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4.3. Designated Routers (DRs) and Hello Messages

PIM Hello messages serve multiple purposes:

  • Discovering PIM neighbors
  • Negotiating capability options
  • Electing a Designated Router (DR)
  • Maintaining neighbor liveness

4.3.1. Sending Hello Messages​

PIM routers periodically send Hello messages on each of their PIM-enabled interfaces. Hello messages are sent to the link-local all-PIM-routers multicast address 224.0.0.13.

Hello Message Contents:

  • Holdtime: Time (in seconds) neighbors should keep this router alive
  • DR Priority: Priority value used for DR election (higher value preferred)
  • Generation ID: Identifier used to detect router reboots
  • Address List: List of secondary addresses on the interface (optional)
  • LAN Prune Delay: Configured propagation delay and override interval (optional)

Hello Timers:

  • Default Hello interval: 30 seconds
  • Triggered Hello: Sent immediately when interface is enabled
  • Holdtime: Typically set to 3.5 × Hello interval

4.3.2. DR Election​

On multi-access networks, a Designated Router must be elected. The DR is responsible for:

  • Sending PIM Register messages on behalf of directly connected hosts
  • Sending Join/Prune messages on behalf of directly connected receivers

DR Election Rules:

  1. The router with the highest DR Priority is elected as DR
  2. If multiple routers have the same highest priority, the router with the highest IP address is selected
  3. If neighbors do not support the DR Priority option, election is based solely on IP address

DR Election Example:

Router A: Priority = 100, IP = 10.1.1.1
Router B: Priority = 100, IP = 10.1.1.2
Router C: Priority = 50,  IP = 10.1.1.3

Election Result: Router B (same priority, higher IP)

4.3.3. Reducing Prune Propagation Delay on LANs​

On multi-access LANs, multiple downstream routers may simultaneously send Join or Prune messages to an upstream router. To avoid unnecessary traffic interruption, PIM uses a Prune delay mechanism.

The LAN Prune Delay option contains:

  • Propagation_Delay: Estimated LAN propagation delay
  • Override_Interval: Time allowed for other routers to override a Prune

When a router receives a Prune message on a LAN, if it still needs that traffic, it will send a Join message within the Override_Interval to override the Prune.

4.3.4. Maintaining Secondary Address Lists​

Routers can configure multiple IP addresses on their interfaces. Hello messages can include an Address List option to advertise the interface's secondary addresses. This is important for correct RPF checking and neighbor discovery.

note

For detailed Hello message format, option encoding, and state machines, please refer to Sections 4.3 and 4.9.2 of RFC 7761 for the complete specification.

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