Multicast Questions
Here you will find answers to Multicast Questions
Question 1
In PIM-SM what control plane signaling must a multicast source perform before it begins to send multicast traffic to a group?
A. The source must send a PIM Register message to the rendezvous point (RP).
B. The source must first join the multicast group using IGMP before sending.
C. The source must perform a Request to Send (RTS) and Clear to Send (CTS) handshake with the PIM designated router (DR).
D. No control plane signaling needs to be performed; the source can simply begin sending on the local subnet.
Answer: D
Question 2
The ip pim autorp listener command is used to do which of these?
A. enable a Cisco router to “passively” listen to Auto-RP packets without the router actively sending or fotwarding any of the packets
B. allow Auto-RP packets in groups 224.0.1.39 and 224.0.1.40 to be flooded in dense mode out interfaces configured with the ip pim sparse-mode command
C. enable the use of Auto-RP on a router
D. configure the router as an Auto-RP mapping agent
Answer: B
Question 3
In order to configure two routers as anycast RPs, which of these requirements, at a minimum, must be satisfied?
A. Multicast Source Discovery Protocol mesh-groups must be configured between the two anycast RPs.
B. The RPs must be within the same IGP domain.
C. Multicast Source Discovery Protocol must be configured between the two anycast RPs.
D. The two anycast RPs must be IBGP peers.
Answer: C
Question 4
Which two of these statements correctly describe classic PIM-SM? (Choose two)
A. The IOS default is for a last-hop router to trigger a switch to the shortest path tree as soon as a new source is detected on the shared tree.
B. The IOS default is for every one of the routers on the shared tree to trigger a switch to the shortest path tree as soon as a new source is detected on the shared tree.
C. The default behavior of switching to the shortest path tree as soon as a new source is detected on the shared tree can be disabled by setting the value in the ip pim spt-threshold command to “infinity”.
D. The default behavior of switching to the shortest path tree as soon as a new source is detected on the shared tree can be disabled by setting the value in the ip pim spt-threshold command to “zero”.
Answer: A C
Question 5
Refer to the exhibit. Router E learned about the PIM RP (designated as 7.7.7.7) from four different sources. Routers A and D advertised the 7.0.0.0 network via EIGRP. Routers B and C advertised the 7.0.0.0 network via OSPF. Considering that all four Ethernet interfaces on router E could potentially lead back to the PIM-RP, when router E receives the first multicast packet down the shared tree, which incoming interface will be used to successfully pass the RPF check?
A. E0
B. E1
C. E2
D. E3
E. None of these interfaces will be used to successfully pass the RPF check.
F. All of these interfaces would successfully pass the RPF check.
Answer: A
Question 6
Refer to the exhibit. Two ISPs have decided to use MSDP and configured routers X and Y (both are PIM RPs) as MSDP peers. In the domain of ISP B, PC A has sent an IGMP membership report for the group 224.1.1.1 and PC B has sent an IGMP membership report for the group 224.5.5.5. Assuming that the MSDP peering relationship between routers X and Y is functional, and given the partial configuration output shown from router X, which two of these statements are true? (Choose two)
A. Router X will contain an entry for 224.1.1.1 in its SA cache and will also have an installed (S,G) entry for this in its mroute table
B. Router X will not contain an entry for 224.1.1.1 in its SA cache but will have an installed (*,G) entry for this in its mroute table.
C. Router X will not contain an entry for 224.5.5.5 in its SA cache but will have an installed (S,G) entry for this in its mroute table.
D. Router X will not contain an entry for 224.5.5.5 in its SA cache but will have an installed (*,G) entry for this in its mroute table.
E. Router X will have no entries for 224.5.5.5 in neither its SA cache nor in its mroute table.
F. Router X will have no entries for 224.1.1.1 in neither its SA cache nor in its mroute table.
Answer: A D
In question 5: If there are two equal EIGRP unicast routes via E0 and E3, why will the RPF check only pass for E0 and not also for E3?
PIM will use the neighbor with the highest IP.
In q 5 E0 and E3 is taken in to account as EIGRP has lower AD the OSPF, if both E0 and E3 cost are the same next there is rule that “When faced with multiple equal cost paths to a source, IP multicast chooses the interface that has a Protocol Independent Multicast (PIM) neighbor with the highest IP address as the incoming interface and then sends prunes to PIM neighbors on the other links.”
Thanks very much for your comments. I understand now.
Question 6,
In this question the presence of RP’s point us to the use of PIM Sparse-Mode. MSDP Peer X is filtering the SA-Cache (source Active) inbound via ACL 101 which would implicit deny group 224.5.5.5 (thus no (S,G) entry in router X SA-Cache) but would permit the join for group 224.1.1.1 thus RP X would have an (S,G) for the group as the source would be known via the MSDP peer 2.2.2.3 (router Y) .
The second answer is that the Host residing in ISP-B domain sent a IGMP join for the group 224.5.5.5 which creates a (*,G) entry on each RPF router towards RP X, but because the source of group 224.5.5.5 is filtered the source would remain unknown, and the (*,G) entry would most likely be pruned from the mroute table.
This is my understanding of the answer – please let me know if I have missed anything?
can someone please explain question 2 and 3. Thanks
Dear Anonymous,
for Q2: this link give the answer, http://www.cisco.com/en/US/docs/ios/12_2/ipmulti/command/reference/1rfmult2.html#wp1090395
Hope this helped you.
..I add only a little info on Q5. The picture shows EIGRP with 1000 of Internal DA so OSPF might be a better choose because is 110. But the maximum da you can canfigure is 255 so I think the picture is a mistake or I don’t understand well why is written
in case of multiple routes to your source.. over here EIGRP is considered since it has a lower AD value than OSPF.. EIGRP=90 , OSPF=120 , and then again for the tie between 2 EIGRP routes, the highest next hop IP address is considered to break the tie
@ Rocky I agree with you but the picture shows EIGRP with 1000 of Internal DA. What does it mean?
Router metrics are always compared after AD. 1000 is the internal EIGRP metric.
Jimbob I think you’re right.
Thanks for the great info.
I’m using it for my CCIE recert (11 yrs) and so far a lot of it is correct and corresponds to other material I’ve seen. A couple of hints when taking these exams, watch for keywords to be out of place, look for wrong answers first and that can narrow down a lot. Also think about what the question is testing you for. Watch for misleading time consuming information that is put there just to confuse you.
Q1. Technically the control plane is used for forwarding with the FIB and LFIB, no signaling takes place, only forwarding so D is the only correct one.
Q2. Auto RP listener actually uses dense mode on 224.0.1.39 and 224.0.1.40 to build everything it needs. It does this first. Thats what they want you to know about Auto RP listener. B is correct
Q3. A. ‘Mess groups’ don’t apply wrong answer, B. Being int the same IGP domain makes no sense, another wrong answer. It leaves C and D. D, Anycast has nothing to do with IBGP or BGP for that matter, C is correct.
Q4. Tough one. answer A is good, Answer B it doesn’t use every router, wrong keyword. Leaves C and D. They are checking you for syntax ip pim spt-threshold command to “infinity” is the right answer. same source as above:
http://www.cisco.com/en/US/docs/ios/12_2/ipmulti/command/reference/1rfmult2.html#wp1020111
Q5. PIM RPF check uses highest IP, only A is correct..easy one ignore all the others. Know that the RPF check uses the highest IP. The example should say EIGRP composite metric, or just metric. Distance is a misleading term. It would take the EIGRP routes over OSPF (AD EIGRP is 90 versus 110(as mentioned)) Then it looks for the highest IP.
Q6. Good answer from Jimbob
For Qn6, A prune happens only when there are no hosts interested in the stream. Since host B in ISPB sent a join message to 224.5.5.5, a (*,G) entry will be in the mroute table of PIM RP X.
Normally I’m against killing but this article slaughetred my ignorance.
for Q6, I have put in the exam A and C. I don’t understand why you say D??
The host B has joined 244.5.5.5. so the entry (S,G) should be created “same as Host A” since The source tree in sparse mode has (S,G) entry not (*,G).
You only get a (S,G) entry when the router actually sees the traffic. The S is the IP address of the source 5.5.5.5 the GROUP is 244.5.5.5. So if router X filters out the group 244.5.5.5 it does not send the join to router Y so router Y never forwards the traffic for this group router X. Because it never gets any traffic it never inserts a s,g. Remember when you have active feed you have both a *,G and a S,G in the mroute table.
In the real world the S,G entries are the key to knowing you are seeing data, how else would the router figure out what the IP of the sender that corresponded to the multicast group
all anon’s comments have good reasons. Q6 answer is A and D. you’ll get your point.
i suggest study more about the concept (like anon did) so it won’t matter to you if the question being modified. currently, this question stays the same (at least three hours ago).
Comment to Q1:
isn´t it necessary that MC-Sources register with the RP. In that case A should be the correct answer instead of D.
I got the point, the MC DR on the LAN receiving the MC request will register to the RP, but
the Workstation itself is not aware of PIM/RPs so it just will send out its multicast traffic.
where i can find CCIE training videos???
Hi Guys,
If you are using GNS3 for your Multicast labs you might encounter some very strange behaviors like ping does not work (or worse, it only works from time to time etc …)
To solve GNS3 multicast issues, you just need to:
no ip cef
interface FastEthernet0/0 ==> on any interfaces involved with multicast
no ip route-cache
no ip mroute-cache
Save your config and reload the routers
@Slavisa: I would say both. Often it helps to read about the same subject from different authors. Each author has his way to explain things. Some topics may be more deeply explored by the one than the other.
>>> “Q1. Technically the control plane is used for forwarding with the FIB and LFIB, no signaling takes place, only forwarding so D is the only correct one.”
As I understand “PIM Register” is a control plane message but it is sent by DR.
As it seems to me correct explanation is “multicast source” (for example TV stream source) doesn’t support PIM. It is a dummy device which just sending multicast traffic.
About Q1: isn’t control plane in general anything that’s needed in order to get routing working? in other words, it is the “signalling” of the network? and data plane is basically anything that goes *through* the router, and not *to* the router?
According to that, my answer would be A, The source must send a PIM Register message to the rendezvous point (RP). Any thoughts?
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Question 47 – the corect asenwr is B: * router R1 Fa0/0 interface **** router R1 S0/0/0 interface * router R2 S0/0/0 interface * router R2 Fa0/0 and S0/0/1 interfaces * router R3 Fa0/0 and S0/0/0 interfaces because from first routing principle Every router makes it’s decision alone, base on the information it’s has in it’s own routing table and because routers R2 and R3 do not have static or dynamic routing enabled, the router R2 does not khows how to get to network 172.16.1.0/24, and therefore router R2 cannot reply to ping request.
Could you any one give some explanation of questions about this sections…..
Q1 is wrong because RP should make STP (S,G) for any multicast sources, DR sent RegMsg to RP if RP need this traffic it do nothing or sent RegStop and sent (S,G) to multicast route table. correct answer is A!!!
Dear none, the Q1 is about the source not the router close to the source.
to none September 25th, 2013
Q1 is right, a multicast source does not have to signal in order to start forwarding traffic. You’re referring to the DR router which is not in question.
I’m quite pleased with the inoiomatfrn in this one. TY!