CertificationZone

Date of Issue: 11-01-2023

Question 1.
What are some advantages of PIM Sparse Mode over PIM Dense Mode?
(Choose 2)

a) With PIM sparse mode, you have a client pull model. Multicast traffic will only be sent to receivers who explicitly request it.
b) With PIM sparse mode, you have a client push model. Multicast traffic will be sent out all multicast ports.
c) PIM sparse mode is optimized for situations where you have many sources but only a few widely dispersed receivers.
d) PIM sparse mode is optimized for situations where you have only a few sources, but many receivers in close proximity to each other.

Answer

Question 2.
You will always see the multicast group 224.0.1.40 present on multicast routers. What is this multicast group used for? Here is an example:

router#sh ip igmp groups
IGMP Connected Group Membership
Group Address    Interface        Uptime    Expires   Last Reporter
239.255.255.254  FastEthernet0/0  00:20:09  00:02:41  172.16.1.36
224.0.1.40       Serial0/0        00:20:30  never     0.0.0.0
224.0.1.60       FastEthernet0/0  00:20:09  00:02:38  172.16.1.225
225.1.2.3        FastEthernet0/0  00:20:09  00:02:38  172.16.1.119
224.0.1.24       FastEthernet0/0  00:20:03  00:02:43  172.16.1.36

a) This group is used for all systems on this subnet.
b) This group is used for RIP v2.
c) This group is used to represent all PIM routers.
d) This group is used for Cisco Auto-RP Discovery.

Answer

Question 3.
When you enter the following command:

router#sh ip igmp int s 0/0
Serial0/0 is up, line protocol is up
Internet address is 172.16.10.5/30
IGMP is enabled on interface
Current IGMP version is 2
CGMP is disabled on interface
IGMP query interval is 60 seconds
IGMP querier timeout is 120 seconds
IGMP max query response time is 10 seconds
Last member query response interval is 1000 ms
Inbound IGMP access group is not set
IGMP activity: 1 joins, 0 leaves
Multicast routing is enabled on interface
Multicast TTL threshold is 0
IGMP querying router is 0.0.0.0 (this system)
Multicast groups joined (number of users):
224.0.1.40(1)

You note that the multicast TTL threshold is 0. What does this mean?
(Choose 2)

a) The "Multicast TTL threshold is 0" response indicates that all multicast packets sent out this interface will have the multicast threshold set to 0.
b) This means that this particular router is a multicast border router.
c) This means that multicast packets that arrive with a TTL value of 1 or less will be dropped.
d) This means that multicast packets must arrive with a TTL value of one or more to be passed to the next router.

Answer

Answers

Answer
a) With PIM sparse mode, you have a client pull model. Multicast traffic will only be sent to receivers who explicitly request it.
c) PIM sparse mode is optimized for situations where you have many sources but only a few widely dispersed receivers.

Explanation
With PIM sparse mode, you have a client pull model. Multicast traffic will only be sent to receivers who explicitly request it. This reduces bandwidth usage.

PIM sparse mode is optimized for situations where you have many sources but only a few widely dispersed receivers.

The other choices describe PIM dense mode.
[4311]

Question 2.
You will always see the multicast group 224.0.1.40 present on multicast routers. What is this multicast group used for? Here is an example:

router#sh ip igmp groups
IGMP Connected Group Membership
Group Address    Interface        Uptime    Expires   Last Reporter
239.255.255.254  FastEthernet0/0  00:20:09  00:02:41  172.16.1.36
224.0.1.40       Serial0/0        00:20:30  never     0.0.0.0
224.0.1.60       FastEthernet0/0  00:20:09  00:02:38  172.16.1.225
225.1.2.3        FastEthernet0/0  00:20:09  00:02:38  172.16.1.119
224.0.1.24       FastEthernet0/0  00:20:03  00:02:43  172.16.1.36

a) This group is used for all systems on this subnet.
b) This group is used for RIP v2.
c) This group is used to represent all PIM routers.
d) This group is used for Cisco Auto-RP Discovery.

Answer
d) This group is used for Cisco Auto-RP Discovery.

Explanation
If you check the IANA website for multicast group address assignments, you will see that 224.0.1.40 is used for Cisco rp-discovery while 224.0.1.39 is used for Cisco rp-send, also called Cisco rp-announce.

The correct multicast group address for all systems on this subnet is 224.0.0.1. The multicast group address for RIP v2 is 224.0.0.9. 224.0.0.13 is used for all PIM routers.
[4321]

Question 3.
When you enter the following command:

router#sh ip igmp int s 0/0
Serial0/0 is up, line protocol is up
Internet address is 172.16.10.5/30
IGMP is enabled on interface
Current IGMP version is 2
CGMP is disabled on interface
IGMP query interval is 60 seconds
IGMP querier timeout is 120 seconds
IGMP max query response time is 10 seconds
Last member query response interval is 1000 ms
Inbound IGMP access group is not set
IGMP activity: 1 joins, 0 leaves
Multicast routing is enabled on interface
Multicast TTL threshold is 0
IGMP querying router is 0.0.0.0 (this system)
Multicast groups joined (number of users):
224.0.1.40(1)

Answer
c) This means that multicast packets that arrive with a TTL value of 1 or less will be dropped.
d) This means that multicast packets must arrive with a TTL value of one or more to be passed to the next router.

Explanation
When a multicast packet arrives, the TTL value is decremented by 1. If the resultant TTL is less than or equal to 0, then the packet will be dropped. After decrementing the TTL by 1, the resultant TTL value is compared to the TTL threshold value. In order for the packet to be passed to the next router, the TTL value of the packet must be greater than the TTL threshold.

The TTL threshold setting does not have any effect on re-setting TTL values for packets. The TTL threshold merely defines a value against which to check the packets after decrementing them by one.

You must use the ip multicast ttl-threshold command to set a TTL value to define border routers.
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[IE-MCST-SQ1-F03]
[2001-11-02-01]