Appendix D. Answers to Review Questions
Answers to Chapter 1 Review Questions
1 What is the current version of EGP?
Answer: The current version of EGP is 2.
2 What is an EGP interior neighbor? An EGP exterior neighbor?
Answer: An EGP gateway’s neighbor is interior if it is within the same AS. An exterior neighbor is in a different AS.
3 What is the primary difference between an EGP stub gateway and an EGP core gateway?
Answer: Stub gateways can advertise only networks that are interior to their own AS. Core gateways can advertise both interior and exterior networks.
4 Why does EGP use the concept of a core, or backbone, AS?
Answer: EGP has no mechanisms for detecting loops. Therefore, a loop-free topology must be engineered physically so that inter-AS traffic must traverse a backbone.
5 What is the difference between an active EGP neighbor and a passive EGP neighbor?
Answer: An active neighbor initiates the peer relationship and sends Hellos to maintain it. Passive neighbors respond to Hellos with I-Heard-You messages.
6 What is the purpose of an EGP Poll message?
Answer: A Poll message is a request to a neighbor for an Update.
7 What is an indirect, or third-party, neighbor?
Answer: An indirect neighbor is a gateway that shares a common data link with another gateway and can reach certain networks through that gateway but is not peered directly with the gateway. Rather, it learns its reachability information from yet another gateway on the data link.
8 How does EGP use its metrics to calculate the best path to a destination?
Answer: Although EGP has a metric, it has no mechanism for determining best paths. Therefore, the metric is used only for indicating an unreachable network.
Answers to Chapter 2 Review Questions
1 What is the most important difference between BGP-4 and earlier versions of BGP?
Answer: BGP-4 is classless. Earlier versions are classful.
2 What two problems was CIDR developed to alleviate?
Answer: CIDR was developed to alleviate the explosion of Internet routing tables and to slow the depletion of Class B network addresses.
3 What is the difference between classful and classless IP routers?
Answer: Classful IP routers perform routing table lookups on the major class network address first and then match the subnet. Classless IP routers ignore the class of the destination address and try to make a longest match on the address prefix.
4 What is the difference between classful and classless IP routing protocols?
Answer: Classful IP routing protocols advertise only a network or subnet address, without any information about the prefix length. As a result, routers receiving the advertisement must make certain assumptions about the address prefix. Classless IP routing protocols include information that allows the receiving router to parse the address prefix. As a result, VLSM and summarization are possible with classless protocols.
5 Given the addresses 172.17.208.0/23, 172.17.210.0/23, 172.17.212.0/23, and 172.17.214.0/23, summarize the addresses with a single aggregate, using the longest possible address mask.
Answer: 172.17.208.0/21
6 What is an address prefix?
Answer: An IP address prefix is the part of an IP address that a router considers when making routing decisions. In a classful environment, the prefix is a major class network address or one of its subnets. In a classless environment, the prefix can be any number of leading bits in the 32-bit address.
7 The routing table in Example 2-16 is taken from a classless router. To what next-hop address does the router forward packets with each of the following destination addresses?
172.20.3.5
172.20.1.67
172.21.255.254
172.16.50.50
172.16.0.224
172.17.40.1
172.17.41.1
172.30.1.1
Example 2-16 The Routing Table for Review Question 7
Stratford#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default
Gateway of last resort is not set
172.20.0.0 is variably subnetted, 6 subnets, 2 masks
D 172.20.0.0 255.255.0.0 [90/409600] via 172.20.5.2, 00:01:50, Ethernet0
D 172.20.2.0 255.255.255.0
[90/409600] via 172.20.6.2, 00:01:50, Ethernet1
D 172.20.3.0 255.255.255.0
[90/5401600] via 172.20.6.2, 00:01:50, Ethernet1
C 172.20.5.0 255.255.255.0 is directly connected, Ethernet0
C 172.20.6.0 255.255.255.0 is directly connected, Ethernet1
C 172.20.7.0 255.255.255.0 is directly connected, Ethernet2
172.16.0.0 is variably subnetted, 3 subnets, 2 masks
D 172.16.50.0 255.255.255.0
[90/409600] via 172.20.6.2, 00:01:50, Ethernet1
D 172.16.0.0 255.255.255.0
[90/460800] via 172.20.6.2, 00:01:51, Ethernet1
D 172.16.0.0 255.255.0.0 [90/409600] via 172.20.7.2, 00:01:51, Ethernet2
172.17.0.0 is subnetted (mask is 255.255.255.0), 1 subnets
D 172.17.40.0 [90/2841600] via 172.20.7.2, 00:01:52, Ethernet2
D 172.16.0.0 (mask is 255.240.0.0) [90/409600] via 172.20.5.2, 00:01:52, Ethernet0
Stratford#
8 Explain how summarization helps hide network instabilities.
Answer: Member addresses, or destination addresses that are summarized by an aggregate address, are not advertised past the summarization point. So if the state of one of the member addresses changes, the change is not advertised past the summarization point.
9 Explain how summarization can cause asymmetric traffic patterns.
Answer: Summarization hides the details of the internetwork behind the summarization point. If a summary address is advertised by more than one router, the routers beyond the summarization points select only the closest summarizing router.
10 Is asymmetric traffic undesirable?
Answer: The answer is subjective. Asymmetric traffic can make baselining and troubleshooting more difficult, and if the internetwork is geographically large, delay-sensitive traffic can be affected. On the other hand, the benefits of summarization might outweigh these problems.
11 What is a NAP?
Answer: A network access point is a LAN or switch through which service providers may interconnect. From the perspective of Internet traffic flow, NAPs are the hierarchically highest points in the Internet topology.
12 What is a route server?
Answer: A route server is a server with which routers may peer via some routing protocol. Each router sends its updates to the route server rather than to the other peers. The route server applies the appropriate routing policies and then sends the updates to the other peers. Route servers are useful when many routers must peer across a common data link, as in a NAP, by reducing the number of peering sessions each router must establish. This can be especially important if the routers are using a unicast protocol such as BGP, in which a separate packet must be sent to each peer. A route server is not a router, because it performs no packet forwarding.
13 What is a provider-independent address space, and why can it be advantageous to have one?
Answer: A provider-independent address space is assigned by the regional IP address registry rather than as part of a service provider’s CIDR block. It proves useful if an AS is multihomed to different service providers. It is also useful because it is portable. That is, the owner of the address space can change ISPs without having to re-address.
14 Why can it be a problem to have a /21 provider-independent address space?
Answer: Some national service providers do not accept IP prefixes longer than a /19. As a result, a /21 might not be advertised to all parts of the Internet.
15 What is a routing policy?
Answer: A routing policy is a predefined set of rules for handling incoming and outgoing routes. Typical tools for setting routing policies are redistribution, route filters, and route maps.
16 What is the underlying protocol that BGP uses to reliably connect to its neighbors?
Answer: BGP uses TCP port 179.
17 What are the four BGP message types, and how is each one used?
Answer: The four BGP message types are Open, Keepalive, Update, and Notification. Open messages are used to initially identify a BGP speaker to its neighbor and begin a peering session. Keepalives maintain the peer connection. Updates are used to advertise routes, and Notification messages advise peers of errors.
18 In what state or states can BGP peers exchange Update messages?
Answer: BGP peers can exchange Update messages only when both are in the Established state.
19 What is NLRI?
Answer: Network Layer Reachability Information is the IP address prefix or prefixes advertised in a BGP Update.
20 What is a path attribute?
Answer: A path attribute is a characteristic of a BGP route.
21 What are the four categories of BGP path attributes?
Answer: The four categories of BGP path attributes are Well-known Mandatory, Well-known Discretionary, Optional Transitive, and Optional Nontransitive.
22 What is the purpose of the AS_PATH attribute?
Answer: The AS_PATH attribute describes the AS numbers that a received Update has crossed after it left the originating router. This information can be used to determine the shortest inter-AS path, and it is also used to detect routing loops.
23 What are the different types of AS_PATH?
Answer: AS_PATH types are AS_SEQUENCE, AS_CONFED_SEQUENCE, AS_SET, and AS_CONFED_SET. AS_SEQUENCE is an ordered set of AS numbers, and AS_SET is an unordered set of AS numbers. AS_CONFED_SEQUENCE and AS_CONFED_SET are the same as AS_SEQUENCE and AS_SET but are used only within BGP confederations.
24 What is the purpose of the NEXT_HOP attribute?
Answer: The NEXT_HOP attribute describes the IP address of the next-hop router that packets should be forwarded to in order to reach the destination advertised as the NLRI in a BGP Update.
25 What is the purpose of the LOCAL_PREF attribute?
Answer: If multiple IBGP speakers are advertising the same route within an AS, the LOCAL_PREF attribute can be used to identify the preferred route. The higher the LOCAL_PREF value, the more preferred the route.
26 What is the purpose of the MULTI_EXIT_DISC attribute?
Answer: When multiple links exist between two autonomous systems, EBGP speakers can use the MED to inform the neighboring AS of the preferred link for incoming traffic.
27 What attribute or attributes are useful if a BGP speaker originates an aggregate route?
Answer: THE ATOMIC_AGGREGATE informs downstream routers that a loss of route information has occurred due to aggregation. The AGGREGATOR attribute identifies the router that originated the aggregate.
28 What is a BGP administrative weight?
Answer: A BGP administrative weight is a Cisco-specific parameter that can be assigned to routes within a single router. The higher the weight, the more preferable the route. Weights are local to the router and are not advertised to peers.
29 Given an EBGP route and an IBGP route to the same destination, which route will a BGP router prefer?
Answer: If the weights, LOCAL_PREFs, AS_PATH lengths, ORIGIN codes, and MEDs are equal, EBGP routes are preferred over IBGP routes.
30 A router has two IBGP routes to the same destination. Path A has a LOCAL_PREF of 300 and three AS numbers in the AS_PATH. Path B has a LOCAL_PREF of 200 and two AS numbers in the AS_PATH. Assuming no other differences, which path will the router choose?
Answer: LOCAL_PREF has a higher priority in the BGP decision process than AS_PATH, so path A is chosen.
31 What is route dampening?
Answer: Route dampening is a mechanism by which BGP routes are assigned a penalty for changing state. The more often the state changes (the route flaps), the greater the accumulated penalties. If the penalties exceed a certain threshold, the route is suppressed for a time. As a result, unstable routes have less adverse effect on the BGP internetwork.
32 Define the penalty, suppress limit, reuse limit, and half-life as they apply to route dampening.
Answer: The penalty is a value assigned to a route by the route-dampening mechanism each time the route changes state. The suppress limit is a threshold that, if exceeded by a route’s accumulated penalties, signifies that the route should not be advertised. Reuse limit is a threshold that, if a suppressed route’s accumulated penalties falls below it, signifies that the route can again be advertised. The half-life is the rate at which a route’s accumulated penalties are reduced. At the end of each half-life, the penalty is reduced by half.
33 What is IGP synchronization, and why is it important?
Answer: IGP synchronization is a rule whereby a BGP router cannot advertise a transit route to an EBGP peer unless the route is found in the IGP routing table. If a BGP router forwards a transit packet to an IBGP peer via an IGP router, and the IGP router does not know the route, the packet is dropped.
34 Under what circumstances can you safely disable IGP synchronization?
Answer: You can safely turn off IGP synchronization if the IBGP peers in an AS are fully meshed, or when the AS is not a transit AS.
35 What is a BGP peer group?
Answer: A BGP peer group is a group of BGP peers that have been identified on a single router to share common routing policies. Peer groups simplify configuration by allowing route policies to be applied to the group rather than to each individual member.
36 What is a BGP community?
Answer: A BGP community is a group of routes that share common routing policies. They work by setting a common COMMUNITY attribute in the routes; peers receiving those routes can recognize the COMMUNITY attribute and apply the appropriate policy.
37 What is a route reflector? What is a route reflection client? What is a route reflection cluster?
Answer: A route reflector is similar to a route server in that it permits IBGP routers to peer with it rather than with each other. Routes from one peer are advertised, or reflected, to the other peers. As a result, the number of peering sessions is reduced from what would be required if the IBGP peers were fully meshed. Route reflectors differ from route servers in that the route reflector is also a router. A route reflection client is an IBGP router that has peered with a route reflector. A route reflection cluster is a route reflector and its clients. A cluster can have more than one route reflector, but all the clients in the cluster must be peered with all the route reflectors in the cluster.
38 What is the purpose of the ORIGINATOR_ID and the CLUSTER_LIST path attributes?
Answer: The ORIGINATOR_ID and CLUSTER_LIST attributes prevent routing loops when route reflectors are being used.
39 What is a BGP confederation?
Answer: A BGP confederation is a large AS that has been subdivided into a group of smaller autonomous systems for easier manageability.
40 Can route reflectors be used within confederations?
Answer: Yes.
41 What is the purpose of the next-hop-self function? Are there any reasonable alternatives to using this function?
Answer: next-hop-self tells a router to change the NEXT_HOP attribute of routes received from an external peer to its own IP address. This function is used when the IGP has no knowledge of the external next-hop address. An alternative method is to run the IGP passively on the external link so that it knows the subnet on which the external next-hop address resides.
Answers to Chapter 5 Review Questions
1 Give several reasons why replicated unicast is not a practical substitution for true multicast in a large network.
Answer: Replicated unicast places a processing burden on the source and can cause severe bottlenecks at the source interface, data link, and connected router. The source also must hold state to remember what addresses to send the replicated packets, and there must be some potentially complex mechanism for members to signal joins and leaves to the source. Finally, replicated unicast can cause queuing problems and unacceptable latency between packets.
2 What range of addresses is reserved for IP multicast?
Answer: The Class D addresses, in which the first four bits are 1110. This address range is 224.0.0.0–239.255.255.255.
3 How many subnets can be created from a single Class D prefix?
Answer: No subnets are created from a Class D prefix. IP multicast uses only single addresses, not subnets.
4 In what way do routers treat packets with destination addresses in the range 224.0.0.1–224.0.0.255 differently from other multicast addresses?
Answer: Routers do not forward packets with destination addresses in the range 224.0.0.1 through 224.0.0.255.
5 Write the Ethernet MAC addresses that correspond to the following IP addresses:
(a) 239.187.3.201
(b) 224.18.50.1
(c) 224.0.1.87
(a) 0100.5E3B.03C9
(b) 0100.5E12.3201
(c) 0100.5E00.0157
6 What multicast IP address or addresses are represented by the MAC address 0100.5E06.2D54?
Answer: The MAC address 0100.5E06.2D54 can represent any of 32 IP addresses in which the first octet is 1 of 15 numbers in the range 224–239, the second octet is either 134 or 6, the third octet is always 45, and the last octet is always 87.
7 Why is Token Ring a poor medium for delivering multicast packets?
Answer: Token Ring is a poor medium for delivery of IP multicast packets because of the Token Ring frame’s little-endian format, which prevents an easy encoding of the multicast IP address into the MAC address. Instead, either a reserved functional MAC address or a broadcast MAC address must be used, either of which can sharply reduce efficiency on the data link.
8 What is join latency?
Answer: Join latency is the time between when a host first signals a desire to join a group and the time the host begins receiving group traffic.
9 What is leave latency?
Answer: Leave latency is the time between when a host first leaves a group and the time the host is removed from the group.
10 What is a multicast DR (or querier)?
Answer: A multicast querier is the router on a subnet responsible for querying the attached hosts for group membership.
11 What device sends IGMP Query messages?
Answer: IGMP Query messages are sent by routers. If more than one router is attached to the subnet, the router with the lowest IP address is the querier.
12 What device sends IGMP Membership Report messages?
Answer: Hosts send IGMP Membership Report messages.
13 How is an IGMP Membership Report message used?
Answer: An IGMP Membership Report is sent by a host to inform the local router that it wants to join a group.
14 What is the functional difference between a General IGMP Query and a Group-Specific IGMP Query?
Answer: A router sends a General IGMP Query to discover members of any and all groups. A Group-Specific IGMP Query is sent to discover members of a specific group, usually after the reception of a Leave Group message.
15 Is IGMPv2 compatible with IGMPv1?
Answer: IGMPv2 is mostly compatible with IGMPv1, although if there is an IGMPv1 router on a subnet, all routers should be set to IGMPv1.
16 What IP protocol number signifies IGMP?
Answer: IGMP uses protocol number 2.
17 What is the purpose of the Cisco Group Membership Protocol (CGMP)?
Answer: CGMP is a protocol by which Ethernet switches can discover which ports group members are connected to and thereby avoid having to forward IP multicast frames out all ports.
18 What is the advantage of using IP Snooping rather than CGMP? What is the possible disadvantage?
Answer: Unlike CGMP, IP Snooping is not proprietary and therefore may be preferable in a mixed-vendor environment. Its potential disadvantage is that if IP Snooping is supported on a switch only in software, it can affect performance.
19 What devices send CGMP messages: routers, Ethernet switches, or both?
Answer: Only routers send CGMP messages. Switches listen for CGMP messages.
20 What is Reverse Path Forwarding?
Answer: RPF is the basic forwarding mechanism of IP multicast routing. Because the routers find the shortest paths to the source rather than the destination, when multicast packets are forwarded toward the destination (or, more accurately, away from the source), they are forwarded in the reverse direction along the shortest path.
21 How many hosts constitute a dense topology, and how many hosts constitute a sparse topology?
Answer: There is no set number differentiating sparse and dense topologies.
22 What is the primary advantage of explicit joins over implicit joins?
Answer: The primary advantage of explicit joins over implicit joins is that routers do not have to hold state for interfaces that are not upstream from any group members.
23 What is the primary structural difference between a source-based multicast tree and a shared multicast tree?
Answer: A source-based tree is rooted at the source subnet or source router, whereas a shared tree is rooted at some common rendezvous point or core and can be, by definition, shared by multiple sources.
24 What is multicast scoping?
Answer: Multicast scoping is the practice of limiting the range of certain multicast packets to a determined topological area.
25 What are the two methods of IP multicast scoping?
Answer: The two methods of IP multicast scoping are TTL scoping and administrative scoping.
26 From the perspective of a multicast router, what is meant by upstream and what is meant by downstream?
Answer: Upstream is the direction toward a multicast source, and downstream is the direction away from the source.
27 What is an RPF check?
Answer: An RPF check is a verification that a multicast packet from a particular source has arrived on the upstream interface toward that source and no other interface.
28 What is a prune? What is a graft?
Answer: A prune is the action of removing a router from a multicast tree. A graft is the action of adding a router to a multicast tree.
29 What is a prune lifetime? What happens when a prune lifetime expires?
Answer: A prune lifetime, used by implicit join protocols, is the amount of time that a router holds an interface in prune state. When a prune lifetime expires, the router again forwards packets on the interface until the downstream neighbor again requests a prune.
30 What is a route dependency? How does DVMRP signal a route dependency?
Answer: A route dependency is a dependency a router has on an upstream neighbor to forward packets for a particular group. DVMRP routers signal a route dependency by using a poison reverse route, in which the metric is the advertised hop count to the source plus 32.
31 Is DVMRP a dense-mode protocol or a sparse-mode protocol?
Answer: DVMRP is a dense-mode protocol.
32 Is MOSPF a dense-mode protocol or a sparse-mode protocol?
Answer: MOSPF is a dense-mode protocol.
33 What is the name and type number of the LSA used exclusively by MOSPF?
Answer: The LSA used exclusively by MOSPF is the Group Membership LSA, which is type 6.
34 Can an MOSPF router establish an adjacency with an OSPF router that does not support MOSPF?
Answer: Yes, although only neighbors whose MC bits are set in their Database Description packets exchange Group Membership LSAs.
35 Define the following MOSPF router types:
(a) Interarea multicast forwarder
(b) Inter-AS multicast forwarder
(c) Wildcard multicast receiver
(a) An interarea multicast forwarder forwards IP multicast packets between areas and is similar to a unicast OSPF ABR.
(b) An inter-AS multicast forwarder forwards IP multicast packets outside of the MOSPF domain and is similar to a unicast OSPF ASBR.
(c) A wildcard multicast receiver is a router to which all multicast packets are forwarded.
36 Is CBT a dense-mode protocol or a sparse-mode protocol?
Answer: CBT is a sparse-mode protocol.
37 What are a CBT parent router and a CBT child router?
Answer: A CBT parent router is an upstream router, and a CBT child router is a downstream router.
38 Describe the two ways a CBT DR can deliver packets from a source to the core and the circumstances under which each method is used.
Answer: If a directly connected source is a member source, its packets are forwarded on the tree. If the source is a nonmember source, a tunnel to the core is created, and the packets are forwarded over the tunnel.
39 What is a PIM prune override?
Answer: A prune override is a Join message sent to an upstream router on a multiaccess network to cancel a prune requested by another router on the same network.
40 What is a PIM forwarder? How is a forwarder selected?
Answer: When multiple upstream routers are connected to the same multiaccess network and are receiving packets for the same group, the PIM forwarder is the router that forwards the packets onto the network. The forwarder is elected by the lowest administrative distance advertised in an Assert message. If the administrative distances are equal, the lowest route metric is used. If the metrics are the same, the lowest IP address is the tiebreaker.
41 What criteria does PIM use to select a DR?
Answer: The PIM router with the highest IP address (according to the PIM Hello messages) is the DR.
42 What is a PIM SPT? What is a PIM RPT?
Answer: A shortest path tree is a source-based tree, and a rendezvous point tree is a shared tree rooted at a rendezvous point.
43 What two mechanisms are available for Cisco routers to automatically discover PIM-SM RPs?
Answer: PIM-SM RPs can be automatically discovered using either Auto-RP or the bootstrap protocol.
44 Of the mechanisms in Question 43, which should be used in multivendor router topologies?
Answer: Auto-RP may not be supported by other vendors, so bootstrap protocol should be used.
45 What is a C-RP?
Answer: A C-RP is a Candidate RP, or a router that is eligible to become an RP for either all groups or a specified set of groups.
46 What is a BSR?
Answer: When the bootstrap protocol is used, a bootstrap router advertises C-RPs throughout the PIM-SM domain in an RP-Set.
47 What is an RP mapping agent?
Answer: When Auto-RP is used, an RP mapping agent advertises group-to-RP mappings.
48 What is the difference between an (S,G) mroute entry and a (*, G) mroute entry?
Answer: An (S,G) entry refers to an SPT, whereas a (*, G) entry refers to an RPT.
49 What is the major drawback with a bidirectional CBT tree between the source and core, as opposed to a PIM-SM unidirectional tree from the RP to the source?
Answer: It is difficult to guarantee a loop-free path with bidirectional trees, because there is no distinct upstream and downstream.
50 What is PIM-SM source registration?
Answer: Source registration is a mechanism whereby a router forwards packets from a multicast source to an RP in PIM Register messages. If there is significant traffic from the source, the RP builds an SPT and then sends a Register Stop.
51 When does a Cisco router switch from a PIM-SM RPT to an SPT?
Answer: Cisco routers switch from an RPT to an SPT immediately after receiving the first packet for a particular (S,G) on the RPT, or when the arrival rate of the packets for the (S,G) exceeds a threshold specified with the command ip pim spt-threshold.
Answers to Chapter 7 Review Questions
1 In the section "Multicast Scoping," a sample configuration is given for administrative scoping. The boundary at interface E0 blocks organization-local packets (destination addresses whose prefixes match 239.192.0.0/14) but passes packets with global scope. Will a packet with a group address 224.0.0.50 pass this boundary?
Answer: Packets with a destination address of 224.0.0.50 pass this boundary only if the local router originates them. Although 224.0.0.50 is permitted by access list 10, it is in the link-local range and so is not forwarded by any next-hop router.
2 How does Cisco IOS Software handle DVMRP Prune messages on point-to-point and multiaccess interfaces that are configured to run PIM?
Answer: DVMRP Prunes are ignored on multiaccess interfaces and are processed normally on point-to-point interfaces.
3 Why does Cisco IOS Software accept DVMRP Probe messages but does not send them?
Answer: The reception of DVMRP Probes is necessary to detect DVMRP neighbors. Probes are not sent because another Cisco PIM router on a multiaccess network would mistake the originator for a DVMRP-only router.
4 What is a PIM (*,*,RP) entry?
Answer: A (*,*,RP) entry is to a PIM Multicast Border Router. MBRs are not supported by Cisco IOS.
5 How does Multiprotocol BGP (MBGP) differ from normal BGP?
Answer: MBGP is extended with two route attributes: MP_REACH_NLRI and MP_UNREACH_NLRI.
6 What is the MBGP AFI?
Answer: The AFI is the Address Family Identifier. When MBGP is used for multicasting, the AFI is always set to 1 (for IPv4), and the sub-AFI will indicate whether the related NLRI is to be used for multicast, unicast, or both.
7 Is the following statement true or false? MSDP carries information about multicast sources and group members between RPs in different PIM domains.
Answer: False. MSDP only communicates information about multicast sources, not group members.
8 What is the transport protocol for MSDP?
Answer: MSDP uses TCP port 639.
9 What is an MSDP SA message?
Answer: An SA is a Source Active message. When a source’s DR registers with an RP, if the RP is running MSDP, it advertises the (S,G) pair to its peers in SA messages.
10 How does an MSDP RP determine whether an SA was received on an RPF interface?
Answer: It checks the BGP next-hop database (MBGP first, and then unicast BGP) for the correct upstream interface.
11 What is SA caching?
Answer: SA caching is the storage of (S,G) state information learned from SA messages. SA caching trades some memory in the router for reduced join latency. By default, SA caching is disabled in Cisco IOS Software.
12 Is there an alternative to reducing join latency without enabling SA caching?
Answer: Yes. If an MSDP peer is caching, you can configure an RP to use SA Request messages to request (S,G) information from the peer as soon as a join is received.
Answers to Chapter 8 Review Questions
1 Which of the following are valid representations for the address 200A 0000 0000 0C00 0000 0000 0000 0000 with a 60-bit prefix?
A. 200A:0000:0000:0C/60
B. 200A::0C00:0:0:0:0/60
C. 200A:0000:0000:0C00::/60
D. 200A::0C00::/60
F. 200A::0C/60
Answer: B, C, E. A is not a complete address. D is ambiguous, with two sets of ::. F doesn’t expand to the correct address.
2 For what is the address 0:0:0:0:0:0:0:0 used?
Answer: This is the unspecified address. It represents the absence of an address. If this is the source address of a packet, the interface has not yet been assigned an address. It is attempting to discover whether its tentative address is being used by another node.
3 You configure your site border routers, connecting to an IPv6 public network, to advertise all your internal network numbers, including FEC0:0020:0:0100::/56. You get a nasty call from the IPv6 public network administrator. What is wrong?
Answer: FEC0:0020:0:0100::/56 is a site-local address. It must never be advertised beyond the boundaries of a site.
4 Which extension headers are processed by every IPv6 node in the path from source to destination?
Answer: Hop-by-Hop.
5 Which extension headers are used to specify a list of routers to visit before reaching the destination and to have each of those routers process the header?
Answer: Destination Options header followed by the Routing header.
6 A router receives a packet larger than its outgoing link’s MTU. Does it fragment the packet and forward the fragments toward the destination?
Answer: No. It drops the packet and sends an ICMP Packet Too Big message back to the source. The source uses these ICMP packets to perform path MTU discovery. It is the sole responsibility of the source to fragment the packet.
7 If set in a Router Advertisement, what affect does the Managed bit have?
Answer: The router sends the RA to all hosts on a link. If the Managed bit is set, the hosts obtain an address from a stateful configuration server.
8 If a router advertises prefix information in its RAs, how is the information used?
Answer: Prefix information included in RAs tells hosts which prefixes are on-link and/or which prefixes to use when they autoconfigure their addresses.
9 In what two states can a host’s IP address reside, and what are the roles of the two states?
Answer: Preferred and deprecated. A preferred address can be used to initiate any IP session. A deprecated address should be used only to maintain an existing connection, not to initiate a new connection, if a preferred address exists.
10 What information does a router advertise in its RA to tell hosts to stop using a particular prefix when initiating IP sessions?
Answer: Either a Valid Lifetime or Preferred Lifetime of value 0. Valid Lifetime 0 says the prefix is no longer valid. Preferred Lifetime 0 says to deprecate the prefix.
11 If a node has a neighbor with state DELAY, can the node send the neighbor packets?
Answer: Yes. A neighbor with state DELAY has not been verified reachable, but the node will send packets to its cached link layer address for the neighbor.
12 A host is not running any routing protocol. It is sending data to a remote node using a default router. The default router fails. Will the host continue to send data into the black hole of the dead router until its TCP connection fails?
Answer: No. The neighbor unreachability process, default router list, and address resolution processes will assist the host in discovering the dead router and finding a new one.
13 What are the scope values for multicast packets, and for what are they used?
Answer: Node, link, site, organization, global. The scope values are used to limit the meaning of a multicast group and to control how far a multicast packet can travel.
14 What Cisco router command enables IPv6 routing?
Answer: ipv6 unicast-routing
15 What interface subcommands enable IPv6 on an interface?
Answer: ipv6 enable, ipv6 address address prefix [eui-64]
16 What commands are used to enable a RIPng process?
Answer: Interface subcommand: ipv6 rip process-name enable
17 How is the BGP-for-IPv6 process enabled between neighbors?
router bgp local-AS
neighbor neighbor-ipv6-address remote-as remote-as-number
!
address-family ipv6
neighbor neighbor-ipv6-address activate
Answers to Chapter 9 Review Questions
1 Explain the difference between SNMP polls and traps.
Answer: A management station requesting information from a router is polling the router. A router that sends unsolicited information about an event that occurred sends a trap.
2 If you specify the severity level of messages logged to be Errors, what other levels of messages are logged?
Answer: Emergencies, Alerts, Critical
3 You look at a router interface and see that there are unusual traffic patterns. Normally, all traffic is inbound, but now there is outbound traffic. How can you quickly determine the source and destination of the traffic?
Answer: Enable IP accounting on the interface. Use the show ip accounting command repeatedly to see which source/destination pair is sending the traffic.