Chapter 16 Do I Know This Already Quiz

12 Mar

Chapter 16 “Do I Know This Already?” Quiz
1. d
2. a
3. d
4. a
5. d
6. b
7. c
8. a, c, d
9. b
10. b, c, d
11. a
12. b
13. a, c, d
14. a, b, c
15. b
16. b
17. b, c, d
18. d

1. If a route reflector hears an update from a nonclient, what action will be taken?
If a route reflector hears an update from a nonclient, it will reflect the update to clients only.

2. In version 11.0 of the Cisco IOS software, what method would be used to restrict routing
information from being received or propagated?

Distribute lists would be used to restrict BGP routing updates in version 11.0 of the Cisco IOS software. Prefix lists became available to ISPs in 11.2 of the IOS and in 12.0 to the general public.

3. Explain the purpose and use of the command show ip prefix-list name [seq seq-number].
This command displays the entry prefix list with a given sequence number.

4. Why would you redistribute static routes into BGP?
The static routes are forwarded to the nonexistent interface null 0 to aggregate the routes to create a supernet. This is then redistributed to the BGP process instead of the hundreds of routes that exist. Because any route redistributed into BGP must be in the IP routing table, this is a way of creating a supernet and having it redistributed.

A static route to null 0 is not needed if the network command is used and no address aggregation is performed, although it can be used to prevent routing loops.

5. Why is it advisable to have the route reflectors fully meshed?
Although clients are not fully meshed within a cluster, it is important that the route reflectors they serve are fully meshed. This is to ensure that the routers pass routing information to each other. Remember that iBGP does not propagate routing information; it generates information only based on the network command. The route reflectors need to pass information to each other to ensure the entire network has a full knowledge of the network. It is possible to create a hierarchical design with route reflectors, but this should be done with great caution, as it can cause routing loops.

6. Why is filtering often required when redistributing BGP into an IGP?
Filtering is often required when redistributing BGP into an IGP because the routing tables can become overwhelmed by the number of routes that are imported.

7. What are the advantages of multihoming?
Multihoming allows for redundancy, which is important when the link is into the Internet and can carry crucial business information for the company. Multihoming also increases performance by allowing the selection of the better paths.

8. Why do iBGP peers need to be fully meshed?
iBGP needs to be fully meshed because it has a fundamental rule stating that iBGP will not propagate back into iBGP a route that it has learned from iBGP. The reason is that this is the only way to prevent routing loops. If each router is connected to every other router (fully meshed), the updates come directly from the source, which removes the need for a router to propagate any updates that it has received.

9. How is a fully meshed network avoided in iBGP?
A fully meshed network can cause some problems to the routers because they have a great deal of information to handle from every directly connected router. The configuration of route reflectors does not use as much bandwidth, CPU, or memory. Route reflectors allow the router to pass routes on to its peers, and a hub-and-spoke topology is thus possible.

10. What is the equation to determine the number of sessions needed in a fully meshed BGP network?

The equation for determining the number of sessions required is n (n – 1) / 2, where n is the number of routers. Thus, 10 routers would mean 10 (10 – 1) / 2 = 10 * 9 / 2 = 45 sessions.

11. Why does a fully meshed network in iBGP cause problems?
A fully meshed network in iBGP causes problems because the network has to accommodate a large number of TCP connections, and this can eat up memory, CPU, and eventually bandwidth.

12. State two benefits to using route reflectors.
Route reflectors have many benefits:

— The use of a router reflector means that fewer TCP peer connections are needed. This streamlines the network traffic and solves the excessive use of network resources
sometimes incurred with a fully meshed network.
— The design and configuration are very straightforward, which means that it is easy to implement and thus to migrate an existing network, particularly because path
attributes are not affected by them.
— Despite the fact that the route reflectors are straightforward, the flexibility in the design means that it is possible to become very sophisticated using redundant route
reflectors and even multiple levels of route reflectors. Complex solutions are possible using route reflectors.

13. If a route reflector sees multiple paths to a destination, what action is taken?
The route reflector chooses the best path to the destination.

14. Explain the difference between a cluster-ID and an originator-ID.
The cluster-ID is an optional, nontransitive BGP attribute (type code 10). A cluster is the route reflector and its clients. If a cluster has more than one route reflector, it needs to be identified by a cluster-ID. The cluster-ID is used to identify the route reflector advertising the prefix. This is listed in the update; if a router receives an update with its own cluster-ID, it will be ignored, thus preventing routing loops.

An originator-ID is the ID given to the source route-reflector client. It serves the same purpose as the cluster-ID in that it prevents routing loops. A router that receives an update that contains its own originator-ID will ignore the update.

15. State two advantages in using prefix lists over access lists.
The advantages of using prefix lists instead of access lists include the following:

— They have a better performance than access lists.
— They allow editing of the lists so that additional lines of code can be inserted anywhere in the list.
— The user interface is easier to use.
— They are more flexible.

16. If the ISP has provided a default route, how will the router within the autonomous system select the exit path in a multihomed environment?

The IGP within the autonomous system will use the metric associated with that routing protocol. The router running EIGRP, for example, will select the nearest router based on the composite metric of bandwidth and delay (by default).

17. What is a disadvantage of an autonomous system receiving full routing updates from all ISPs?
The disadvantage is that a great deal of network resources, such as memory, bandwidth, and CPU, is required.

18. What is the danger of redistributing BGP into the IGP?
The danger is that the autonomous system routers receiving the BGP updates will be overwhelmed by the amount of routing information that they receive. This could result in an unstable network or even a network exhausted to death.

19. What are the advantages of a fully meshed iBGP network?
The advantage of a fully meshed network is that the network will receive full routing information from the directly connected peers. This means that the IGP does not need to carry routing information to the BGP peers, and no redistribution is necessary. This does not run the risk of overloading the IGP. It also means that the network will converge more quickly and that synchronization can be turned off, which will improve performance.

20. In configuring a route reflector, how is the client configured?
The client of a route reflector is configured at the same time as the route reflector. The command issued on the router that is to become the route reflector identifies the router that is to become the client. The following command is the syntax issued at the router reflector:

21. What commands are used to display the BGP router ID that identifies the router that is sending the updates and peering with its neighbor?

The commands that display the BGP router ID are show ip bgp neighbor or show ip bgp .

Chapter 17
“Do I Know This Already?” Quiz
1. a, c
2. a, c
3. b
4. a
5. a, c
6. c
7. a, b
8. c, d
9. b
10. c
11. b
12. a, b, c
13. c
14. a, b, c

1. State two of the methods that Cisco recommends for controlling routing protocol traffic.
The methods that Cisco discusses as useful methods of controlling routing updates are as follows:

— Passive interfaces: Prevent routing protocol communication and thus an adjacency from being formed with neighboring routers.
— Changing the administrative distance on the route: Changes the natural order as laid down by Cisco on which routing protocol is more plausible than the others. A scale of weighting is applied to the protocols.
— Default routes: Instruct the router on where to send the destination traffic if the routing table has no entry for that destination.
— Static routes: Offers the ability to configure manually the path to a destination network.
— Route update filtering: Offers the use of access lists to control the exchange of routing information.

2. What is the default administrative distance for RIP?
The administrative distance for RIP is 120; it has the highest distance of interior routing protocols and is therefore the least likely to be selected.

3. State two instances when you do not want routing information propagated.
The two occasions that you do not want routing information to be propagated are as follows:

— If there is a WAN link where the cost of the link is based on network traffic. This might also have the added disincentive of being a WAN link that is a dial-on-demand link,
which is raised and maintained by the presence of traffic attempting to flow across the interface.

— When you are trying to prevent routing loops. If the routing domain has redundant paths that will be learned by different routing protocol, it is advisable to filter the propagation of one of the paths.

4. In what instances will EIGRP automatically redistribute?
EIGRP will automatically redistribute between itself and IGRP as long as both processes are running the same autonomous system number.

EIGRP for IPX automatically redistributes; IPX RIP/SAP and Enhanced IGRP for AppleTalk similarly redistribute automatically into RTMP.

5. Which command is used to view the administrative distance of a route in the routing table?
The command show ip route displays the administrative distance for each route; two numbers are shown in brackets: the route metric, followed by the administrative distance.

6. When is redistribution required?
Redistribution is required when there is more than one routing protocol for IP running within the organization and when every part of the network needs connectivity to all the networks.

7. Why does Cisco recommend that you not overlap routing protocols?
The reason not to overlap routing protocols is that it will increase network traffic, router CPU processing, and memory because of the additional protocol updates. This additional traffic and CPU and memory requirements complicate the routing process. The decision is not simply between multiple paths, but also between the various routing protocols that are advertising them.

8. Why would you want to prevent routing updates across an on-demand WAN link?
A WAN link that is a dial-on-demand link is raised and maintained by the presence of interesting traffic attempting to flow across the interface. Thus, every time a RIP update is sent, the path would be raised or, more likely, kept up all the time.

9. What is the metric used for in a routing protocol?
The metric is used to select the best path when multiple paths are available to a remote network.

10. Give two reasons for using multiple routing protocols.
The main reasons for multiple protocols existing within an organization are as follows:

— The organization is transitioning from one routing protocol to another because the network has grown and there is a need for a more sophisticated protocol that will scale.

— Although a vendor solution is preferred, there is a mix of different vendors within the network, so the vendor solution is used in the areas available. This is particularly true in client/server networks.
— Historically, the organization was a series of small network domains that have recently been tied together to form one large enterprise network. The company may
well have plans to transition to a single routing protocol in the future.
— Often after a merger or a takeover, when several companies become one, it takes planning, strategy, and careful analysis to determine the best overall design for the
— Politically, there are ideological differences among the different network administrators, which until now have not been resolved.

11. In a very large environment, the various domains might have different requirements, making a single solution inefficient. A clear example is the case of a large multinational corporation, where EIGRP is the protocol used at the access and distribution layer, but BGP is the protocol connecting the core. When implementing redistribution, state one possible problem that you might experience, and explain why it is a problem.

The problems experienced as a result of multiple routing processes and their redistribution include the following:

— The wrong, or less efficient, routing decision is made, referred to as the suboptimal path.
— A routing loop occurs, in which the data traffic is sent in a circle without ever arriving at the destination.
— The convergence time of the network increases because of the different technologies involved. If the routing protocols converge at different rates, this can also cause problems. In some cases, this might result in timeouts and the temporary loss of networks.

12. Which has a lower administrative distance, IGRP or OSPF?
IGRP has an administrative distance of 100, whereas OSPF has an administrative distance of 110. The IGRP path will be entered into the routing table if there are paths offered to the same destination from both protocols.

13. What command is used to configure an outbound route filter?
The command for configuring a route filter is as follows:

14. What is a passive interface?
A passive interface is an interface that will listen to routing updates but that will not propagate any updates for the protocol configured, although OSPF and EIGRP will not learn routes over a passive interface. It is used to prevent unnecessary traffic from being sent out of an interface. Usually a passive interface is configured when there are no routers to hear the updates on that network.

15. What is the purpose of administrative distance?
When the routing table is populated with networks that are provided by multiple routing protocols, the administrative distance is used to choose the best path to the remote network.

16. What is the concern of redistributing into a redundant network?
The concern with redistributing into a redundant network is that it is possible to select suboptimal routes. That is, if two routing protocols both have a path to the same destination, the path for the routing table will be selected based on administrative distance. This is a blanket solution that does not always render the best decision.

17. What is a default network?
A default network is a route used when there is no entry for the remote network in the routing table. It is used to connect to the Internet and other routing domains when it is not practical to know all the available networks. It is sufficient to have an exit point from your network identified.

18. Why is it necessary to configure a default metric when redistributing between routing protocols?

The metric is used within a routing protocol to select the best path to a remote network when there are multiple paths. When redistributing, it is not always possible to port the metric across because the metric is protocol-specific. The default metric throws the original metric away and substitutes a new metric for the new routing protocol.

19. Which command is used to modify the administrative distance of a route?
The command to configure the administrative distance of a route is as follows:

20. What is the difference in processing for an inbound and an outbound route filter?
If a filter is applied to an incoming interface, the routing table is not checked unless the route is permitted. However, the router has to examine every incoming packet, which is expensive in resources. The outbound filter must go through the routing decision process the first time, after which the result is cached.

Inbound filters are wider ranging because they prevent routes from entering the router instead of filtering on each outgoing interface that is affected.

Chapter 18
“Do I Know This Already?” Quiz
1. c
2. a, b, d
3. b
4. d
5. a
6. a, c, d
7. c
8. b
9. d
10. a
11. a
12. c
13. c
14. b
15. c
16. a
17. b

1. Explain the command match ip address {access-list-number | name} [access-list number | name].
The command is used to match criteria in establishing the policy-based routing. Access lists are used to specify the addressing of the packets to be affected.

2. Explain the command ip route-cache policy .
This command is set on an incoming interface and enables the fast switching of policy-based routing. Before version 11.2 of the Cisco IOS software, policy-based routing was processswitched. This caused some applications to time out, but the problem has now been resolved. Fast switching of policy-based routing is disabled by default. Therefore, it is necessary to manually configure it.

3. State two benefits of using policy-based routing.
The benefits of policy-based routing include the following:

— Organizations can determine traffic flow based on the origin of the traffic. They can send traffic owned by different groups across different paths.
— QoS can be set in the IP header using the precedence or TOS bits. This allows certain traffic to be prioritized through the network.
— High-cost links can be raised or made active on more specific criteria, which allows an efficient use of the resources available.
— Traffic can be sent across multiple paths based on traffic characteristics.

4. How are matching routes modified in a route map?
Using the set command modifies matching routes. If the criteria are met in the match command and the action was to permit, the set criteria is initiated to control the routing as specified.

5. Explain the command set ip default next-hop [ip-address...ip-address].
This command provides a list of IP addresses for traffic if there is no explicit route in the routing table for the destination address of the packet. These addresses are those of next-hop routers or of the interfaces of adjacent routers. If multiple next-hop addresses are listed, then the first address is tried. If it is unavailable, the others are tried in turn.

6. Which command displays route maps that are configured on interfaces?
The command show ip policy displays the route maps used for policy-based routing on the router’s interfaces. The command show route-map [map-name] displays the route maps.

7. What command is used to attach a route map to an incoming interface?
The command ip policy route-map map-tag assigns the route map to the incoming interface where it examines all incoming packets and issues set commands on all packets that match at least one of the match criteria.

8. What is a map tag?
A map tag is the name of the route map to be used in policy-based routing. This must match a map tog specified by a route-map command.

9. Explain briefly the difference between the match and set commands.
The match command determines whether the packet will be routed using the route map, and the set command determines how the packet will be routed.

10. What are the criteria by which policy-based routes are determined?

Instead of routing by the destination address, policy-based routing allows you to determine and
implement routing policies to allow or deny paths based on the following:

— The identity of a particular end system
— The application being run
— The IP protocol in use
— The size of packets

11. How would you block traffic that found no match?
The way to block traffic that is not matched in the route map is to add an extra line of configuration. Adding a set command at the end of the route map routes all unmatched routes to the interface null0.

12. What is the purpose of the sequence number in a route map?
Within a route map, each route map statement is numbered with sequence numbers and, therefore, can be edited individually. The sequence number is also used to specify the order in which conditions are checked. Thus, if there are two statements in a route map named BESTEST, one with sequence 5 and the other with sequence 15, sequence 5 is checked first. If there is no match for the conditions in sequence 5, then sequence 15 will be checked.

13. What logic is used if there are multiple match statements in the route map?
A route map statement might contain multiple match statements. All match statements in the route map statement must be considered true for the route map statement to be considered matched. This is a logical AND.

14. What parameters can an extended access list define in the route map selection process?
An extended IP access list is used to specify criteria based on source and destination, application, protocol type, TOS, and precedence. If multiple access list statements are specified, matching any one will result in a match.

15. How can policy-based routing save money for the network?

The very nature of policy-based routing allows the network to be streamlined and to forward traffic down cheaper circuits. The bulk traffic generated by a specific activity can be diverted to use a higher-bandwidth, high-cost link for a short time. Meanwhile, interactive traffic is provided basic connectivity over a lower-bandwidth, low-cost link. For example, a dial-ondemand ISDN line might be raised in response to traffic to a finance server for file transfers selected by policy-based routing.

16. What are some of the potential disadvantages of using policy-based routing?
The following are disadvantages of policy-based routing:

— You need a backup path in place in case the defined next-hop router goes down. If there is no alternative defined, policy-based routing will default to dynamic routing decisions.
— Additional CPU is required to examine every source address to affect the defined policy.
— Extra configuration is required.
— The possibility exists that other traffic will be disrupted.

17. Explain the use of the command match length min max.
This command is used to define the criteria based on the Layer 3 length of the packet. The min parameter states the minimum inclusive length of the packet allowed for a match. The max parameter states the maximum inclusive length of the packet allowed for a match.

In this way, interactive traffic that is time-sensitive, such as SNA traffic tunneled in IP, can be sent on a dedicated route. Interactive traffic uses small packets, so the links could be dedicated by packet size, allowing file transfers using large packets to use a separate link so that the terminal sessions are not starved of resources.

18. Explain the following sample configuration.

The serial interface has been configured with an IP address and has the route map tryout assigned to make decisions about inbound packets.

The route map tryout is configured to send all packets that are destined for network with the address in the network address space. All routes that have this destination address will be sent out of the interface e1, whereas all unknown addresses, including broadcasts, will be sent to the default next hop of

19. Briefly explain the use of the command show ip policy .
This command displays the route maps used for policy-based routing on the router’s interfaces.

20. Configuring route maps is complex, and it is easy to confuse the logic by which they work. State
one of the things you should be aware of when configuring a route map. When editing a route map statement with the no version of the existing command line, if you forget to type in the sequence number, you will delete the entire route map.

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