Multicast Questions 2

Kindly provide the salution so other student can have benifited……

It is baffling to me why answer A is correct for Question 3. 255.0.0.11 is not in the multicast range (224.0.0.0 to 239.255.255.255) and even if it were, it would map to the MAC address of 01-00-5e-00-00-0b). Now the question were asking why an admin would not use 224.0.0.11 then that would make more sense to me as this is a reserved link local mulitcast address for “Mobile Agent”. Is it really true that cisco will ask about 255.0.0.11 or could this be a type-o?

Maybe they mean 225.0.0.11. But Cisco exam questions themselves are not without error.

Because the L2 multicast address of 255.0.0.11 and 224.0.0.11 are the same. The multicast MAC is generated from the lower 23 bits of the IP address which in this case is 0.0.11 for both hence they will both generate the same multicast MAC address of 01:00:5E:00:00:0B hence will be flooded out all port of a L2 switch. Remember they are talking L2 the multicast IP means nothing. This is Cisco’s way of having a ‘trick’ question – it is written by someone who doesn’t really understand what they are on about.

Thanks Geoff, you explanation is very good 😀

guys,i did not see these quests in latest 195Q.wtf?

hi yarrak, i have same concern, but wat i have done is i have also prepared these questions…lets be prepared for the worse rather than just relying on that 195 q sheet 😉

Is that 195Q are sufficient???

I still think the best answer to Q3 is the Link-Local answer C.

Saying it’s even a L3 Multicast Address is clearly wrong because it’s not even in Class D.

As the mapping to and back from the MAC address means it represents any IP address in the range 224-239.x.0.11 (x = 0 or 128) any device that is listening to that L2 adddress would then have to take a look at the L3 address encapsulated in the L2 frame to decide what to do with the packet i.e. if it is 224.0.0.x it is Link Local but otherwise not.

So, as C is only talking about L2 it does map to the Link Local and is therefore the least wrong of the options between A & C. B & D are clearly wrong because the L3 addresses for them are:

SAP – 224.2.0.0/16
Multicast Address Dynamic Client Allocation Protocol – is a L5 protocol on port 2535

question 3 makes no sense. doesnt flooding occur anyway unless CGMP or IGMP snooping is configured?

Question 3 must be a typo !! Guys, hold on. That address belongs to Class E. There is not a iota of detail in the Internet of what the router will do when it sees such an address. We CANNOT talk about L2 or L3 multicast here because that IP is NOT in the multicast range. That is, the router will NOT create a multicast layer 2 frame for it simply because that address is NOT multicast.

225.0.0.0 range is “Reserved” by IANA. RFC 5771 doesn’t say for what though … so we don’t know.

The only logical option left is that the “real” question asks about 224.0.0.11 and then answer C is correct and makes sense in that case.

Hi everyone, studying for my recert for what seems like the 100th time. I found typing some solutions helps me remember the ‘heart of the question” better.

Good stuff here, I’ll try to add my two cents:

Q1 A multicast address is chosen for the receivers in a multicast group. Senders use that address as the destination address of a datagram to reach all members of the group. Source:

http://www.cisco.com/en/US/docs/ios/12_2/ip/configuration/guide/1cfmulti.html#wp1000871

IGMP packets are transmitted using IP multicast group addresses as follows:
•IGMP general queries are destined to the address 224.0.0.1 (all systems on a subnet).
•IGMP group-specific queries are destined to the group IP address for which the router is querying.
•IGMP group membership reports are destined to the group IP address for which the router is reporting.
•IGMP Version 2 (IGMPv2) Leave messages are destined to the address 224.0.0.2 (all routers on a subnet).
Answers A and B are wrong because report messages are not sent to the RP or Default gateway. Keyword is “Report”. That leaves C and D. Answer C is wrong, because leave messages are sent to 224.0.0.2. Leaves answer D which is correct. See Source…

Q2 Bidir-PIM is derived from the mechanisms of PIM-SM and shares many shortest-path tree (SPT) operations. Bidir-PIM also has unconditional forwarding of source traffic toward the RP upstream on the shared tree, but no registering process for sources as in PIM-SM. These modifications are necessary and sufficient to allow forwarding of traffic in all routers solely based on the (*, G) multicast routing entries. This feature eliminates any source-specific state and allows scaling capability to an arbitrary number of sources. source:
http://www.cisco.com/en/US/docs/ios/12_2/ip/configuration/guide/1cfbipim.html
Answer A is right on forwards up the Tree..not down the tree. that also leaves out answer D too. Answer B is wrong because the “local subnet” is not correct, doesn’t really apply to multicast in this case. Answer C is partly correct with the PIM portion, but its not the “only” router to do this. Again watch for keywords that are wrong.

Q3. Trick question. First it’s out of the multicast range, and has noted above it won’t map well to a Layer 2 Multicast address: So so far Answer A is good.
http://www.rfc-editor.org/rfc/rfc3171.txt
Answer B and D are partially right, it is reserved by the IANA but I can’t verify it for these two services, it just says reserved.
Answer C is wrong, because link local address is an IPV6 term, and doesn’t apply at all to IPv4.
http://tools.ietf.org/html/rfc5771

Q3: Looks to me Answer A is correct because 255.0.0.11 belongs to class E which is reserved anyways and not understood by any application. And guess what happens when a Cisco Layer 2 switch sees an “unknown unicast” packet. It always floods out to all ports which is Answer A. The reference to this IP address as multicast address might be to confuse us and to steer away from the fact that it will be treated as unknown unicast.

hello ,

I am not able to understand the mapping from MAC to multicast IP.

This is the senario.

which 2 ports multicast stream 225.230.57.199 will be forwarded on switch.

sw#show mac-address-table multicast
vlan mac address type ports
2 0100.5ee6.39c7 IGMP gi3/7,fa6/28,fa7/20
2 0100.5e00.0123 igmp Fa5/7 fa6/28 ,fa7/20
2 0100.5e66.39c7 igmp g3/4,g3/7 f4/10 f4/14 f7/31 f7/40
3 0100.5e00.39c7 igmp g3/7,g6/21
3 0100.5e50.4453 igmp g3/7,f4/2,f4/3 f4/14 f4/38 f5/3

Answers are : G3/7 & G4/14

How they have reached to these answer is mystery
can anyone help me regarding this?

Thanks & Regards,

Amol

you need to change from IP to MAC. let me help u. First you need the first 23 BITs. ONLY 23.
230.57.199
11100110.00111001.11000111 Since we need 23. this is what happens (24 is always 0)
01100110.00111001.11000111 now covert to Hex
0110.0110.0011.1001.1100.0111
6.6.3.9.c.7
66.39.c7
add the first part 0100.5e

0100.5e66.39.c7

hey Night_wolf

I got this mapping from IP to MAC multicast.
I am confuse with the answers they have provided as the port G3/7 & G4/14.
I am unable to get any relation in between them.

Regards,
Amol

0100.5e66.39c7 igmp g3/4,g3/7 f4/10 (f4/14) f7/31 f7/40- I think that is typo F4/14 is typo. Should be G. G3/4 is there in the list

http://www.cisco.com/en/US/tech/tk828/technologies_white_paper09186a00802d4643.shtml

The range of 224.0.0.0 through 224.0.0.255 is considered the Local Network Control Block—more commonly known as the Link-Local Block…

…it is important to note that most Layer 2 switches flood all multicast traffic that falls within the MAC address range of 0x0100.5E00.00xx (which corresponds to Layer 3 addresses in the Link-Local block) to all ports on the switch even if IGMP Snooping is enabled.

Dude, right on there brohetr.

Q3. B,D incorrect. A and C are correct statements. but C doesn’t answer the question. The reason why Network administrator always avoid applying multicast address that will map to 0x0100.5E00.00xx MAC (including 224.0.0.11, 225.0.0.11, 225.128.0.0.11, and so on) is Layer-2 flooding. and A has this “flooding” keyword. A is correct.

Q4. explanation : http://www.cisco.com/en/US/docs/ios/solutions_docs/ip_multicast/White_papers/anycast.html

In Anycast RP, all the RPs are configured to be MSDP peers of each other. When a source registers with one RP, an SA message will be sent to the other RPs informing them that there is an active source for a particular multicast group. The result is that each RP will know about the active sources in the area of the other RPs. If any of the RPs were to fail, IP routing would converge and one of the RPs would become the active RP in more than one area. New sources would register with the backup RP. Receivers would join toward the new RP and connectivity would be maintained.

Q5. explanation: http://www.ietf.org/rfc/rfc3376.txt

“filter-mode” may be either INCLUDE or EXCLUDE. In INCLUDE mode,
reception of packets sent to the specified multicast address is
requested *only* from those IP source addresses listed in the
source-list parameter. In EXCLUDE mode, reception of packets sent
to the given multicast address is requested from all IP source
addresses *except* those listed in the source-list parameter.

A.2 Host Suppression

In IGMPv1 and IGMPv2, a host would cancel sending a pending
membership reports if a similar report was observed from another
member on the network. In IGMPv3, this suppression of host
membership reports has been removed

hmm funny, i didn’t get any of these questions (q1-7) in this page only, today in my exam.

Even if they do not use these questions its good revision anyway

Hi mates, this question is not in rstut, but I believe this may be an exam question. Every pratical exames that I found, answer is A… BUT :), In the books and in real as I know, “The first 8 bits of an IPv6 multicast address are always FF (11111111)”.

Can rstut, or some coment this question please? Thanks a lot.

Whichstatement is correct in reference to IPv6 multicast?
A. IPv6 multicast uses Multicast Listener Discovery.
B. The first 8 bits of an IPv6 multicast address are always FF (11111111).
C. IPv6 multicast requires MSDP.
D. PIM dense mode is not part of IPv6 multicast.

@Cereso
I think both A and B are correct but IPv6 definitely uses MLD so let’s say A.

What does a unicast prefix-based IPv6 multicast address start with?
A. FF80
B. FF3
C. FF
D. FF7

What is the RP for IPv6 multicast address FF76:0130:2001:db8:3811::2323?
A. 2001:db8:3811::1
B. 2001:db8:3811::2323
C. 2001:db8:3811::0130
D. 2001:db8:3811::3
E. 2001:db8:3811::6
Answer: A

Superior thinking detatsmroned above. Thanks!