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Chapter 10
Configuring Multicast
IP PIM Dense Mode
Dense mode was discussed in Chapter 9. PIM dense mode functions by using
the source root shared tree. It also assumes that all PIM neighbors have
active multicast members directly connected, and therefore, it initially for-
wards multicast group data out all PIM-enabled interfaces.
The syntax for this command is very simple, ip pim dense-mode. An
example of placing an interface in PIM dense mode follows:
RouterA#conf t
Enter configuration commands, one per line. End with
CNTL/Z.
RouterA(config)#interface FastEthernet3/0
RouterA(config-if)#ip pim dense-mode
RouterA(config-if)#^Z
RouterA#
This is what the interface configuration looks like now:
!
interface FastEthernet3/0
ip address 172.16.21.4 255.255.255.0
no ip directed-broadcast
ip pim dense-mode
!
IP PIM Sparse Mode
Sparse mode was described to use shared root source tree distribution and
relies on the knowledge of an RP. If an RP cannot be found, the router is
unable to forward multicast information, strictly because it does not know
where the source of the multicast traffic should come from. If it can't deter-
mine where the traffic is supposed to be coming from, the Reverse Path For-
warding (RPF) check will fail and no interfaces will be added to the multicast
forwarding table.
Configuration of PIM sparse mode is just as simple as it was for IP dense
mode. The command that needs to be used to enable IP PIM sparse mode is
ip pim sparse-mode
. Sparse mode PIM will also activate IGMP on the
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