Information Technology Reference
In-Depth Information
Mark as close to the source as possible.
PC
IP Phone
Access Layer
Distribution Layer
Figure 3-18
Marking of Frames or Packets
Multicast Traffic Considerations
Internet Group Management Protocol (IGMP) is the protocol between end workstations
and the local Layer 3 switch. IGMP is the protocol used in multicast implementations be-
tween the end hosts and the local router. RFC 2236 describes IGMP Version 2 (IGMPv2).
RFC 1112 describes the first version of IGMP. IP hosts use IGMP to report their multicast
group memberships to routers. IGMP messages use IP protocol number 2. IGMP mes-
sages are limited to the local interface and are not routed.
RFC 3376 describes IGMP Version 3 (IGMPv3) IGMPv3 provides the extensions required
to support source-specific multicast (SSM). It is designed to be backward compatible with
both prior versions of IGMP. All versions of IGMP are covered in Chapter 11, “OSPF,
BGP, Route Manipulation, and IP Multicast.”
When campus LANs use multicast media, end hosts that do not participate in multicast
groups might get flooded with unwanted traffic. Two solutions are
Cisco Group Management Protocol (CGMP)
■
IGMP snooping
■
CGMP
Cisco Group Management Protocol is a Cisco proprietary protocol implemented to con-
trol multicast traffic at Layer 2. Because a Layer 2 switch is unaware of Layer 3 IGMP
messages, it cannot keep multicast packets from being sent to all ports.
As shown in Figure 3-19, with CGMP, the LAN switch can speak with the IGMP router to
find out the MAC addresses of the hosts that want to receive the multicast packets. You
must also enable the router to speak CGMP with the LAN switches. With CGMP,
switches distribute multicast sessions to the switch ports that have group members.
When a CGMP-enabled router receives an IGMP report, it processes the report and then
sends a CGMP message to the switch. The switch can then forward the multicast mes-
sages to the port with the host receiving multicast traffic. CGMP Fast-Leave processing al-
lows the switch to detect IGMP Version 2 leave messages sent by hosts on any of the
supervisor engine module ports. When the IGMPv2 leave message is sent, the switch can
then disable multicast for the port.
