Cisco Systems 3560 Manual De Usuario

Cisco routers and multilayer switches run PIM and can forward multicast packets to and receive from a 
DVMRP neighbor. It is also possible to propagate DVMRP routes into and through a PIM cloud. The 
software propagates DVMRP routes and builds a separate database for these routes on each router and 
multilayer switch, but PIM uses this routing information to make the packet-forwarding decision. The 
software does not implement the complete DVMRP. However, it supports dynamic discovery of 
DVMRP routers and can interoperate with them over traditional media (such as Ethernet and FDDI) or 
over DVMRP-specific tunnels.

DVMRP neighbors build a route table by periodically exchanging source network routing information 
in route-report messages. The routing information stored in the DVMRP routing table is separate from 
the unicast routing table and is used to build a source distribution tree and to perform multicast forward 
using RPF. 

DVMRP is a dense-mode protocol and builds a parent-child database using a constrained multicast 
model to build a forwarding tree rooted at the source of the multicast packets. Multicast packets are 
initially flooded down this source tree. If redundant paths are on the source tree, packets are not 
forwarded along those paths. Forwarding occurs until prune messages are received on those parent-child 
links, which further constrain the broadcast of multicast packets.

This software release provides CGMP-server support on your switch; no client-side functionality is 
provided. The switch serves as a CGMP server for devices that do not support IGMP snooping but have 
CGMP-client functionality. 

CGMP is a protocol used on Cisco routers and multilayer switches connected to Layer 2 Catalyst 
switches to perform tasks similar to those performed by IGMP. CGMP permits Layer 2 group 
membership information to be communicated from the CGMP server to the switch. The switch can then 
can learn on which interfaces multicast members reside instead of flooding multicast traffic to all switch 
interfaces. (IGMP snooping is another method to constrain the flooding of multicast packets. For more 
information, see 

CGMP is necessary because the Layer 2 switch cannot distinguish between IP multicast data packets and 
IGMP report messages, which are both at the MAC-level and are addressed to the same group address.

CGMP is mutually exclusive with HSRPv1. You cannot enable CGMP leaving processing and HSRPv1 
at the same time. However, you can enable CGMP and HSRPv2 at the same time. For more information, 
see the 

These sections contain this configuration information:

 (optional)

 (required if the interface is in sparse-dense mode, and 

you want to treat the group as a sparse group)