5. Protocol Description for Multicast Routers

This section describes the part of MLDv2 that applies to multicast routers.

5.1. Conditions for Querier Election

Multicast routers on a link must elect a single Querier to send periodic General Queries. The election process is as follows:

1. Initially, every router starts as a Querier.
2. If a router receives a Query from another router with a lower IP address, it becomes a Non-Querier.
3. If a Non-Querier does not receive a Query from a router with a lower IP address for a certain period (Other Querier Present Interval), it resumes the role of Querier.

5.2. Querier Action on Reception of a Query

When a Querier receives a Query:

• If the source IP address is lower than its own, it ceases to be a Querier.
• If the source IP address is higher than its own, it remains the Querier.
• If the source IP address is equal to its own, the Query is ignored.

5.3. Sending of Queries

The Querier periodically sends General Queries to solicit membership information. The default [Query Interval] is 125 seconds.

The Querier also sends Group-Specific or Multicast-Address-and-Source-Specific Queries when it receives State Change Reports indicating that a listener is leaving a group or blocking a source. These queries are used to verify if there are any other listeners interested in the group or source.

5.4. Action on Reception of Reports

Routers maintain a state for each attached link, which tracks the multicast addresses and sources of interest. When a router receives a Report, it updates this state.

• Current State Reports: Used to refresh the state of existing listeners.
• State Change Reports: Used to update the state immediately when a listener's interest changes.

Based on the state, the router decides whether to forward multicast packets for a specific (S,G) or (*,G) to the link.