Part VIII: Appendixes
Appendix A Answers to Chapter “Do I Know This Already?” Quizzes and Q&A Sections
Appendix B Suggested Reading
Answers to Chapter
“Do I Know This Already?”
Quizzes and Q&A Sections
“Do I Know This Already?” Quiz
1. a, c, d
6. a, c
12. a, d
1. In the routing table, a field indicates the source of the routing information. If the field showed the letter C, what would this mean?
A ﬁeld showing the letter C would mean that the network is directly connected.
2. In the routing table, how is the next hop indicated?
In the routing table, the next hop is indicated by the word via followed by an IP address. This is the address of the next logical hop.
3. Cisco distinguishes between the routing and the switching functions. What is the difference?
The routing function is how the router learns the logical topology of the network. It decides whether the datagram can be routed, which path to select if there is a choice, and to which outgoing interface to queue the datagram. It operates at Layer 3 of the OSI stack.
The switching function is the forwarding of the frame from the inbound interface to an outbound interface. It operates at Layer 2 of the OSI stack, not at Layer 3 like a router, because the routing or Layer 3 function has already been completed.
4. Name the interior IP routing protocols that send the mask with the routing update.
The interior IP routing protocols that send the mask with the routing update are EIGRP, OSPF, IS-IS, and RIPv2.
5. Does VLSM require a classful or classless routing protocol, and why?
VLSM requires a classless routing protocol because it needs the subnet mask to be sent with the update.
6. State one of the characteristics of a classful routing protocol.
The characteristics of a classful routing protocol are as follows:
— It summarizes at the network boundary.
— Routes exchanged between foreign networks are summarized to the NIC number.
— Within the same network (IANA classful network), subnet routes are exchanged by routers.
— All the interfaces on all the routers within a NIC number must share the same subnet mask.
— VLSM is not possible within the network
7. What is the command to show whether a specific network, such as 220.127.116.11, is present in the routing table?
The command to show whether a speciﬁc network, such as 18.104.22.168, is present in the routing table is as follows:
show ip route 22.214.171.124
8. State one major difference between a classful protocol and a classless routing protocol.
Major differences between classful and classless routing protocols include:
— The capability to use VLSM
— The capability to summarize at an administratively deﬁned boundary, as opposed to summarizing at the classful boundary
— The capability to maximize the logical address space
9. Describe briefly the difference between a routing and routed protocol.
The routed protocol is the Layer 3 protocol used to transfer data from one end device to another across the network. The routed protocol is the Layer 3 datagram that carries the application data in addition to the upper-layer information.
The routing protocol is the protocol used to send updates between the routers about the networks that exist in the organization, thereby allowing the routing process to determine the path of the datagram across the network.
10. Describe the processes used to build and maintain the routing table.
There are three steps involved in building and maintaining the routing table. These three processes are independent and include:
— The routing protocol, which actually sends the information about the routes or networks within the autonomous system, such as RIPv1, IGRP, EIGRP
— The routing table, which receives updates from the routing protocol and provides the forwarding process with information on request
— The forwarding process, which determines which path to select from the routing table in order to forward a datagram
11. Give a brief explanation of the switching function.
The switching function does the following:
— Checks the incoming frame for validity
— Checks whether the frame is addressed (at Layer 2) to the router
— Checks whether the frame is within the scope of the framing criteria (too big or too small)
— Checks whether the frame passes CRC
— Strips the Layer 2 header and trailer from the frame and checks the destination address against the cache entries
— Creates the appropriate frame header and trailer (if there is an entry in cache for the destination address) and forwards the frame to the outbound interface queue
12. What information is contained in the fast switching cache?
If fast switching is enabled, the datagram is then examined again, and an entry is put into a route cache. The entry in this cache consists of the following:
— An IP preﬁx
— The output interface
— The link-layer header to be used in forwarding the datagram
13. When would you use the command no auto-summary ?
Some routing protocols, such as BGP, RIPv2, and EIGRP, summarize at the network boundary automatically. Summarization within the NIC number boundary must be conﬁgured manually, and therefore autosummarization must ﬁrst be disabled with the no auto-summary command.
14. When does OSPF send updates, and do they contain the entire routing table?
Each OSPF router refreshes its LSA every 30 minutes. An LSA will be sent earlier if there is an adjacency change.
15. Network convergence is when a network domain has learned about the new network topology after a change has occurred in the domain. What is considered a network change that would require network convergence?
The accuracy of the table will be affected by how quickly it responds to changes in the network. These changes include the following:
— Learning new networks
— Learning a better path to an existing network
— Learning that a network is no longer available
— Learning an alternative route to a network
16. When would you consider using a static route in your network?
A static route would be conﬁgured for the following reasons:
— Links that have very low bandwidth, such as dialup links
— The administrator needs control over the link
— The link is a backup to the dynamically learned route
— There is only one path to the remote network, such as a stub network
— The router has very limited resources and cannot run a routing protocol
— The administrator needs to control the routing table to allow a classful protocol and a classless routing protocol to populate the routing table
17. When would you consider using a default route in your network?
Occasions for using a default route include:
— Connecting to the autonomous system from a stub network
— Connecting to the Internet
18. What command is used to configure ODR on a hub router?
router odr is the only command needed to be conﬁgured on the hub router.
19. When would you consider using a floating static route in your network?
Floating static routes are used when a backup route is required.
20. Describe some of the characteristics of classless routing protocols.
Classless routing protocols include OSPF, EIGRP, RIPv2, IS-IS, and BGP.
— The characteristics of a classless routing protocol are:
— Router interfaces within the same network can have different subnet masks (VLSM).
— They support the use of classless interdomain routing (CIDR).
— Some routes can be summarized within the major NIC number. This is done manually.
“Do I Know This Already?” Quiz
11. a, c, d
1. Identify one criterion to help determine a subnet mask for classless addressing when designing a network-addressing scheme.
Questions to ask include the following:
— How many networks are there in the network?
— How many hosts are there on the largest subnet?
2. With a classless address of 126.96.36.199/20, what is the range of classful addresses that are included in the address? Write your answer in dotted decimal and the third octet in binary notation.
The address 188.8.131.52 /20 includes the Class C addresses 184.108.40.206 to 220.127.116.11; this is illustrated in both dotted decimal and binary notation in the following table.
A discontiguous network is a network in which a classful network is separated by another classful network. Therefore, the original classful network is no longer contiguous because an intervening Internet number has segmented or divided it.
4. For VLSM to be available as a design option in the network, what characteristic must the routing protocol possess?
The routing protocol must send the preﬁx or subnet mask as part of the routing update.
5. If summarization is to be implemented in the network, name one design criterion for the addressing scheme that must be in place.
For VLSM to work, the addressing scheme must be hierarchical, allowing the upstream devices
to share the same high-order bits as the downstream devices.
6. If the host portion of a subnet has been used to identify end devices, can that subnet be used again for VLSM?
It is not possible to use a subnet for addressing hosts or to further subnet the network using VLSM. The addresses would be seen as duplicate addresses.
7. Give one example of when route summarization would not be a good solution.
Route summarization is not useful in the following circumstances:
— There are discontiguous networks in the organization.
— A speciﬁc subnet needs to be seen throughout the network.
— The addressing scheme does not support summarization. No common high-order bits are shared in the network-addressing scheme.
— Access lists require detailed information, which summarization suppresses.
8. Give one reason for implementing route summarization.
Route summarization is useful for the following reasons:
— To keep the routing tables small
— To keep the network overhead low
— To hide the network details from the rest of the organization
— To prevent ﬂapping links from affecting the rest of the network
9. Given an address of 18.104.22.168 and a prefix mask of /25, how many networks can be addressed, and how many hosts can exist on each network? Write the first and last possible subnets in binary and decimal notation.
For the network address of 22.214.171.124, the subnet mask of 255.255.255.128 would enable you to address 510 subnets with 126 hosts on each subnet. This complies with the subnetting rule of not allocating addresses with all 0s or all 1s. The following table illustrates the ﬁrst and last subnet in their binary and decimal notation formats.
10. What class of address is 126.96.36.199, and how many hosts can be addressed if no subnetting is used?
188.8.131.52 is a Class B address and can address more than 65,000 hosts on one network if no subnetting is used.
11. Write out the decimal notation of the following subnet mask presented in the binary notation of 11111111.11111111.11111111.11111000.
The decimal notation of the subnet mask would be 255.255.255.248.
12. Is 184.108.40.206/20 a valid subnet mask?
Yes, this is a valid mask, and it will provide 16 consecutive Class C addresses to the organization.
13. Briefly define route summarization.
Route summarization is the method of including many subnets in a few routing entries.
14. What sort of design scheme does route summarization require?
Route summarization requires a hierarchical addressing scheme.
15. In route summarization, in which direction is the network/host boundary in the subnet mask moved?
In route summarization, the subnet mask is shifted to the left.
16. Explain how summarization allows for smaller routing tables.
Summarizing is the consolidation of multiple routes into a single advertisement.
17. What is the subnet mask for a /21 prefix?
The subnet mask is 255.255.248.0.
18. What is the default subnet mask for the IP address 220.127.116.11?
The subnet mask is 255.255.255.0.
19. State whether 18.104.22.168/13 is an example of CIDR or VLSM routing.
The address 22.214.171.124/13 is an example of CIDR routing because it is summarizing Class B addresses within the Internet routing tables.
20. State how many classful addresses are summarized in the address 126.96.36.199/13.
Eight Class B addresses are summarized in the address 188.8.131.52 /13. The range of addresses is 184.108.40.206–220.127.116.11/16.
“Do I Know This Already?” Quiz
1. a, b, d
5. b, c
6. b, d
1. What is an Inside Global address, and when is it used?
The Inside Global address connects your organization indirectly to the Internet. Typically, these are the addresses provided by the ISP. These addresses are propagated outside the organization. They are globally unique and are the addresses used by the outside world to connect to inside the organization. Simply explained, they are the addresses that deﬁne how the inside addresses are seen globally by the outside.
2. When is the Inside Local address used?
The Inside Local address is the address that allows every end device in the organization to communicate. Although these addresses are unique within the organization, they are probably not globally unique. They may well be private addresses that conform to RFC 1918. They are the inside addresses as seen locally within the organization.
3. Explain the difference between the Outside Global address and the Outside Local address.
The Outside Global address is the Internet address (all the addresses outside the domain of the organization). They are the outside addresses as they appear to the global Internet.
The Outside Local address, however, is external to the organization. This is the destination address used by a host inside the organization connecting to the outside world. This will be the destination address of the packet propagated by the internal host. This is how the outside world is seen locally from inside the organization.
4. Give one example of when NAT might be employed.
NAT is useful in the following circumstances:
— To connect organizations that use address space issued to other organizations to the Internet
— To connect organizations that use private address space deﬁned in RFC 1918 and want to connect to the Internet
— To connect together two organizations that have used the same private address, in line with RFC 1918
— When the organization wants to hide its addresses and is using NAT as part of ﬁrewall capabilities or is using additional security features
5. Explain what PAT is.
Port Address Translation (PAT) translates different local addresses (within the organization) into one address that is globally signiﬁcant for use on the Internet. The additional identiﬁer of a TCP or UDP port unravels the multiple addresses that have been mapped to single addresses. The uniqueness of the different local addresses is ensured by the use of the port number mapped to the single address.
6. Why is NAT often configured on the organization’s firewall?
NAT performs Network Address Translation from private addressing to global addressing. This is required only when connecting to the outside world when a globally unique address is required. A ﬁrewall is installed on the edge of the autonomous system to protect the organization from unauthorized access. Therefore, both applications would reside on the edge of the network as it connects into the Internet.
7. Give one example of when private addressing would be a good solution for an organization.
Private addressing was designed as an addressing method for an organization that has no intention of ever connecting to the Internet. If Internet connectivity were not required, there would be no requirement for a globally unique address from the Internet. The individual organization could address its network without any reference to the Internet, using one of the address ranges provided. Private addressing is often used now by companies that do not want to make a submission for an address from the IANA or do not want the labor of stretching the small allocation of addresses that they might receive. Instead, they can use a Class A address for their organization and use NAT to connect to the outside world.
8. Why does summarization need a hierarchical addressing structure?
Summarization of IP addresses needs a hierarchical addressing structure to hide the downstream subnets. A summarized address is where many subnets that share the same leftmost bits of the IP address might be represented by a single subnet. This can only occur when the minor subnets are physically downstream of the summarized network.
9. When might you use Destination Address Rotary Translation?
You use destination address rotary translation for trafﬁc entering the organization from the outside. The destination address is matched against an access list, and the destination address is replaced by an address from the rotary pool. This is used only for TCP trafﬁc, unless other translations are in effect. This is required when creating TCP connections that require a connection into the network. It allows Telnet, FTP, and other connection-oriented sessions.
10. Why do NAT and private addressing tend to be implemented together?
NAT performs Network Address Translation and is implemented when private addresses need to be translated into a global address to allow connectivity to the Internet with a globally unique address.
11. Which routing protocols for IPv6 does Cisco IOS support?
Cisco supports RIPng, BGP-4+, IS-IS, and OSPF in Cisco IOS 12.2T.
12. How many bits are there in the IPv6 address space?
There are 128 bits in the IPv6 address space.
13. Explain why it is important to remember to identify how many hosts and subnets are required when designing an IPv4 network.
It is important to determine how many hosts and subnets are required so that you can adequately address the network, allowing for the physical topology of the network to marry the logical topology and to build in a hierarchical addressing scheme that can be summarized to add efﬁciency into the network.
14. Why does IPv6 addressing allow for more effective security and QoS to be implemented?
IPv6 addressing allows for more effective security and QoS to be implemented because the address space allows for end-to-end connectivity. Without having to traverse ﬁrewalls and NAT servers, both security and QoS are much easier to create and maintain.
15. Give one reason it might be advisable to implement private addressing.
The reasons for addressing your organization’s network using private addressing include the following:
— There is a shortage of addressing within the organization.
— You need security. Because the network must go through a translation gateway, it will not be visible to the outside world.
— There is an ISP change. If the network is connecting to the Internet through an ISP, the addresses allocated are just on loan or are leased to your organization. If the
organization decides to change its ISP, the entire network will have to be readdressed. If the addresses provided deﬁne just the external connectivity and not the internal subnets, however, readdressing is limited and highly simpliﬁed.
16. In the following address
which part of the address is autoconfigured?
The MAC address 0AC0:3428:121C provides the system ID for the address.
17. Why is IPv6 multicasting more efficient than IPv4 broadcasting?
IPv4 broadcasting forces every end system to interrupt its process to view the incoming packet, because the broadcast address is an address to every system. This saturates not only the media resources, but also the resources of the end systems. Multicasting is an address to multiple end systems, but only to those systems that need to receive the packet.
18. State one of the main benefits of IPv6.
The main beneﬁts of IPv6 are as follows:
— Larger address space, allowing for a larger number of systems that can be globally addressed and a more scalable network
— Increased address space, allowing for a deeper hierarchical structure
— Simpliﬁed header, allowing for greater routing efﬁciency and, thus, network performance
— Policies for network architecture ﬂexibility, allowing evolution and growth of the protocol
— Support for routing and route aggregation
— Simple administration through serverless autoconﬁguration, the ability to renumber with ease, multihoming—all of which allow a level of plug-and-play support
— Security using IP Security (IPSec) support for all IPv6 devices
— Support for Mobile IP and mobile computing devices (direct-path)
— Multicast support built into the protocol using a greater number of addresses and efﬁcient mechanisms
19. What is an IPv6 extension header?
The IPv6 extension header is the same as the Options ﬁeld in the IPv4 header. However, instead of including the Options ﬁeld within the header as IPv4 does, IPv6 attaches the Options ﬁeld to the end of the header, indicating with the Next Header ﬁeld whether there is something to additional to process. This speeds up the processing and also allows for protocol evolution, because many extension ﬁelds can be chained together.
20. What are the two most common methods of transitioning an IPv4 network to an IPv6 network?
Dual stacks and 6to4 tunneling are the two most common methods of transitioning an IPv4 network to an IPv6 network.
“Do I Know This Already?” Quiz
1. a, b, c
5. c, d
12. a, b, c, d
16. b, c, d
1. Name one routing protocol that sends periodic updates.
RIPv1, RIPv2, and IGRP all send periodic updates.
2. What is an incremental update, and how often is it sent out?
An incremental update is an update that is sent out only when there is a change in the network. It contains only the information about the change. The change could be either the loss of a network or the addition of a network. EIGRP sends out incremental updates, as does OSPF, IS-IS, and BGP.
3. Distance vector routing protocols naturally summarize at which boundary?
Distance vector routing protocols naturally summarize at the IANA or major network boundary. They do this by following the ﬁrst octet rule.
4. What is the algorithm used by distance vector protocols?
Distance vector routing protocols use the Bellman Ford algorithm.
5. Give three reasons why RIPv1 has problems working in a large network.
RIPv1 has problems working in a large network because of the following reasons:
— It has a maximum hop count of 15.
— It sends updates of its routing table out of every interface every 30 seconds, which increases the network overhead and leads to link congestion.
— To avoid routing loops, it uses holddown and poison reverse, and thereby increases the time that it takes to propagate the changes in the network.
6. What is the destination address of the distance vector periodic update in RIPv1?
The destination address of the distance vector periodic update is 255.255.255.255 (the broadcast address).
7. State two ways that a route is selected as the preferred path.
A route is selected for the following reasons:
— Because it is the only available path
— Because the administrative distance is lower
— Because the metric is lower
8. What is administrative distance?
Administrative distance is the mechanism used by the routing process to select a path offered by two different routing protocols. The administrative distance is a set of values, in which a value is given to each IP routing protocol. This allows a hierarchy to be established so that when multiple protocols offer a path to the same remote network, one path can be chosen. The path that is chosen will be the one offered by the routing protocol with the lowest administrative distance. The administrative distance can be manually conﬁgured.
9. If IGRP has three paths to a remote network in which each path has an equal metric, what will happen?
If IGRP sees equal-cost paths to a remote network, it will load balance between those paths by default.
10. A distance vector routing protocol uses the mechanism of poison reverse. What is poison reverse?
When the routing process suspects that a route in its routing table is no longer valid, it sets the metric so high for that route that it renders it unusable. This metric will be propagated in the routing updates to other routers. This mechanism is used in triggered updates.
It is also used with split horizon. Instead of suppressing routes from being advertised out of the interface from which they were heard, it advertises the routes but sets the metric so high as to be rendered unusable. This is called split horizon with poison reverse.
11. Name two distance vector routing protocols.
Distance vector routing protocols include RIPv1, RIPv2, IGRP, and EIGRP (an advanced distance vector routing protocol).
12. Describe the mechanism of split horizon.
Split horizon is a routing technique in which information about routes is prevented from exiting the router interface through which that information was received. Split-horizon updates are useful in preventing routing loops.
13. What is meant by the phrase routing by rumor?
Distance vector routing protocols are said to be “routing by rumor” because information is never sent or ﬂooded to all routers but rather sent to neighbors who in turn forward their routing table.
Distance vector protocols send periodic updates. These updates are sent to directly connected neighbors. The update is periodic because it waits for a timer to expire before it sends an update.
After receiving a neighbor’s routing table, the router updates its own table and sends the modiﬁed table in subsequent updates.
14. Why does the use of multicast addressing in RIPv2 overcome some of the limitations of RIPv1?
The use of the multicast addressing saves network resources because all the nodes in the network can discard the packet at either Layer 2 or Layer 3 instead of taking the packet all the way up to the transport layer, where the port number is rejected. Although this saves system resources, the same amount of bandwidth is consumed on each link that the multicast traverses.
15. Explain the use of holddown in distance vector routing protocols to create stability in the network.
The use of holddown is crucial to network stability. The term holddown refers to when and how the routing process decides that a route in the routing table is no longer valid. After deciding that a network in the routing table is no longer available, the routing process waits for three routing updates (by default) before it believes a routing update with a less-favorable metric. Again, this is to prevent routing loops from generating false information throughout the network.
16. What is the maximum hop count in RIPv1 and RIPv2?
The maximum hop count is 15 with inﬁnity stated at 16.
17. Both EIGRP and IGRP use a composite metric. What are the main components of this metric?
Bandwidth, delay, reliability, and load are the interface values that are used for IGRP and EIGRP.
18. Explain briefly how RIPv2 differs from RIPv1.
RIPv2 differs from RIPv1 in the following ways:
— The metric is a composite calculated from bandwidth, delay, loading, reliability, and MTU. In fact, although MTU was originally designed as part of the metric, it is tracked but not used in the calculation. It is possible to conﬁgure the use of all the calculated elements of the metric. If these are not conﬁgured, the system will use only bandwidth and delay by default.
— The hop count is 100 and is conﬁgurable to 255 (although this is not used as a metric, but to age out datagrams).
— The update timer is set by default to 90 seconds (three times that of RIPv1).
— Unequal-cost load balancing occurs on multiple paths.
— A more efﬁcient packet structure is used.
— Autonomous systems are used to allow multiple processes within a routing domain, which allows the network to scale.
19. What is meant by the term convergence?
Convergence occurs when all the routers in the routing domain agree on the available routes. Convergence time is the time that it takes for every router’s routing table to synchronize after there has been a change in the network topology.
It is important to ensure that the time taken is as short as possible, because, while the routers disagree on the available networks, they cannot route data correctly or efﬁciently.
20. Give the configuration commands to turn on the process for RIPv2.