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.
A:	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?
A:	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?
A:	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?
A:	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: 239.187.3.201 224.18.50.1 224.0.1.87
A:	[click here] 0100.5E3B.03C9 0100.5E12.3201 0100.5E00.0157
6:	What multicast IP address or addresses are represented by the MAC address 0100.5E06.2D54?
A:	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?
A:	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?
A:	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?
A:	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)?
A:	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?
A:	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?
A:	Hosts send IGMP Membership Report messages.
13:	How is an IGMP Membership Report message used?
A:	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?
A:	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?
A:	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?
A:	IGMP uses protocol number 2.
17:	What is the purpose of the Cisco Group Membership Protocol (CGMP)?
A:	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 ?
A:	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?
A:	Only routers send CGMP messages. Switches listen for CGMP messages.
20:	What is Reverse Path Forwarding?
A:	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?
A:	There is no set number differentiating sparse and dense topologies.
22:	What is the primary advantage of explicit joins over implicit joins?
A:	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?
A:	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?
A:	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?
A:	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?
A:	Upstream is the direction toward a multicast source, and downstream is the direction away from the source.
27:	What is an RPF check?
A:	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 ?
A:	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 ?
A:	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?
A:	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?
A:	DVMRP is a dense-mode protocol.
32:	Is MOSPF a dense-mode protocol or a sparse-mode protocol?
A:	MOSPF is a dense-mode protocol.
33:	What is the name and type number of the LSA used exclusively by MOSPF?
A:	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?
A:	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: Interarea multicast forwarder Inter-AS multicast forwarder Wildcard multicast receiver
A:	(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?
A:	CBT is a sparse-mode protocol.
37:	What are a CBT parent router and a CBT child router?
A:	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.
A:	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?
A:	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?
A:	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?
A:	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?
A:	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?
A:	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?
A:	Auto-RP may not be supported by other vendors , so bootstrap protocol should be used.
45:	What is a C-RP?
A:	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?
A:	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?
A:	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?
A:	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?
A:	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?
A:	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?
A:	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.