Multicast Questions

Here you will find answers to Multicast questions

Question 1

A network administrator assigns a multicast address of 239.255.8.5 to an application running on a device with an Ethernet MAC address of 01.b2.7d.05.f1.80. Which Layer 2 multicast address will this device use?

A. 01.00.5e.7F.08.05
B. 01.b2.7d.05.f1.80
C. 01.b2.7d.0a.08.05
D. 01.00.5e.05.f1.80
E. ff.ff.ff.ff.ff.ff

Answer: A

Explanation

+ First convert the IP multicast address 239.255.8.5 to binary form:
1110 1111.1111 1111.0000 1000. 0000 0101

+ Next we separate the 23 low-order binary bits from the converted address:
1110 1111.1111 1111.0000 1000. 0000 0101

+ Drop other bits:
111 1111.0000 1000. 0000 0101

+ Convert these bits to hexadecimal:7F.08.05

+ We knew that the first 24 bits of this MAC address always begin with 01.00.5e (the organizationally unique identifier or OUI). So we just need to append the converted result with 01.00.5e to get the result:01.00.5e.7F.08.05. That is the official way to convert multicast IP address to multicast MAC address.

There is a shorter way to do this:
+ Convert 3rd and 4th octet directly to hex.
+ If the decimal value of 2nd octet is great than 128 then subtract it with 128 and convert it to hex. If not, convert it directly to hex.
+ Append your results with 01.00.5e.

Question 2

Which three statements are true about IP multicast configuration? (Choose three)

A. PIM sparse-dense mode and PIM dense mode require an RP on the network.
B. PIM sparse mode and PIM dense mode require an RP on the network.
C. PIM sparse mode interfaces are always added to the multicast routing table in a router.
D. PIM sparse mode and PIM sparse-dense mode require an RP on the network.
E. PIM dense mode interfaces are always added to the multicast routing table in a router.
F. PIM sparse-dense mode acts as PIM dense mode if an RP is not known.

Answer: D E F

Explanation

A rendezvous point (RP) is required only in networks running Protocol Independent Multicast sparse mode (PIM-SM) -> A & B are not correct.

In PIM-SM, only network segments with active receivers that have explicitly requested multicast data will be forwarded the traffic. This method of delivering multicast data is in contrast to the PIM dense mode (PIM-DM) model. In PIM-DM, multicast traffic is initially flooded to all segments of the network. Routers that have no downstream neighbors or directly connected receivers prune back the unwanted traffic -> C is not correct

In populating the multicast routing table, dense-mode interfaces are always added to the table. Sparse-mode interfaces are added to the table only when periodic join messages are received from downstream routers, or there is a directly connected member on the interface -> E is correct.

An interface configured in sparse-dense mode is treated in either sparse mode or dense mode of operation, depending on which mode the multicast group operates. If a multicast group has a known RP, the interface is treated in sparse mode. If a group has no known RP, the interface is treated in dense mode and data will be flooded over this interface -> F is correct.

In fact answer D and F seem to contradict with each other. But if we have to choose 3 answers, D is the best option left after eliminating the three other answers.

(Reference: Cisco IOS 12.0 Solutions for Network Protocols, Volume 1)