Cisco CGMP protocol and RGMP protocol

Today I will talk about Cisco CGMP and RGMP.

CGMP protocol, Cisco group management protocol:

CGMP is used to limit multicast traffic on Layer 2 networks. Because the switch cannot view Layer 3 data packets, and cannot distinguish between IGMP packets. With CGMP, the router tells the switch the interface of the multicast group users that only the router can generate CGMP packets, and the switch just listens for CGMP packets.

Mainly provide the following services:

1. Allow IP multicast packets to be switched to those ports that have IP multicast clients.

2. Keep the network bandwidth in the user field so that unnecessary IP multicast traffic will not be relayed.

3. No additional overhead is incurred when creating a separate VLAN for each multicast group in the switched network.

CGMP has two types of data packets:

Join

The router announces to the switch to add a member to the multicast group

Leave

The router informs the switch to delete a member from the multicast group

Once CGMP is activated, it can automatically identify the port connected to the CGMP-Capable router. CGMP is activated by default, and it supports registration of IP multicast groups up to 64.

Multicast routers that support CGMP periodically send CGMP JoinMessages to notify themselves of network switching behavior. The receiving switch saves information and sets a Timer similar to the router Holdtime.

Each time the switch receives a CGMP join message, the timer is continuously updated with it. When the router hold time expires, the switch is responsible for moving all known multicast groups out of CGMP.

RGMP protocol, Cisco router port group management protocol:

The Cisco Router Port Group Management Protocol (RGMP) makes up for the shortcomings of the Internet Group Management Protocol (IGMP: Internet Group Management Protocol) in the Snooping technical mechanism.

The RGMP protocol acts between the multicast router and the switch. Through RGMP, the multicast data packets forwarded in the switch can be fixed in the required router. The design goal of RGMP is to apply to BackboneSwitched Networks with multiple routers connected.

The limitation of IGMP Snooping technology is mainly reflected in: this technology can only fix multicast traffic to the exchange port directly or indirectly connected between other receivers through other switches. Under IGMP Snooping technology, multicast traffic cannot be fixed at least with one Multicast routers are connected to ports, which causes the multicast traffic of these ports to spread.

IGMP Snooping is an inherent limitation of the mechanism. Based on this, the router cannot report the traffic status, so the switch can only know the type of multicast traffic requested by the host, but not the type of traffic received on the router port.

The RGMP protocol supports fixing multicast traffic to router ports. In order to efficiently achieve fixed traffic, network switches and routers must support RGMP. 

Through RGMP, the backbone switch can know the type of group required by each port, and then the multicast router transmits this information to the switch. However, the router only sends RGMP information and ignores the received RGMP information.

When the group no longer needs to receive communication traffic, the router sends an RGMP Leave Message. In the RGMP protocol, a network switch needs to consume network ports to reach RGMP information and process it. In addition, the switch in RGMP does not allow forwarding/diffusion of received RGMP information to other network ports.

The design goal of RGMP is to be used in conjunction with the multicast routing protocol that supports distribution tree Join/Prune. The typical protocol is PIM-SM. The RGMP protocol only specifies IP v4 multicast routing operations, and does not include IPv6.

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