OECG – Chapter 19

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Router-Port Group Management Protocol (RGMP) restricts the multicast destinations that a switch will forward to a router. This is intended to reduce router overhead. RGMP and CGMP, both Cisco proprietary, are mutually exclusive. Enabling one disables the other. RGMP does work with IGMP snooping, however. Remember that while IGMP snooping helps prevent hosts from receiving unwanted multicast traffic, IGMP snooping doesn’t save a multicast router from this same fate. RGMP fills in this gap.

RGMP is a mechanism whereby a router can tell a switch to only forward specific multicast group traffic to him, but not all multicast traffic. The scenario is when you have a switch upstream from a router. The hosts downstream from the router might only need one particular multicast group, but the upsteam switch on the far side of the router doesn’t know that. The router will send RGMP messages to the switch to join/leave a multicast group.

RGMP Messages (Hello, Join, Leave, Bye)

  • RGMP Hello messages are sent by routers every 30 seconds. When the switch receives an RGMP hello, it stops forwarding multicast traffic on that port.
  • When the router wants to receive traffic for a multicast group, it sends an RGMP join group message. The switch will then forward traffic for that multicast group to the router.
  • When the router no longer wants to receive traffic for a multicast group, it sends an RGMP leave group message. The switch no longer forwards traffic for that multicast group to the router.
  • When RGMP is disabled on the router, the router sends an RGMP Bye message. When the switch receives the Bye message, it begins forwarding all multicast traffic to the router again.
By Ethan Banks

Ethan Banks is a podcaster and writer with a BSCS and 20+ years in enterprise IT. He's operated data centers with a special focus on infrastructure — especially networking. He's been a CNE, MCSE, CEH, CCNA, CCNP, CCSP, and CCIE R&S #20655. He's the co-founder of Packet Pushers Interactive, LLC where he creates content for humans in the hot aisle.

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