Appendix. Answers to Review Questions
This appendix provides answers to the review questions that appear at the end of Chapter 1 and the even-numbered chapters that cover the key aspects of RIP, IGRP, EIGRP, OSPF, IS-IS, PIM, and BGP.
Chapter 1
1 What is connectionless data networking?
Connectionless networking refers to transferring data in independent units referred to as packets, without the need to predefine the path of data flow. Instead, the packets are forwarded using a hop-by-hop routing paradigm between the source and destination.
2 Why is routing needed in a connectionless networking environment? List two means by which routers obtain information for routing packets toward their destinations.
The packets used in connectionless transfer of data have addressing information for their intended destination in packet headers. Routing is needed to provide information for forwarding packets along optimal paths to their target destinations.
Various mechanisms exist for forwarding packets on Cisco routers. However, forwarding decisions ultimately are based on information in the routing table, which is populated manually with static routes or dynamically by routing protocols.
3 What is the difference between functionalities of Interior Gateway Protocols (IGPs) versus Exterior Gateway Protocols (EGPs)?
IGPs exchange routes between routers belonging to a single network domain. EGPs support routing between domains.
4 List the two main groups of IP routing protocols based on the method of operation and routing algorithm. Also, list two examples of each type.
Distance vector and link-state protocols. RIP and Cisco IGRP are distance vector-based; OSPF and Integrated IS-IS are link-state protocols. EIGRP falls under yet a third group, called advance distance vector protocols.
5 Briefly describe the operation of link-state routing protocols.
Link-state routing protocols share and collect network topology information by means of link-state advertisements. Link-state information is stored in a database, which is fed as input to the shortest path algorithm for determining the best routes.
6 What is the key difference between classless and classful routing protocols? Give an example of each.
Classful protocols operate under the notion of the rigid boundaries of classful addressing, whereas classless protocols are more flexible in this, regarding allowing them to support VLSMs and CIDR.
RIP is an example of a classful routing protocol. OSPF is an example of a classless protocol.
7 What is the use of routing protocol administrative distances on Cisco routers?
The concept of administrative distance is used to distinguish between routing sources and assign relative preferences between them.
8 What are the values of administrative distance of IS-IS and OSPF, respectively?
The administrative distance of IS-IS is 115; the administrative distance of OSPF is 110.
9 If a router is running both OSPF and IS-IS protocols and has the same route from each of them, which protocol’s information will be used in the IP routing table?
The lower administrative distance is preferred, so only the OSPF route will make it into the IP routing table.
Chapter 2
1 What is the maximum metric in RIP?
The maximum metric is 15 because RIP was designed for small networks.
2 Why doesn’t RIP support discontiguous networks?
RIP is a classful protocol, so it summarizes the update at the major network boundary.
3 Why doesn’t RIP support VLSM?
When RIP sends the update, it checks to see whether the network being advertised has the same mask. If the advertised network has a different mask, RIP doesn’t advertise that network.
4 What is the default update interval for RIP?
The entire routing table is updated every 30 seconds.
5 What transport protocol and port number do RIP use for sending updates?
RIP uses UDP port 520 to transport its update packets.
6 What is the purpose of the split-horizon technique?
Split horizon is used in RIP to avoid routing loops.
7 Does RIP Version 2 solve the discontiguous network problem by default?
No, the command no auto-summary is needed under router rip.
8 Does RIP Version 2 also use broadcast for sending updates?
No, RIP Version 2 uses a multicast address of 224.0.0.9 to send its routing updates.
9 Does RIP support authentication?
RIP Version 1 does not support authentication, but RIP Version 2 does support it.
Chapter 4
1 What is the default update timer period for IGRP?
The default update timer period is 90 seconds.
2 What variables does IGRP use to calculate its metrics by default?
By default, IGRP considers only the bandwidth and the delay of the link when calculating its metrics.
3 What are the K values in the IGRP metric equation?
The K1 through K5 variables are constant numbers used in the IGRP metric equation. The default value of the K values are K1 = K3 = 1, K2 = K4 = K5 = 0. The network administrator can change the default K value to other numbers so that other components of the metric equation, such as load and reliability, can be used; however, such a change is highly not recommended.
4 What command is used in IGRP to perform unequal-cost load balancing?
IGRP uses the variance command to perform unequal-cost load balancing.
5 What is split horizon? Does IGRP support this feature?
Split horizon, supported by IGRP, is the technique used to avoid routing loops. With split horizon, the router does not advertise a route over the interface in which the route is learned from.
6 Does IGRP support VLSM?
Because IGRP does not send subnet mask information as part of the routing update, IGRP does not support VLSM.
Chapter 6
1 What is the difference between metric calculations in IGRP versus EIGRP?
The EIGRP metric is the IGRP metric multiplied by 256.
2 What is an EIGRP query, and what is it used for?
An EIGRP query is sent when the successor is gone and the feasible successor is not available. An EIGRP query is used so that EIGRP can have fast convergence.
3 What is the meaning of the term active route?
Active routes are routes in which the primary path is gone and no feasible successors are available. The router is actively searching for an alternate path.
4 What is a feasible successor?
A feasible successor is an EIGRP neighbor that does not satisfy the feasible condition. Feasible successors can also be thought of as EIGRP backup routes that are used when the primary route is gone.
5 What is EIGRP’s multicast address?
EIGRP’s multicast address is 224.0.0.10.
6 What is the feasible condition?
The feasible condition is a condition in which the reported distance is less than the feasible distances. This condition ensures a loop-free topology.
7 What is stuck in active?
Stuck in active is a condition in which the router has sent out queries for a lost route and has not received a reply within the active timer. By default, the active timer is three minutes.
Chapter 8
1 How many types of packet are there in OSPF?
OSPF has five types of packets.
2 Which of the LSAs have a field called Forwarding Address?
External LSAs have a Forwarding Address field.
3 Which of the LSAs are not allowed in a totally stubby area?
External and summary LSAs are not allowed in a totally stubby area.
4 What is the multicast address for AllSPFRouters?
224.0.0.5 is the multicast address.
5 Which of the OSPF protocol packets are used to elect a master and a slave?
Type 2 DBD packets are used to elect a master and a slave.
6 Which of the OSPF protocol packets implement flooding of the LSA?
Link-state update packets implement flooding of the LSA.
7 What is the time limit in seconds before an LSA is declared as MAXAGED?
The limit is 3600 seconds.
8 How many bytes long is a common LSA header?
A common LSA header is 20 bytes long.
Chapter 10
1 Name the three network layer protocols that form the basis of ISO connectionless network services.
CLNP, ES-IS, and IS-IS.
2 How many levels are there in the routing hierarchy supported by the IS-IS routing protocol?
ISO 10589 specifies two levels: Level 1 and Level 2.
3 What is the general layout of the IS-IS packet format?
All IS-IS packets consist of a header to which special routing information fields, known as TLVs, are appended.
4 What does the acronym NSAP stand for, and what is it used for?
NSAP stands for network service access point. NSAPs are network layer address OSI nodes running the CLNP protocol.
5 What are the three major components of an NSAP? Describe the significance of each.
The three components are area ID, system ID, and N-selector. The area ID defines the area that the node belongs to, the system ID is a unique address of the node within its area, and the N-selector specifies a network service user. A 0 value specifies the routing layer.
6 What is the maximum length of an NSAP, and what is the minimum length that can be configured on a Cisco router?
The maximum length of an NSAP is 160 bits, or 20 bytes. The minimum size that can be configured on a Cisco router is 8 bytes. The 8 bytes include 1 byte of N-selector, 6 bytes of system ID, and 1 byte of area ID.
7 What is the significance of the IS-IS link-state database?
Link-state protocols such as IS-IS require each router in an area to have the same view of the area’s topology. Each router creates a link-state packet that describes its immediate environment shared with other routers in the area. LSPs are collected in the link-state database. When pieced together, the LSPs in an area’s link-state database describe the topology of the entire area.
8 What is the basic difference between Level 1 and Level 2 link-state databases?
A Level 1 link-state database describes a single IS-IS area and contains only LSPs from the routers in that area. The Level 2 database describes the interconnection between areas in the IS-IS domain and contains LSPs from the Level 2 routers. The Level 2 LSPs are intended to provide interarea information.
9 How are flooding and database synchronization different between a point-to-point link and a broadcast link?
LSPs are flooded reliably with acknowledgments on point-to-point links, whereas they are not acknowledged on broadcast links. Database synchronization occurs on broadcast media through the designated router, which sends periodic CSNPs over the link to assist with synchronization.
10 Describe the two steps for enabling basic IS-IS routing on a Cisco router.
First, an IS-IS routing process is configured with the router isis [tag] command. Then, IS-IS routing is enabled on the relevant interfaces with the ip router isis [tag] interface-level command.
11 List some show commands that you can use to verify configuration and operation of IS-IS.
show clns neighbors, show clns interface, show isis topology, and show is-is database.
Chapter 12
1 What is the difference between unicast, broadcast, and multicast?
Unicast packets are destined for only one host. Broadcast packets are destined for all hosts on the same segment, regardless of whether the host is interested in the packet. Multicast packets are sent with one copy, and only hosts that are interested in the multicast packet process the packet.
2 What are the different modes of PIM?
PIM dense mode and PIM sparse mode are the two modes.
3 What mechanism does PIM dense mode operate on?
PIM dense mode operates on the flood-and-prune mechanism. The router first floods the multicast packets on all interfaces, and the neighbors that don’t want the multicast packet prune the interface.
4 What mechanism does PIM sparse mode operate on?
PIM sparse mode operates on the prune-and-join mechanism. The router won’t forward the multicast packet until it receives a PIM join on the interface.
5 What is the difference between IGMP version 1 and version 2 concerning the group leave mechanism?
IGMP version 1 doesn’t have a specific group leave mechanism. IGMP version 1 group members simply leave the group silently. IGMP version 2 has a specific group leave mechanism in which the host sends a specific IGMP leave message to the router indicating that it’s leaving the multicast group.
6 What multicast address does IGMP use for IGMP queries?
224.0.0.1 is used.
7 How does RPF check work?
When a router receives a multicast packet on an interface, it checks its routing table on the source address of the multicast packet. If the routing table corresponds with the interface from which the multicast packets are received, RPF check succeeds and packets are forwarded; otherwise, multicast packets are silently discarded.
8 What is the rendezvous point (RP)?
The rendezvous point (RP) is where the multicast sender and receiver meet first before the shortest-path tree is established.
Chapter 14
1 Does BGP have its own transport mechanism to ensure the guarantee of BGP updates?
A. BGP has its own transport mechanism to deliver BGP packets to its neighbors.
B. UDP is a preferred method because BGP neighbors are in most cases directly connected and the loss of packets is unlikely.
C. BGP uses TCP as its transport mechanism.
Answer: C. BGP uses TCP as its transport mechanism.
2 Assuming no Route-Reflection or Confederations are used, what problems might occur if IBGP neighbors are not fully meshed?
A. An IBGP update will not be propagated to BGP routers in the AS because the IBGP learned update is not announced to other IBGP neighbors.
B. Everything will run fine.
C. Only external BGP neighbors won’t receive the BGP updates.
Answer: A. An IBGP update will not be propagated to BGP routers in the AS because the IBGP learned update is not announced to other IBGP neighbors.
3 What BGP technique is used to penalize flapping of BGP routes in some other AS?
A. Route-Reflection
B. Dampening
C. Peer groups
Answer: B. Dampening.
4 The BGP process can exchange updates with its neighbors after passing which neighbor state?
A. Established
B. OpenSent
C. Active
Answer: A. Established.
5 Which of the following techniques are used in solving the IBGP full mesh requirement?
A. Dampening
B. Aggregation
C. Route Reflection and Confederation
Answer: C. Route Reflection and Confederation.