When is it necessary to map CLNS to the DLCI?
20. In the show clns interface command, it is possible to identify the DIS on a multiaccess link. Which ﬁeld in the output screen of this command would show the DIS for the segment?
The following scenarios and questions are designed to draw together the content of the chapter and to exercise your understanding of the concepts. There is not necessarily a right answer. The thought process and practice in manipulating the concepts are the goals of this section. The answers to the scenario questions are found at the end of this chapter.
The ISP Flying Data has recently converted from OSPF to Integrated IS-IS. The migration was relatively painless. The company uses the private network 10.0.0.0 for IP and the private ISO addressing with AFI 49. They created a hierarchical addressing structure. See Figure 12-7 to see this addressing scheme.
Figure 12-7 Diagram for Scenario 12-1
The addressing of the network was a large project, with all the pitfalls that accompany such a major exercise. The network is now stable, and it is time to conﬁgure the WAN connections using multipoint Frame Relay.
1. Issue the commands that will allow Router A to use Integrated IS-IS routing across the NBMA cloud as if the cloud were a broadcast medium. Refer to Figure 12-7 for the addressing scheme.
2. The WAN is a Frame Relay cloud, and Router G has a point-to-point link with Router C. Issue the commands for Router C that conﬁgure the link for Integrated IS-IS as a point-to-point link.
3. To reduce bandwidth consumption and to hide some network detail, summarization has been suggested as a solution over the WAN links. Issue the commands for Router A that will summarize the networks behind this router with a preﬁx of /16 across the WAN.
Given the conﬁguration of Integrated IS-IS in Example 12-14 and the output screen in Example 12-15, perform the tasks and answer the questions listed. The WAN has light user trafﬁc and has a fully meshed conﬁguration, as shown in Figure 12-8.
Figure 12-8 Diagram for Scenario 12-2
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Example 12-14 Router B’s Configuration File
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Example 12-15 The show clns interface Command from Router B (Continued)
1. Identify the DIS on the Ethernet segment. How was this information apparent?
2. If Router A died, what would the effect be on the network?
3. Is summarization possible only on the routers entering the WAN cloud, or is it possible on the networks not shown in the ﬁgure, but on the other side of the routers? Give reasons for your answers.
The answers provided in this section are not necessarily the only possible answers to the questions. The questions are designed to test your knowledge and to give practical exercise in certain key areas. This section is intended to test and exercise skills and concepts detailed in the body of this chapter.
If your answer is different, ask yourself whether it follows the tenets explained in the answers provided. Your answer is correct not if it matches the solution provided in the book, but rather if it has included the principles of design laid out in the chapter.
In this way, the testing provided in these scenarios is deeper: It examines not only your knowledge, but also your understanding and ability to apply that knowledge to problems.
If you do not get the correct answer, refer back to the text and review the subject tested. Be certain to also review your notes on the question to ensure that you understand the principles of the subject.
Scenario 12-1 Answers
1. Issue the commands that will allow Router A to use Integrated IS-IS routing across the NBMA
cloud as if the cloud were a broadcast medium. Refer to Figure 12-7 for the addressing scheme.
The highlighted portion of the conﬁguration ﬁle in Example 12-16 shows the conﬁguration of IS-IS across the Frame Relay cloud, using the broadcast technology and LAN Hellos. The frame-relay map ip command maps the IP destination address to the outgoing DLCI and deﬁnes the interface as a broadcast interface.
The frame-relay map clns command maps to the CLNS process on the destination router. Without the second command, no routes appear in the IP routing table because CLNS does not receive the frames to populate the IP routing table. Remember that these are IP routes carried in the IS-IS routing protocol. IS-IS updates the IP routing table.
Example 12-16 Configuration for Router A: Integrated IS-IS Broadcast Across an NBMA Frame Relay Cloud
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Example 12-16 Configuration for Router A: Integrated IS-IS Broadcast Across an NBMA Frame Relay Cloud (Continued)
2. The WAN is a Frame Relay cloud, and Router G has a point-to-point link with Router C. Issue the commands for Router C that configure the link for Integrated IS-IS as a point-to-point link.
The highlighted portion of the conﬁguration ﬁle in Example 12-17 shows the conﬁguration of IS-IS across the Frame Relay cloud, using the point-to point technology and point-to-point Hellos. Because the link is point-to-point, there is no need to conﬁgure frame-relay map commands (there is no choice of destination to deﬁne). The point-to-point link is just a pipe that goes to one destination. As shown in the conﬁguration, it is only necessary to conﬁgure the interface as point-to-point, start Frame Relay, and deﬁne the DLCI. In addition to conﬁguring Frame Relay, you must start the IS-IS process for the interface.
Example 12-17 Configuration for Router C: Integrated IS-IS Point-to-Point Across an NBMA Frame Relay Cloud
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Example 12-17 Configuration for Router C: Integrated IS-IS Point-to-Point Across an NBMA Frame Relay Cloud (Continued)
3. To reduce bandwidth consumption and to hide some network detail, summarization has been
suggested as a solution over the WAN links. Issue the commands for Router A that will
summarize the networks behind this router with a prefix of /16 across the WAN.
The highlighted portion of the conﬁguration ﬁle in Example 12-18 shows the summaryaddress command that is used to hide the routes within area 0001 from the other areas. This conﬁguration is possible on Router A because it sits on the boundary between areas. Summarizing routes reduces the network resources required by the network.
Example 12-18 Configuration for Router A: Summarizing Networks from Router A in Area 0001 to All Other Areas
Example 12-18 Configuration for Router A: Summarizing Networks from Router A in Area 0001 to All Other Areas (Continued)
Scenario 12-2 Answers
1. Identify the DIS on the Ethernet segment. How was this information apparent?
On examining the show interface output screen in Example 12-19, you can see the circuit ID is 0000.0000.000B.01. This is the system ID of the pseudonode, as is apparent because the last octet has a nonzero value. The system ID of the pseudonode is the system ID of the DIS plus the nonzero octet. Therefore, the DIS is 49.0002.0000.0000.000B.00.
Example 12-19 Identifying the DIS in Scenario 12-2
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Example 12-19 Identifying the DIS in Scenario 12-2 (Continued)
2. If Router A died, what would the effect be on the network?
If Router A died, Routers B and C would not be able to communicate with Router A or with Router D. However, Routers B and C would be able to communicate with each other. The network behind Router A would function, but it would be isolated from the others.
The neighbor tables would fail to hear the Hellos from Router A, and Routers B and C would time out all routes that they had heard from this router. Routers behind Router A would time out Router A from the neighbor table. All the former neighbors of Router A would send LSPs. The LSPs from Router A would be purged from all the databases, new LSPs would ﬂood the network, and the SPF algorithm would be run. Router A and the network behind it would be annexed from the larger network of Routers B and C.
3. Is summarization possible only on the routers entering the WAN cloud, or is it possible on the
networks not shown in the figure, but on the other side of the routers? Give reasons for your
Summarization is only possible on the Level 1-2 routers, acting in a similar way to an OSPF ABR, and then only if the addressing scheme allows for it to be implemented. The Level 1-2 router would summarize the routes and inject them into the Level 2 network.
NEXT: Part IV: EIGRP