Chap 13 - Multicast (part 2)

Question 1

What are the 2 type of Multicast Distribution Trees?

Answer 1

• Shortest Path Trees (SPT) aka Source Trees
• Shared Trees

Question 2

What is a Source Tree or Shortest Path Tree?

Answer 2

• Source is the root of the tree
• Tree is built using the Shortest Path from the Source to the leaves of the tree
• Every multicast Source needs a SPT because every Source is the root of the tree. Different source, different tree.
• Uses S, G notation

Question 3

What is meant by S,G notation?

Answer 3

It’s a way of describing the forwarding state of a SPT. S, G stands for Source IP address, Group IP address (ex. 10.1.1.2, 239.1.1.1)

Question 4

What is meant by *, G notation?

Answer 4

It’s a way of describing the forwarding state of a Shared Tree, also known as a Rendezvous Point Tree (RPT)

Question 5

What is a Shared Tree?

Answer 5

• aka Rendezvous Point Tree (RPT)
• Multicast traffic is forwarded down the Shared Tree according to the Group Address that the packets are addressed to
• Has a router known as a RP or Rendezvous Point
• Uses *,G notation with * meaning any source and G meaning the Group IP address

Question 6

What is an advantage of a Shared Tree (RPT)?

Answer 6

They require fewer multicast entries.

Question 7

What are the disadvantages of a Shared Tree?

Answer 7

• Receivers receive traffic from ALL sources even if they are sending traffic to the same Multicast address thereby wasting bandwidth
• Network security issues since any source can send what may be unwanted traffic

Question 8

What is an RPF?

Answer 8

• Reverse Path Forwarding Interface
• The interface with the lowest-cost path (administrative distance and metric) to the address of the source (SPT) or Rendezvous Point (RPT)

Question 9

When identifying an RFP interface and multiple interfaces have the same metric what is the tie-breaker?

Answer 9

Interface with the Highest IP address wins

Question 10

What is an RPF neighbor?

Answer 10

The neighbor of the RPF interface.

Question 11

What is Upstream?

Answer 11

Upstream is toward the source or if RPT upstream would be in the direction of the RP router

Question 12

What is an Upstream Interface?

Answer 12

Another name for the RPF interface aka IIF (Incoming Interface)

Question 13

What is Downstream?

Answer 13

In the direction AWAY from the source and toward the Receiver

Question 14

What is a Downstream Interface?

Answer 14

Any interface used to forward traffic away from the source aka OIF or Outgoing Interface

Question 15

What is the Incoming Interface?

Answer 15

• IIF
• aka RPF
• The only type of interface that can accept traffic coming from the source

Question 16

What is an Outgoing Interface List?

Answer 16

• OIL
• A group of Outgoing Interfaces that are forwarding traffic to the same group

Question 17

What is a Last Hop Router?

Answer 17

• LHR - A router that is directly attached to receivers
• aka Leaf Router
• Its responsible for sending PIM Joins upstream toward the RP or the Source

Question 18

What is a First Hop Router?

Answer 18

• FHR
• aka Root Router
• A router that is directly attached to the source
• It is responsible for sending Register Messages to the RP

Question 19

What is the Multicast Routing Information Base?

Answer 19

• aka Multicast Route Table
• aka MRIB
• aka mroute
• Topology table derived from the unicast routing table and PIM.

Question 20

What is in the mroute table?

Answer 20

• For every multicast route it contains
• The S, G, Source, Group info
• Incoming Interfaces (IIF)
• Outgoing Interfaces (OIF)
• RPF Neighbor info

Question 21

What is the Multicast Forwarding Table?

Answer 21

• MFIB
• A forwarding table that uses MRIB to program multicast forwarding info in hardware for faster forwarding

Question 22

What is the Multicast State?

Answer 22

• Multicast forwarding state that is used by the router to forward multicast traffic
• Multicast state is composed of entries found in the mroute table (S, G, IIF, OIF, etc)

Question 23

What are the 5 PIM Operating Modes?

Answer 23

• PIM Dense Mode (PIM-DM)
• PIM Sparse Mode (PIM-SM)
• PIM Sparse Dense Mode
• PIM Source Specific Multicast (PIM-SSM)
• PIM Bi-directional mode (Bidir-PIM)

Question 24

Whis is another name for PIM-DM and PIM-SM?

Answer 24

Any Source Multicast (ASM)

Question 25

What are the 9 PIM Control Messages along with their message types?

Answer 25

* Type 0 - Hello * Type 1 - Register * Type 2 - Register Stop * Type 3 - Join/Prune * Type 4 - Bootstrap * Type 5 - Assert * Type 8 - Candidate RP Advertisement * Type 9 - State Refresh * Type 10 - DF Election

Question 26

What is the PIM Hello message used for and how often is it sent?

Answer 26

* It is sent out all PIM-enabled interfaces every 30 seconds by default * It is used to detect other PIM routers * It is also used to elect a Designated Router

Question 27

Who sends the PIM Register message and what is it used for?

Answer 27

* A FHR will send the RP a unicast Register message when it detects a new multicast source * It is sent to let the RP know about the new source

Question 28

Who sends the PIM Register Stop message, where is it sent to, and what is its purpose?

Answer 28

* Sent by the RP to FHR with unicast * Used to stop the register messages when the RP either doesn't have a subscriber to the source or when the SPT between the source and the RP has been built

Question 29

What are 2 times when the PIM Join/Prune message is sent, and what are its purposes ?

Answer 29

* This is sent by a PIM router when it gets a membership report from a host and is then relayed by other PIM routers until it gets to the source or RP * It is also sent by a PM router when it wants to stop a multicast stream * Sent to 224.0.0.13 (all PIM routers)

Question 30

Who sends the PIM Bootstrap message, what is its destination, and what does it contain?

Answer 30

* Sent by Bootstrap router * Sent to 224.0.0.13 (all PIM routers) with TTL of 1 * Contains information about the BSR and RP-to-group mappings

Question 31

When is the PIM Assert message sent and what does it cause to happen?

Answer 31

* This is sent by a PIM router when it detects multicast traffic from another PIM router on the same subnet. * It is used to facilitate an election between the 2 to decide who the PIM Forwarder will be. (Winner has best AD, then metric, then highest IP address)

Question 32

What is the PIM Candidate RP Advertisement?

Answer 32

* Sent by a router that wants to be an RP * Sent to Bootstrap Router (BSR) in unicast

Question 33

What is the PIM State Refresh message?

Answer 33

This message keeps the pruned state in PIM dense mode from timing out, which saves network bandwidth by greatly reducing the reflooding of unwanted multicast traffic to pruned branches of the PIM dense mode network

Question 34

When is PIM-DM used?

Answer 34

When there are a lot of receivers on the network.

Question 35

What 3 steps describe how is a PIM-DM tree built?

Answer 35

* When a multicast stream starts it is flooded out to every PIM-DM router in the network. * The tree takes shape as routers decide if they have any receivers * If not, they send Prune messages and the flooding stops for that router

Question 36

Which interface is allowed to accept multicast flooding?

Answer 36

The RPF interface

Question 37

In PIM-DM, what happens if a router receives multicast flooding on a non-RPF interface?

Answer 37

* Router discards the traffic on that interface * Sends the neighbor a Prune message to stop the flooding.

Question 38

In PIM-DM when do Prunes expire?

Answer 38

Every 3 minutes

Question 39

How often is a PIM-DM multicast tree rebuilt?

Answer 39

Every 3 minutes

Question 40

In PIM-DM what remains on a router even after it has been pruned?

Answer 40

The S, G state remains on the router.