Implementing and Operating Cisco Enterprise Network Core Technologies (ENCOR) v1.2InfrastructureMedium
A network engineer is troubleshooting a multicast video distribution system. Users in different VLANs are unable to receive the multicast stream, even though the source is active and PIM is configured on the router interfaces. The engineer suspects a Layer 2 issue preventing multicast traffic from reaching the PIM-enabled interfaces. Which Layer 2 protocol is essential for forwarding multicast traffic to interested hosts within a VLAN and should be checked?
- ASTP (Spanning Tree Protocol)
- BLLDP (Link Layer Discovery Protocol)
- CCDP (Cisco Discovery Protocol)
- DIGMP (Internet Group Management Protocol)
Show answer & explanationAnswer & explanation
Correct answer: D. IGMP (Internet Group Management Protocol)
IGMP (Internet Group Management Protocol) is crucial at Layer 2 for multicast. It allows hosts to signal their interest in receiving multicast traffic to local routers. Without IGMP, the switch might flood multicast traffic inefficiently or not forward it to the correct ports, preventing PIM from receiving the necessary join messages.
Why the other options are wrong
- A. STP prevents Layer 2 loops but doesn't directly manage multicast group membership.
- B. LLDP is an industry-standard protocol for discovering directly connected devices, similar to CDP.
- C. CDP is a Cisco proprietary protocol for discovering directly connected Cisco devices.
Internet Group Management Protocol (IGMP)
A communication protocol used by hosts and adjacent routers on IP networks to establish multicast group memberships.
- Operates at the network layer (Layer 3) but is critical for Layer 2 multicast forwarding decisions (e.g., IGMP snooping).
- Allows hosts to 'join' or 'leave' multicast groups.
- Routers use IGMP to determine which multicast groups have interested members on a given subnet.
Memory trick: Multicast Needs: PIM for Routing, IGMP for Joining.