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Flashcards in Multicast Deck (106):

In PIM-SM what control plane signaling must a multicast source perform before it begins to send multicast traffic to a group?

No control plane signaling needs to be performed; the source can simply begin sending on the local subnet.


The ip pim autorp listener command is used to do what?

Allow Auto-RP packets in groups and to be flooded in dense mode out interfaces configured with the ip pim sparse-mode command


In order to configure two routers as anycast RPs, what minimal requirements must be satisfied?

Multicast Source Discovery Protocol must be configured between the two anycast RPs.


What action must be taken by a host if it wants to join a multicast group?

send an IGMPv2 membership report using multicast on the local subnet with the destination IP address set to the multicast group being joined


What is the designated forwarder in bidirectional PIM?

It has the best route to the rendezvous point and is the only router on the local subnet that may forward multicast traffic up the shared tree


Why does the network administrator always avoid applying the multicast address to multicast applications?

This Layer 3 IP multicast address is mapped to a layer 2 MAC address that will always be flooded to all ports of a Cisco Layer 2 switch


What happens after a failure of one of the anycast RPs?

the PIM network will reconverge on the remaining anycast RP or RPs in roughly the same time that it takes unicast routing to reconverge


Which improvements does IGMPv3 offer over IGMPv2?

IGMPv3 added the ability for a host to specify which sources in a multicast group it wishes to receive

IGMPv3 added the ability for a host to specify which sources in a multicast group it does not wishes to receive

IGMPv3 removed the report-suppression feature for IGMP membership reports


IP multicast addresses range reserved for administratively scoped multicast?


Each SPT (S,G) and shared tree (*,G) is defined as an entry in the multicast routing table. Once the table is built, any multicast packets received that match a specific (S,G) or (*,G) route entry will be forwarded out the outgoing interface list. Which addresses can be used in the S entries?

any class A, class B, or class C host addresses


Give several reasons why replicated unicast is not a practical substitution for true multicast in a large network.

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.


What range of addresses is reserved for IP multicast?

No subnets are created from a Class D prefix. IP multicast uses only single addresses, not subnets.


In what way do routers treat packets with destination addresses in the range– differently from other multicast addresses?

Routers do not forward packets with destination addresses in the range through


What is join latency?

the time between when a host first signals a desire to join a group and the time the host begins receiving group traffic.


What is leave latency?

the time between when a host first leaves a group and the time the host is removed from the group.


What is a multicast DR (or querier)?

A multicast querier is the router on a subnet responsible for querying the attached hosts for group membership.


What device sends IGMP Query messages?

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.


What device sends IGMP Membership Report messages?

Hosts send IGMP Membership Report messages.


How is an IGMP Membership Report message used?

An IGMP Membership Report is sent by a host to inform the local router that it wants to join a group.


What is the functional difference between a General IGMP Query and a Group-Specific IGMP Query?

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.


Is IGMPv2 compatible with IGMPv1?

IGMPv2 is mostly compatible with IGMPv1, although if there is an IGMPv1 router on a subnet, all routers should be set to IGMPv1.


What IP protocol number signifies IGMP?



What is the purpose of the Cisco Group Membership Protocol (CGMP)?

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.


What is the advantage of using IP Snooping rather than CGMP? What is the possible disadvantage?

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.


What devices send CGMP messages: routers, Ethernet switches, or both?

Only routers send CGMP messages. Switches listen for CGMP messages.


What is Reverse Path Forwarding?

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.


How many hosts constitute a dense topology, and how many hosts constitute a sparse topology?

There is no set number differentiating sparse and dense topologies.


What is the primary advantage of explicit joins over implicit joins?

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.


What is the primary structural difference between a source-based multicast tree and a shared multicast tree?

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.


What is multicast scoping?

Multicast scoping is the practice of limiting the range of certain multicast packets to a determined topological area.


What are the two methods of IP multicast scoping?

The two methods of IP multicast scoping are TTL scoping and administrative scoping.


From the perspective of a multicast router, what is meant by upstream and what is meant by downstream?

Upstream is the direction toward a multicast source, and downstream is the direction away from the source.


What is an RPF check?

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.


What is a prune? What is a graft?

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.


What is a prune lifetime? What happens when a prune lifetime expires?

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.


What is a PIM prune override?

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.


What is a PIM forwarder? How is a forwarder selected?

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.


What criteria does PIM use to select a DR?

The PIM router with the highest IP address (according to the PIM Hello messages) is the DR.


What is a PIM SPT? What is a PIM RPT?

A shortest path tree is a source-based tree, and a rendezvous point tree is a shared tree rooted at a rendezvous point.


What two mechanisms are available for Cisco routers to automatically discover PIM-SM RPs?

PIM-SM RPs can be automatically discovered using either Auto-RP or the bootstrap protocol.


Which RP discovery mechanism is Cisco proprietary

Auto-RP is cisco proprietary


What is a C-RP?

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.


What is a BSR?

When the bootstrap protocol is used, a bootstrap router advertises C_RPs throughout the PIM-SM domain in an RP-Set.


What is an RP mapping agent?

When Auto-RP is used, an RP mapping agent advertises group-to-RP mappings.


What is the difference between an (S, G) mroute entry and a (*, G) mroute entry?

An (S, G) entry refers to an SPT, whereas a (*, G) entry refers to an RPT.


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?

It is difficult to guarantee a loop-free path with bidirectional trees, because there is no distinct upstream and downstream.


What is PIM-SM source registration?

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.


When does a Cisco router switch from a PIM-SM RPT to an SPT?

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.


Is the following statement true or false? MSDP carries information about multicast sources and group members between RPs in different PIM domains.

False. MSDP only communicates information about multicast sources, not group members.


What is the transport protocol for MSDP?

MSDP uses TCP port 639.


What is an MSDP SA message?

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.


How does an MSDP RP determine whether an SA was received on an RPF interface?

It checks the BGP next-hop database (MBGP first, and then unicast BGP) for the correct upstream interface.


What is SA caching?

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.


Is there an alternative to reducing join latency without enabling SA caching?

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.


Define multicasting

Sending a message from a single source or multiple sources to selected multiple destinations across a Layer 3 network in one data stream.


Define multicast address range through


Define multicast address structure

The first 4 bits of the first octet must be 1110. The last 28 bits are unstructured.


Define permanent multicast group

The multicast addresses assigned by IANA.


Define source-specific addresses

The range through that is allocated by IANA for SSM destination addresses and is reserved for use by source-specific applications and protocols.


Define GLOP addressing

The range through that IANA has reserved (RFC 2770) on an experimental basis. It can be used by anyone who owns a registered autonomous system number to create 256 global multicast addresses.


Define administratively scoped addresses

The range through that IANA has assigned for use in private multicast domains.


Define Transient multicast group

Multicast addresses that are not assigned by IANA.


Define multicast MAC address

A 48-bit address that is calculated from a Layer 3 multicast address by using 0x0100.5E as the multicast vendor code (OUI) for the first 24 bits, always binary 0 for the 25th bit, and copying the last 23 bits of the Layer 3 multicast address.


Define joining a group

The process of installing a multicast application; also referred to as launching an application.


Define IGMP

A communication protocol between hosts and a multicast router by which routers learn of which multicast groups’ packets need to be forwarded onto a LAN.


Define MRT

After a host receives an IGMP Query, the amount of time (default, 10 seconds) the host has to send the IGMP Report.


Define report suppression mechanism

When a Query is received from a router, each host randomly picks a time between 0 and the Maximum Response Time period to send a Report. When the host with the smallest time period first sends the Report, the rest of the hosts suppress their reports.


Define IGMPv2 host membership query

A message sent by a multicast router, by default every 125 seconds, on each of its LAN interfaces to determine whether any host wants to receive multicast traffic for any group.


Define IGMPv2 leave

A message sent by a host when it wants to leave a group, addressed to the All Multicast Routers address


Define IGMPv2 group-specific query

A message sent by a router, after receiving a Leave message from a host, to determine whether there are still any active members of the group. The router uses the group address as the destination address.


Define IGMPv2 Host membership report

A message that each host sends, either in response to a router Query message or on its own, to all multicast groups for which it would like to receive multicast traffic.


Define SSM

Receivers subscribe to an (S,G) channel when they request to join a multicast group. That is, they specify the unicast IP address of their multicast source and the group multicast address. SSM is typically used in very large multicast deployments such as television video.


Define querier election

When multiple routers are connected to a subnet, only one should be sending IGMP queries. It is called a querier. IGMPv1 does not have any rules for electing a querier. In IGMPv2 and IGMPv3, a router with the lowest interface IP address on the subnet is elected as a querier.


Define CGMP

A Cisco-proprietary feature. After a Cisco multicast router receives IGMP Join or Leave messages from hosts, it communicates to the connected Cisco switches, telling them which hosts (based on their unicast MAC addresses) have joined or left each multicast group. Switches examine their CAM tables and determine on which ports these hosts are connected and either forward multicast traffic or stop forwarding on those ports only.


Define IGMP snooping

A method for optimizing the flow of multicast IP packets passing through a LAN switch. The switch using IGMP snooping examines IGMP messages to determine which ports need to receive traffic for each multicast group.


Define RGMP

A Cisco-proprietary Layer 2 protocol that enables a router to communicate to a switch which multicast group traffic the router does and does not want to receive from the switch.


Define dense-mode protocol

A multicast routing protocol whose default action is to flood multicast packets throughout a network.


Define RPF check

Designed to solve the problems of multicast duplication and multicast routing loops. For every multicast packet received, a multicast router examines its source IP address, consults its unicast routing table, determines which interface it would use to go in the reverse direction toward the source IP address, compares it with the interface on which the packet was received, and, if they match, accepts the packet and forwards it; otherwise, the router drops the packet.


Define sparse-mode protocol

A multicast routing protocol that forwards the multicast traffic only when requested by a downstream router.


Define RP

In the PIM-SM design, the central distribution point to which the multicast traffic is first delivered from the source designated router.


Define multicast scoping

The practice of defining boundaries that determine how far multicast traffic will travel in your network.


Define TTL scoping

Controls the distribution of multicast traffic by checking the TTL values configured on the interfaces. It forwards the multicast packet only on those interfaces whose configured TTL value is less than or equal to the TTL value of the multicast packet.


Define administrative scoping

Controls the distribution of multicast traffic for the private multicast address range to by configuring a filter and applying it on the interfaces.


Define PIM-DM

PIM-DM is a method of routing multicast packets that depends on a flood-and-prune approach. PIM Dense Mode gets its name from the assumption that there are many receivers of a particular multicast group, close together (from a network perspective). Does not depend on any particular unicast routing protocol to perform its multicast functions.


Define PIM hello message

Sent by a PIM router, by default every 30 seconds, on every interface on which PIM is configured to discover neighbors, establish adjacency, and maintain adjacency.


Define designated router

With PIM on a multiaccess network, the PIM router with the highest IP address on the subnet. With OSPF, the OSPF router that wins an election amongst all current neighbors. The DR is responsible for flooding on the subnet, and for creating and flooding the type 2 LSA for the subnet.


Define source-based distribution tree

Method by which a dense-mode routing protocol distributes multicast traffic from a source to all the segments of a network. Also called shortest-path tree (SPT), because it uses the shortest routing path from the source to the segments of the network.


Define Multicast state information

The information maintained by a router for each multicast entry in its multicast routing table, such as incoming interface, outgoing interface list, Uptime timer, Expire timer, etc.


Define Join/Prune message

Sent by a PIM router to its upstream router to either request that the upstream router forward the group traffic or stop forwarding the group traffic that is currently being forwarded. If a PIM router wants to start receiving the group traffic, it lists the group address under the Join field. If it wants the upstream router to stop forwarding the group traffic, it lists the group address under the Prune field.


Define upstream router

From one multicast router’s perspective, the upstream router is another router that has just forwarded a multicast packet to that router.


Define downstream router

The router that will receive the group traffic when a multicast router forwards group traffic to another router.


Define graft message

Message sent by a PIM-DM router to its upstream router asking to quickly restart forwarding the group traffic; sent using the unicast address of the upstream router.


Define Graft ack message

Message sent by a PIM-DM router to a downstream router when it receives a Graft message from the downstream router; sent using the unicast address of the downstream router.


Define prune override

On a multiaccess network, when a PIM-DM or PIM-SM router receives a Prune message, it starts a 3-second timer. If it receives a Join message on the multiaccess network from another router before the timer expires, it considers the message as an override to the previously received Prune message and continues forwarding the group traffic on the LAN interface; otherwise, it stops forwarding the traffic on the LAN interface.


Define assert message

Sent by a PIM-DM or PIM-SM router when it receives a multicast packet for a group on a LAN interface that is in the outgoing interface list for the group; includes the administrative distance of the unicast routing protocol used to learn the network of the source with its metric value.


Define DVMRP

Operates in dense mode and depends on its own unicast routing protocol that is similar to RIP to perform its multicast functions.


Define MOSPF

A multicast routing protocol that operates in dense mode and depends on the OSPF unicast routing protocol to perform its multicast functions.


Define PIM-SM

PIM-SM is a method of routing multicast packets that requires some intelligence in the network about the locations of receivers so that multicast traffic is not flooded into areas with no receivers. PIM Sparse Mode gets its name from the assumption that relatively few receivers of a particular multicast group, widely scattered (from a network perspective), want to receive that multicast traffic. Does not depend on any unicast routing protocol to perform its multicast functions.


Define source DR

A designated router that is directly connected with a source of the multicast group.


Define source registration

In the PIM-SM design, the process by which a source DR, after it starts to receive the group traffic, encapsulates the multicast packets in the unicast packets and sends them to the RP.


Define shared distribution tree

In PIM-SM, the path of the group traffic that flows from the RP to the routers that need the traffic. It is also called the root-path tree (RPT), because it is rooted at the RP.


Define shortest-path tree switchover

In the PIM-SM design, the process by which a PIM-SM router can build the SPT between itself and the source of a multicast group and take advantage of the most efficient path available from the source to the router as long as it has one directly connected group member. Once it builds an SPT, it sends a PIM-SM (S, G) RP-bit Prune toward the upstream router on the shared tree.


Define PIM-SM (S,G) RP bit prune

When a PIM-SM router switches from RPT to SPT, it sends a PIM-SM Prune message for the source and the group with the RP bit set to its upstream router on the shared tree. RFC 2362 uses the notation PIM-SM (S, G) RP-bit Prune for this message


Define Auto-RP

Auto-Rendezvous Point. Cisco-proprietary protocol that can be used to designate an RP and send RP-Announce messages that advertise its IP address and groups. Also, it can be used to designate a mapping agent that interprets what IP address RP is advertising and for what groups. A mapping agent sends this information in the RP-Discovery messages so that all PIM-SM routers can learn the IP address of the RP and groups it is supporting automatically.


Define BSR

A standards-based way of helping routers find Rendezvous Points (RP). RPs notify BSRs of the groups they handle. BSRs in turn flood the group-to-RP mappings throughout the network. Each router individually determines which RP to use for a particular group.


Define MSDP

Enhances RP redundancy by providing a method for RPs to exchange multicast source information, even between multicast domains.