Scenario 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.

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 10-1 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 10-7 for the addressing scheme.
The serial configuration in Example 10-15 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.

Example 10-15. Configuration for Router A

hostname A
!
Interface Ethernet0
 ip address 10.1.128.1 255.255.255.0
 ip router isis
 isis circuit-type level-1
 Interface Serial0
 ip address 10.100.100.34 255.255.255.0
 ip router isis
 encapsulation frame-relay
 frame-relay map clns 629 broadcast
 frame-relay map clns 931 broadcast
 frame-relay map ip 10.100.100.33 931 broadcast
 frame-relay map ip 10.100.100.35 629 broadcast
 isis circuit-type level-2-only
!
router isis

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 configuration file in Example 10-16 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 10-16. Configuration for Router C

hostname C
!
Interface Serial0
 no ip address
 encapsulation frame-relay
!
Interface Serial0.1 point-to-point
 ip address 10.100.100.66 255.255.255.0
 ip router isis
 frame-relay interface-dlci 333
!
Interface Serial1
 ip address 10.100.100.35 255.255.255.0
 ip router isis
 encapsulation frame-relay
 frame-relay map clns 629 broadcast
 frame-relay map clns 631 broadcast
 frame-relay map ip 10.100.100.33 631 broadcast
 frame-relay map ip 10.100.100.34 629 broadcast
 isis circuit-type level-2-only
!
router isis
 net 49.0003.0000.0000.000c.00

					  

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 IS-IS routing protocol configuration in Example 10-17 shows the summary-address 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 10-17. Configuration for Router A

hostname A
!
Interface Ethernet0
 ip address 10.1.128.1 255.255.255.0
 ip router isis
 isis circuit-type level-1
!
Interface Serial0
 ip address 10.100.100.34 255.255.255.0
 ip router isis
 encapsulation frame-relay
 frame-relay map clns 629 broadcast
 frame-relay map clns 931 broadcast
 frame-relay map ip 10.100.100.33 931 broadcast
 frame-relay map ip 10.100.100.35 629 broadcast
 isis circuit-type level-2-only
!
router isis
 summary-address 10.1.0.0 255.255.0.0
 net 49.0001.0000.0000.000a.00

Scenario 10-2 Answers

1.Identify the DIS on the Ethernet segment. How was this information apparent?
On examining the show interface output screen in Example 10-18, 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 10-18. Identifying the DIS in Scenario 10-2

Router B
B# show clns interface
Ethernet0 is up, line protocol is up
  Checksums enabled, MTU 1497, Encapsulation SAP
  ERPDUs enabled, min. interval 10 msec.
RDPDUs enabled, min. interval 100 msec., Addr Mask enabled
Congestion Experienced bit set at 4 packets
CLNS fast switching enabled
CLNS SSE switching disabled
DEC compatibility mode OFF for this interface
Next ESH/ISH in 15 seconds
Routing Protocol: IS-IS
    Circuit Type: level-1-2
    Interface number 0x0, local circuit ID 0x1
    Level-1 Metric: 10, Priority: 64, Circuit ID: 0000.0000.000B.01
    Number of active level-1 adjacencies: 1
    Level-2 Metric: 10, Priority: 64, Circuit ID: 0000.0000.000B.01
    Number of active level-2 adjacencies: 1
    Next IS-IS LAN Level-1 Hello in 678 milliseconds
    Next IS-IS LAN Level-2 Hello in 1 seconds

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 answers.
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.



 
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