352-001 · Question #104
A company plans to establish a new network using Cisco Catalyst switches for its multicast applications. What is the disadvantage when two multicast applications are using the multicast IP addresses…
The correct answer is B. Multicast packets from both applications are flooded to ports where one multicast receiver from. The multicast addresses 234.17.4.5 and 234.145.4.5 map to the same Layer 2 MAC address because only the lower 23 bits of the IP are used, so IGMP snooping cannot separate the two streams and forwards both to ports that subscribed to only one.
Question
A company plans to establish a new network using Cisco Catalyst switches for its multicast applications. What is the disadvantage when two multicast applications are using the multicast IP addresses 234.17.4.5 and 234.145.4.5 inside the same network?
Options
- AMulticast packets from both applications are flooded to all Layer 2 ports in a segment where a
- BMulticast packets from both applications are flooded to ports where one multicast receiver from
- COnly one multicast stream is received at the port where the receivers from both applications are located.
- DBoth multicast senders will always receive the multicast packets from the other multicast application.
- EThe routers doing PIM-SM cannot distinguish between the two multicast applications.
How the community answered
(25 responses)- A4% (1)
- B64% (16)
- C16% (4)
- D12% (3)
- E4% (1)
Why each option
The multicast addresses 234.17.4.5 and 234.145.4.5 map to the same Layer 2 MAC address because only the lower 23 bits of the IP are used, so IGMP snooping cannot separate the two streams and forwards both to ports that subscribed to only one.
Flooding to all Layer 2 ports is the behavior when IGMP snooping is disabled - Catalyst switches enable IGMP snooping by default, so traffic is constrained to relevant ports, but the MAC collision still causes cross-delivery between the two groups rather than a full flood.
Ethernet multicast MAC addresses for IP multicast are formed by mapping the lower 23 bits of the multicast IP into the OUI prefix 01:00:5e - specifically, MAC = 01:00:5e:(octet2 & 0x7F):octet3:octet4. For 234.17.4.5 octet2 is 17 (0x11) and for 234.145.4.5 octet2 is 145 (0x91); masking both with 0x7F yields 0x11, so both IPs resolve to MAC 01:00:5e:11:04:05. Because IGMP snooping tracks MAC addresses at Layer 2, it cannot distinguish the two streams, causing packets from both multicast applications to be forwarded to ports that hold a receiver for only one of the two groups.
Both streams are forwarded to the affected port, not just one - the MAC address collision causes the switch to treat both groups identically, so receivers at that port get traffic from both applications rather than being restricted to one.
The senders are not affected by the MAC address collision - the problem manifests at the receiver side where IGMP snooping forwards unintended streams to subscribed ports, not at the sender side.
PIM-SM operates entirely at Layer 3 using IP group addresses and correctly distinguishes between 234.17.4.5 and 234.145.4.5 - the collision is a Layer 2 phenomenon caused by the lossy 32-to-48 bit address mapping and does not affect IP-layer multicast routing.
Concept tested: IP multicast to Layer 2 MAC address mapping collision
Source: https://www.cisco.com/c/en/us/support/docs/ip/ip-multicast/9041-multicast-mac.html
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