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Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
OL-8978-04
Chapter 51 Configuring Multicast Services
Understanding How RGMP Works
All routers on the network must be RGMP capable. RGMP-capable routers send an RGMP hello
message to the switch periodically. The RGMP hello message tells the switch not to send the multicast
data to the router unless an RGMP join message has also been sent to the switch from that router. When
an RGMP join message is sent, the router is able to receive the multicast data. To learn how to set a router
to receive the RGMP data, see the
“RGMP-Related CLI Commands” section on page 51-34
.
To stop receiving the multicast data, a router must send an RGMP leave message to the switch. To disable
RGMP on a router, the router must send an RGMP bye message to the switch.
Table 51-4
provides a summary of the RGMP packet types.
These restrictions apply to RGMP:
•
Sparse mode only—RGMP supports PIM sparse mode only. RGMP does not support PIM dense
mode. RGMP explicitly supports the two AutoRP groups in dense mode by not restricting the traffic
to those groups but by flooding it to all router ports. For this reason, you should configure PIM
sparse-dense mode. If you configure groups other than the AutoRP groups for dense mode, their
traffic will not be correctly forwarded through the router ports that have been enabled for RGMP.
•
To effectively constrain the multicast traffic with RGMP, connect the RGMP-enabled routers to
separate the ports on the RGMP-enabled switches.
•
RGMP constrains only the traffic that exits through the ports on which it detects an RGMP-enabled
router. If a non-RGMP enabled router is detected on a port, that port receives all multicast traffic.
•
RGMP does not support the directly connected sources in the network. A directly connected source
will send the traffic into the network without signaling this through RGMP or PIM. This traffic will
not be received by an RGMP-enabled router unless the router already requested receipt of that group
through RGMP. This restriction applies to the hosts and to the functions in the routers that source
the multicast traffic, such as the
ping
and
mtrace
commands, and the multicast applications that
source the multicast traffic, such as UDPTN.
•
RGMP supports the directly connected receivers in the network. The traffic to these receivers will
be constrained by IGMP snooping, or if the receiver is a router, by PIM and RGMP. CGMP is not
supported in the networks where RGMP is enabled on the routers. Enabling RGMP and CGMP on
a router interface is mutually exclusive. If RGMP is enabled on an interface, CGMP is silently
disabled or vice versa.
Table 51-4
RGMP Packet Types
Description
Action
Hello
When RGMP is enabled on the router, no multicast data traffic is sent to the router
by the switch unless an RGMP join is specifically sent for a group.
Bye
When RGMP is disabled on the router, all multicast data traffic is sent to the router
by the switch.
Join
Multicast data traffic for a multicast MAC address from the Layer 3 group address G
is sent to the router. These packets have group G in the Group Address field of the
RGMP packet.
Leave
Multicast data traffic for the group G is not sent to the router. These packets have
group G in the group address field of the RGMP packet.
