Scenario 19-6

15 Mar

Scenario 19-6
Part A of Scenario 19-6 begins with some planning guidelines about the implementation of a new IS-IS network. This requires a sound addressing scheme and decisions about the location of the Level 2 routers to allow communication between the areas. BGP is the method of gaining access into the Internet, which requires some consideration of policy-based routing.

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 19-6, Part A—Planning
Your job is to deploy a new network with three sites, as shown in Figure 19-8.

This is a new network that is to run Integrated IS-IS with connectivity into the Internet, using BGP. In due time, the rest of the network will be transitioned into the IS-IS network. Use the addressing scheme presented Table 19-5. For Part A of this scenario, perform the following tasks:

1. Summarization is necessary between the areas. Give a brief explanation of where summarization would be implemented in the IS-IS network.
2. Using the addressing scheme provided, determine the type of IS-IS routing that should be implemented across the Frame Relay cloud.

3. Explain why the election of the DIS on the NBMA cloud would happen, and why it is important to ensure that Router A is the elected DIS.
4. Explain the method BGP will employ to connect to the ISP from Router A.
5. Explain how IS-IS on Router A will propagate a default route to the rest of the IS-IS domain that points to the ISP.

Figure 19-8 Diagram for Scenario 19-6

Table 19-5 Addressing Scheme for Scenario 19-6, Part A

Table 19-5 Addressing Scheme for Scenario 19-6, Part A (Continued)

Table 19-5 Addressing Scheme for Scenario 19-6, Part A (Continued)

Table 19-5 Addressing Scheme for Scenario 19-6, Part A (Continued)

Solutions to Scenario 19-6, 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.

1. Summarization is necessary between the areas. Give a brief explanation of where summarization would be implemented in the IS-IS network.

Summarization in IS-IS is straightforward. The complexity is in the addressing scheme design. In Figure 19-8, the IP routes from Router A are summarized into Areas 2 and 3. Router A is capable of summarizing routes because the router is both a Level 1 and Level 2 router, acting as an ABR in OSPF.

Likewise, Router B can summarize routes 131.99.192.0 through to 131.99.207.254 with a mask summarized from /28 to /20 into Areas 1 and 3.

Router C can summarize routes 131.99.224.0 through to 131.99.239.254 with a mask summarized from /28 to /20 into Areas 1 and 2.

2. Using the addressing scheme provided, determine the type of IS-IS routing that should be implemented across the Frame Relay cloud.

Frame Relay is a WAN technology that has evolved beyond the point-to-point capabilities. When configured in a full mesh, the cloud is multiaccess, although each circuit is discrete and therefore not a true broadcast medium. However, IS-IS acknowledges only two types of network topologies, broadcast and point-to-point. If the network link is not a serial line connecting to a single router, IS-IS automatically defines the link to be broadcast.

3. Explain why the election of the DIS on the NBMA cloud would happen and why it is important to ensure that Router A is the elected DIS.

In a truly fully meshed configuration, it is important to consider which is to be elected as the DIS router. In this scenario, Router A must be chosen as the DIS because it is the only router that can communicate with the Internet. The DIS is determined by configuring the priority of the outgoing interface on the hub router.

An illustration of this configuration is shown in Example 19-9, which you see in the next section “Solutions to Scenario 19-6, Part B—Configuration.” The frame-relay map ip command maps the IP destination address to the outgoing DLCI and defines the interface as a broadcast interface. Integrated IS-IS uses the links as if they were truly broadcast links and elects a DIS.

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. It is IS-IS that updates the IP routing table.

4. Explain the method that BGP will employ to connect to the ISP from Router A.

Given that there is only one connection into the ISP, and that the IS-IS domain is a stub network from the viewpoint of BGP, use the simplest configuration: Set a static route from the ISP and a default route from the IS-IS domain. However, this company plans to eventually configure a dual-homed network to provide a redundant path from another router, and the administrators wish to ensure that the BGP configuration works. Therefore, you should opt for a BGP configuration on Router A. Because the company is using a private autonomous system number, this number will be stripped from the updates when they arrive at the ISP.

5. Explain how IS-IS on Router A will propagate a default route to the rest of the IS-IS domain that points to the ISP.

Configure a default route that is to be propagated throughout the entire network so that the rest of the IS-IS network can see the path into the Internet.

Scenario 19-6, Part B—Configuration
The next step in your job is to deploy the network designed in Scenario 19-6, Part A. Use the solutions for Part A of Scenario 19-6 to direct you in configuring the addressing and summarization at the ABRs. For Scenario 19-6, Part B, perform the following tasks:

1. Configure IS-IS on Routers A, B, and C based on the design in Scenario 19-6 Part A.
2. Configure IS-IS across the NBMA Frame Relay cloud using broadcast IS-IS.
3. Configure IS-IS summarization on Routers A, B, and C.
4. Configure Router A for BGP with its neighbor in the ISP.
5. Advertise the company’s networks into the ISP using the network command.
6. Configure the default network for the rest of the IS-IS domain toward the ISP.

Solutions to Scenario 19-6, Part B—Configuration
Example 19-9, Example 19-10, and Example 19-11 show the configurations for Scenario 19-6, Part B, given the preceding criteria. The examples show the complete configurations for each router in turn. Within the configuration for each router, the solutions to each question are highlighted and identified by using comments in the configuration.

Example 19-9 Scenario 19-6 Router A Configuration

Example 19-9 Scenario 19-6 Router A Configuration (Continued)

Example 19-9 Scenario 19-6 Router A Configuration (Continued)

Example 19-11 Scenario 19-6 Router C Configuration

Scenario 19-6, Part C—Verification and Questions
Answer the questions following Example 19-12. Use Example 19-12 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 19-12 The show and debug Output Screens for Scenario 19-6, Router A

Example 19-12 The show and debug Output Screens for Scenario 19-6, Router A (Continued)

Example 19-12 The show and debug Output Screens for Scenario 19-6, Router A (Continued)

Answer the following questions:

1. Figure 19-8 shows three IS-IS areas. Which routers in this topology could be configured as Level 1 routers? State your reasons for your answers.
2. It is possible to configure the IS-IS level of routing at the interface level. Which routers in the topology could be configured in this manner? State the levels of routing that would be chosen, and give reasons for your answers.
3. What command is used to configure interface level routing?
4. What are the different methods of configuring IS-IS across an NBMA cloud? Which configuration does Cisco recommend, and why?
5. In examining the show clns neighbor command, identify the neighbors to Router A. When is the next Hello due from Router B, assuming that the interface is using the defaults?
6. The show clns neighbor command shows the Layer 2 address of the neighbors. Give the Layer
2 addresses for the neighbors and the name of the field that provided the information.
7. Which command reveals how long a neighbor has had an adjacency with the router and both the Layer 2 and Layer 3 addresses of the neighboring router?
8. Which command shows exactly the same information on every router in the same area that is operating at the same level of routing?
9. Which command reveals that the OL bit has been set, and what does this mean?
10. What is the purpose of the command show isis spf-log ?
11. Explain briefly the synchronization rule for BGP.
12. What does the command show ip bgp reveal about attributes?
13. What does the status code *> mean in the output of the show ip bgp command?
14. What is the purpose of a prefix list and when might it be used?
15. Why does Cisco advise that you do not redistribute dynamically learned routes from the IGP into the BGP process?
16. Many organizations want to connect to more than one ISP for redundancy. What are some of the concerns about following this strategy?
17. When and why would you issue the command clear ip bgp * ?
18. In the command show ip bgp neighbors , what does the notifications field show?

Solutions to Scenario 19-6, Part C—Verification and Questions
The answers to the questions for Scenario 19-6, Part C, are as follows:

1. Figure 19-8 shows three IS-IS areas. Which routers in this topology could be configured as Level 1 routers? State your reasons for your answers.

The routers within the areas, that is, A, A2, B, B2, C1, and C2, could be configured as Level 1 routers because they are internal routers to their area.

2. It is possible to configure the IS-IS level of routing at the interface level. Which routers in the topology could be configured in this manner? State the levels of routing that would be chosen, and give reasons for your answers.

The routers in the backbone would be configured as Level 1-2 routers. The serial interfaces forming a full mesh in the Frame Relay cloud have no communication with any Level 1 routers, so they could be configured as Level 2 at the interface. The Ethernet interfaces would need to be configured as Level 1 routers so that they could receive the Level 1 updates from the areas.

The routers A2, B2, and C2 could be configured as Level 1 routers.

3. What command is used to configure interface level routing?

The command used to configure the IS-IS router level on the interface is isis circuit-type {level- 1| level-1-2| level-2-only} .

4. What are the different methods of configuring IS-IS across an NBMA cloud? Which configuration does Cisco recommend, and why?

IS-IS acknowledges only two types of network topologies, broadcast and point-to-point. If the network link is not a serial line connecting to a single router, IS-IS automatically defines the link to be broadcast.

WAN technology has evolved beyond point-to-point capabilities. When an NBMA cloud is configured in a full mesh, the cloud is multiaccess, although each circuit is discrete and therefore not a true broadcast medium.

In a true hub-and-spoke configuration, it is important that the hub router is elected as the DIS router, because it is the only router that can communicate with the other routers. This is done by configuring the priority of the outgoing interface on the hub router.

You are recommended to configure subinterfaces and a point-to-point IS-IS network. Avoid NBMA multipoint topologies. They can be made to work, but they require complex configuration and do not work with the strengths of the routing protocol.

5. In examining the show clns neighbor command, identify the neighbors to Router A. When is the next Hello due from Router B, assuming that the interface is using the defaults?

The neighbors shown in the show clns neighbor command are identified by the system ID. Because the command is issued at Router A, the neighbors shown are 0000.0000.000B and 0000.0000.000C.

6. The show clns neighbor command shows the Layer 2 address of the neighbors. Give the Layer 2 addresses for the neighbors and the name of the field that provided the information.

The field labeled SNPA shows the Layer 2 addresses of the neighbors. Because the links are using Frame Relay, the Layer 2 addresses are DLCI addresses. The neighbor addresses are DLCI 526 and DLCI 527.

7. Which command reveals how long a neighbor has had an adjacency with the router and both the Layer 2 and Layer 3 addresses of the neighboring router?

The show clns neighbor detail command shows the uptime as well as the subnetwork point of attachment (SNPA), which is the Layer 2 address, and the IP address.

8. Which command shows exactly the same information on every router in the same area that is operating at the same level of routing?

The show isis database command shows the same information on every router in the same area that is running the same level of routing. This is because the routing protocol is a link-state protocol that creates a database from every router’s updates, as opposed to the distance vector method of sending a routing table to neighbors.

9. Which command reveals that the OL bit has been set, and what does this mean?

The show isis database command shows that the Overload (OL) bit has been set. The OL bit indicates that the router has an incomplete database because of memory overload and is therefore not used for transit data.

10. What is the purpose of the show isis spf-log command?

The show isis spf-log command shows how often and why the router has run a full shortest path first (SPF) calculation. Use the show isis spf-log command in EXEC mode.

11. Explain briefly the synchronization rule for BGP.

A simple rule states that before iBGP can propagate a route into another autonomous system it must hand the route over to eBGP. The route must be totally known within the autonomous system. That is, the Internal Gateway Protocol (IGP) or internal routing protocol must be synchronized with BGP.

This is to ensure that if traffic is sent into the autonomous system, the interior routing protocol can direct it to its destination. This rule is on by default and should be turned off only if all routers in the autonomous system are running BGP.

12. What does the command show ip bgp reveal about the attribute?

The show ip bgp command shows all the values of all the attributes and their status. Therefore, this is a good command to verify any configurations that change attributes to tune the system and to effectively manage the traffic flow to and from the autonomous system.

13. What does the status code *> mean in the output of the show ip bgp command?

The status code shows the status of the table entry. The status is displayed at the beginning of each line in the table. It can be one of the following values:

s—The table entry is suppressed.
*—The table entry is valid.
>—The table entry is the best entry to use for that network.
Therefore, *> indicates the entries are valid and the best path according to BGP.

14. What is the purpose of a prefix list, and when might it be used?

Prefix lists are used for filtering prefixes. From Cisco IOS software version 11.2, ISPs were given prefix lists, which are a more efficient form of filtering. Prefix lists filter based on the prefix of the address. This option was made a part of the general release IOS in version 12.0. Prefix lists are used on network numbers, not autonomous system paths, for which access lists are used.

15. Why does Cisco advise that you do not redistribute dynamically learned routes from the IGP into the BGP process?

Redistributing dynamically learned routes from the IGP is not advised because there is a great reliance on the IGP table. It is imperative that external routes carried in iBGP are filtered out; otherwise, routing loops are generated when BGP routes are fed into IGP, only to be advertised back into BGP farther down the network. Also, an IGP causes instability of BGP advertised routes. IGPs will not handle the full Internet routing table.

16. Many organizations want to connect to more than one ISP for redundancy. What are some of the concerns about following this strategy?

The following are some concerns about connecting to more than one ISP:

— Each provider might not be propagating the same routes into or from the Internet. If the providers are sending subsets of the required routes, there could be a major problem with connectivity if the link to one of the providers fails.

— If you are connected to two different providers, your autonomous system could become a transit autonomous system between the ISPs. This could happen if a router
in the autonomous system of one provider sees a path to a destination via the other provider’s autonomous system, and your autonomous system gives the best route to
the autonomous system of the other provider.

Configuration at the ISP level is the solution to these concerns and is dealt with when setting up the service. Therefore, it is important that you raise the need for multihoming during the negotiations with your ISP so that the ISP is aware of the need for additional configuration.

17. When and why would you issue the command clear ip bgp * ?

After configuration changes in BGP, it is necessary to reset the TCP session between neighbors. This can be forced with the following command:

This command disconnects the session between the neighbors and reestablishes it using the new configuration that has been entered. The soft option does not tear down the sessions, but resends the updates. The in and out options allow the configuration of inbound or outbound soft updates. The default is for both.

18. In the command show ip bgp neighbors , what does the notifications field show?

The notifications field shows the number of error messages that the router has sent to this peer.

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