Cisco Catalyst 4500 Series, Software Configuration Manual

Page: 976 / 2086

38-4
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
Chapter 38 Configuring IP Multicast
About IP Multicast
Rendezvous Point (RP)
If you configure PIM to operate in sparse mode, you must also choose one or more routers to be
rendezvous points (RPs). Senders to a multicast group use RPs to announce their presence. Receivers of
multicast packets use RPs to learn about new senders. You can configure Cisco IOS software so that
packets for a single multicast group can use one or more RPs.
The RP address is used by first hop routers to send PIM register messages on behalf of a host sending a
packet to the group. The RP address is also used by last hop routers to send PIM join and prune messages
to the RP to inform it about group membership. You must configure the RP address on all routers
(including the RP router).
A PIM router can be an RP for more than one group. Only one RP address can be used at a time within
a PIM domain for the same group. The conditions specified by the access list determine
for which groups
the router is an RP (as different groups can have different RPs).
IGMP Snooping
IGMP snooping is used for multicasting in a Layer 2 switching environment. With IGMP snooping, a
Layer 3 switch or router examines Layer 3 information in the IGMP packets in transit between hosts and
a router. When the switch receives the IGMP Host Report from a host for a particular multicast group,
the switch adds the host's port number to the associated multicast table entry. When the switch receives
the IGMP Leave Group message from a host, it removes the host's port from the table entry.
Because IGMP control messages are transmitted as multicast packets, they are indistinguishable from
multicast data if only the Layer 2 header is examined. A switch running IGMP snooping examines every
multicast data packet to determine whether it contains any pertinent IGMP control information. If IGMP
snooping is implemented on a low end switch with a slow CPU, performance could be severely impacted
when data is transmitted at high rates. On the Catalyst 4500 series switches, IGMP snooping is
implemented in the forwarding ASIC, so it does not impact the forwarding rate.
IP Multicast Implementation on the Catalyst 4500 Series Switch
The Catalyst 4500 series switch supports an ASIC-based Integrated Switching Engine that provides
Ethernet bridging at Layer 2 and IP routing at Layer 3. Because the ASIC is specifically designed to
forward packets, the Integrated Switching Engine hardware provides very high performance with ACLs
and QoS enabled. At wire-speed, forwarding in hardware is significantly faster than the CPU subsystem
software, which is designed to handle exception packets.
The Integrated Switching Engine hardware supports interfaces for inter-VLAN routing and switch ports
for Layer 2 bridging. It also provides a physical Layer 3 interface that can be configured to connect with
a host, a switch, or a router.
Figure 38-2 shows a logical view of Layer 2 and Layer 3 forwarding in the Integrated Switching Engine
hardware.