Cisco Group Management Protocol
| Cisco Group Management
Protocol |
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Overview |
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The
Cisco Group Management Protocol (CGMP) is a proprietary layer 2 protocol
that is used between Cisco routers and switches to limit multicast traffic on a
virtual LAN (VLAN). CGMP was developed to address the problem illustrated in
Figures 4-1 and 4-2. |
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Figure 4-1: At least one IGMP-registered receiver is required
for a router to forward multicast traffic. |
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Figure 4-2: Multicast traffic is received by all hosts on a
shared hub network. |
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In
Figure 4-1, the network consists of a router and three ethernet network
segments. Each segment contains an ethernet hub or repeater, and a packet
transmitted by the router onto one of the segments is received by every host on
the segment. Assume a host on network 2 wishes to receive the multicast traffic
from the source on network 1. The host on network 2 sends an IGMP Join message
to the router, and the router installs state for network 2, indicating that
there is at least one receiver for traffic from the indicated multicast
group. |
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Remember from Chapter 3, “Internet Group Management
Protocol,” that the router does
not need to know how many receivers are on a network, only that there is at
least one receiver. Network 3 has no receivers for the multicast group, so the
router does not forward multicast traffic onto network 3. When the sender on
network 1 transmits a multicast packet, the router forwards the traffic onto
network 2, but not onto network 3. The hub on network 2 sends a copy of the
packet and all subsequent packets to all hosts attached to the hub. The hosts
that do not want to receive the multicast traffic must process the frame in
order to determine that the frame was not intended for them. |
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Obviously, this is not an ideal situation. The ideal situation is to
limit the multicast traffic not only to networks that have receivers, but also
to limit the traffic to receivers on a network that want to receive
it. |
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Layer-three multicast routing protocols are used to limit multicast
traffic to networks that have receivers which have indicated their desire to
receive the traffic. Later chapters cover layer three multicast routing
protocols and their implementation. |
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In
order to remedy the situation depicted in Figure 4-2, we will replace the hub
with an ethernet switch. Assume we have an ethernet switch with 50 attached
users and that virtual LANs are not being implemented. Without VLANs, every host
is on the same IP subnet, and broadcast traffic from one host is flooded to all
hosts on the switch (see Figure 4-3). |
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Figure 4-3: Without VLANs, broadcast traffic is forwarded to
all hosts. |
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The
situation in Figure 4-3 can be improved by reducing the size of the broadcast
domain using VLANs. A VLAN is comprised of hosts in a common IP subnet. For
example, if we want to reduce the size of the broadcast domains in Figure 4-3
from 50 to 25 hosts, we would need two VLANs or two logical IP subnets (LIS).
Figure 4-4 contains a network where we can accomplish the same broadcast domain
size reduction using two switches and no VLANs. Whenever you have more than one
LIS, you need a router for intersubnet traffic. |
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