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352-001 · Question #190

A company wants to use SSM as the multicast routing protocol inside its network. Some of its multicast applications do not support IGMPv3. In which two ways can the mapping be done when these…

The correct answer is B. The Layer 3 multicast routers can send a request to a DNS server. D. The mapping can be done statically at the Layer 3 multicast routers. SSM requires IGMPv3 to specify a source address in joins; when applications only support IGMPv2, Layer 3 routers must map the (*,G) join to an (S,G) entry via static configuration or DNS-based SSM mapping.

Designing Network Services

Question

A company wants to use SSM as the multicast routing protocol inside its network. Some of its multicast applications do not support IGMPv3. In which two ways can the mapping be done when these applications send IGMPv2 join messages? (Choose two.)

Options

  • AThe Layer 2 switches can send a request to a DNS server.
  • BThe Layer 3 multicast routers can send a request to a DNS server.
  • CThe mapping can be done statically at the Layer 2 switches.
  • DThe mapping can be done statically at the Layer 3 multicast routers.
  • EThe Layer 2 switches can transform the IGMPv2 join to an IGMPv3lite join.
  • FThe Layer 3 multicast routers can transform the IGMPv2 join to an IGMPv3lite join.

How the community answered

(28 responses)
  • A
    11% (3)
  • B
    57% (16)
  • C
    25% (7)
  • E
    4% (1)
  • F
    4% (1)

Why each option

SSM requires IGMPv3 to specify a source address in joins; when applications only support IGMPv2, Layer 3 routers must map the (*,G) join to an (S,G) entry via static configuration or DNS-based SSM mapping.

AThe Layer 2 switches can send a request to a DNS server.

Layer 2 switches do not participate in multicast routing decisions and have no capability to perform DNS-based SSM mapping - this is strictly a Layer 3 router function.

BThe Layer 3 multicast routers can send a request to a DNS server.Correct

Layer 3 multicast routers support DNS-based SSM mapping, querying a DNS server to resolve a group address (*,G) to a specific source (S,G) entry, enabling SSM to function with IGMPv2-only applications.

CThe mapping can be done statically at the Layer 2 switches.

Layer 2 switches forward frames based on MAC addresses and have no role in translating IGMPv2 (*,G) joins to (S,G) entries required for SSM.

DThe mapping can be done statically at the Layer 3 multicast routers.Correct

Static SSM mapping can be configured directly on Layer 3 multicast routers, providing a manually defined (S,G) mapping for each multicast group used by IGMPv2-only applications.

EThe Layer 2 switches can transform the IGMPv2 join to an IGMPv3lite join.

IGMP message processing and SSM mapping are Layer 3 functions; Layer 2 switches cannot inspect or transform IGMPv2 join messages into IGMPv3lite joins.

FThe Layer 3 multicast routers can transform the IGMPv2 join to an IGMPv3lite join.

The correct router-level mechanism is SSM mapping (static or DNS-based), not a conversion to IGMPv3lite; routers map (*,G) joins to (S,G) entries without reformatting the IGMP message.

Concept tested: IGMPv2 to SSM mapping mechanisms on Layer 3 routers

Source: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipmulti_igmp/configuration/xe-16/imc-igmp-xe-16-book/imc-igmp-ssm-map.html

Topics

#SSM#IGMPv2 to IGMPv3 mapping#DNS mapping#static multicast mapping

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