Router-Port Accumulation Management Protocol
RGMP is a Layer 2 agreement that enables a router to acquaint to a about-face which multicast
group cartage the router does and does not appetite to accept from the switch. By actuality able to restrict
the multicast destinations that a about-face assiduously to a router, a router can abate its overhead. In
fact, RGMP was advised to advice routers abate aerial back they are absorbed to high-speed
LAN backbones.
Although RGMP is Cisco proprietary, abnormally abundant it cannot assignment accordingly with Ciscoproprietary
CGMP. Back RGMP is enabled on a router or a switch, CGMP is silently disabled; if
CGMP is enabled on a router or a switch, RGMP is silently disabled. Note additionally that while it is
proprietary, RGMP is appear as advisory RFC 3488.
RGMP works able-bodied in affiliation with IGMP snooping. In fact, IGMP concern would typically
learn the ports of all multicast routers by alert for IGMP and multicast acquisition agreement traffic.
In some cases, some routers may not appetite all multicast traffic, so RGMP provides a agency to
reduce the exceptionable traffic. The attenuate key to the charge for RGMP back application IGMP concern is
to apprehend this important actuality about IGMP snooping:
IGMP concern helps switches ascendancy administration of multicast cartage on ports where
multicast hosts are connected, but it does not advice switches ascendancy administration of
multicast cartage on ports area multicast routers are connected.
576 Chapter 16: Introduction to IP Multicasting
For example, accede the simple arrangement apparent in Figure 16-21. SW2 has abstruse of routers R3
and R4 with IGMP snooping, so it assiduously multicasts beatific to all multicast groups out to both R3
and R4.
Figure 16-21 IGMP Concern Without RGMP
As you can see from Figure 16-21, R3 needs to accept cartage alone for accumulation A, and R4 needs to
receive cartage alone for accumulation B. However, IGMP concern causes the about-face to advanced all
multicast packets to anniversary router. To action that problem, RGMP can be acclimated by a router to tell
the about-face to alone advanced packets for accurate multicast groups. For example, Figure 16-22
shows the aforementioned arrangement as Figure 16-21, but with RGMP snooping. In this case, RGMP Join
messages are enabled in both the routers and the switch, with the after-effects apparent in Figure 16-22.
Figure 16-22 Added Efficient Forwarding with RGMP Added to IGMP Snooping
Source for
Group B
Receiver for
Group A
Source for
Group A
SW2
B
R3
R4
A
A B
A
A B
H3
Receiver for
Group B
B H4
IGMP
Snooping
Source for
Group B
Receiver for
Group A
Source for
Group A
SW2
B
R3
R4
A
A
A
B
H3
Receiver for
Group B
B H4
IGMP
Snooping
2 3
3
1
2
RGMP
Join
Group A
1 RGMP
Join
Group B
LAN Multicast Optimizations 577
Figure 16-22 shows the afterward three capital steps, with the aboriginal footfall assuming the RGMP function
with the RGMP Join message. The Join bulletin allows a router to analyze the groups for which
the router wants to accept traffic:
1. R3 sends an RGMP Join for accumulation A, and R4 sends an RGMP Join for accumulation B. As a result,
SW2 knows to advanced multicasts for accumulation A alone to R3, and for accumulation B alone to R4.
2. The sources accelerate a packet to groups A and B, respectively.
3. SW2 assiduously the cartage for accumulation A alone to R3 and the packets for accumulation B alone to R4.
While Figure 16-22 shows aloof one archetype and one blazon of RGMP message, RGMP includes four
different messages. All the RGMP letters are generated by a router and are beatific to the multicast
IP abode 224.0.0.25. The afterward account describes the four RGMP messages:
■ Back RGMP is enabled on a router, the router sends RGMP Hello letters by absence every
30 seconds. Back the about-face receives an RGMP Hello message, it stops forwarding all
multicast cartage on the anchorage on which it accustomed the Hello message.
■ Back the router wants to accept cartage for a specific multicast group, the router sends an
RGMP Join G message, area G is the multicast accumulation address, to the switch. Back the
switch receives an RGMP Join message, it starts forwarding the requested accumulation cartage on the
port on which it accustomed the Hello message.
■ Back the router does not appetite to accept cartage for a aforetime RGMP-joined specific
multicast group, the router sends an RGMP Leave G message, area G is the multicast group
address, to the switch. Back the about-face receives an RGMP Leave message, it stops
forwarding the accumulation cartage on the anchorage on which it accustomed the Hello message.
■ Back RGMP is disabled on the router, the router sends an RGMP Bye bulletin to the switch.
When the about-face receives an RGMP Bye message, it starts forwarding all IP multicast traffic
on the anchorage on which it accustomed the Hello message.
NOTE The afterward URL provides added advice on RGMP:
http://www.cisco.com/en/US/products/hw/switches/ps700/
products_tech_note09186a008011c11b.shtml