HP 3500YL - Manual (Page 81)

PIM-DM (Dense Mode)
Configuring PIM-DM
Syntax: ip pim-dense [ ttl-threshold < 0 - 255 > ]
vlan < vid > ip pim-dense [ ttl-threshold < 0 - 255 > ]
Sets the multicast datagram time-to-live (router hop-count)
threshold for the VLAN. Any IP multicast datagrams or state
refresh packets with a TTL less than this threshold will not
be forwarded out the interface. The default value of 0 means
all multicast packets are forwarded out the interface.
This parameter provides a method for containing multicast
traffic within a network, or even within specific areas of a
network. Initially, the multicast traffic source sets a TTL
value in the packets it transmits. Each time one of these
packets passes through a multicast routing device, the TTL
setting decrements by 1. If the packet arrives with a TTL
lower than the
mroute ttl-threshold, the routing switch does
not forward the packet. Changing this parameter on a rout­
ing switch requires knowledge of the TTL setting of incoming
multicast packets. A value that is too high can allow multi­
cast traffic to go beyond your internal network. A value that
is too low may prevent some intended hosts from receiving
the desired multicast traffic. (Default: 0 — forwards multi­
cast traffic regardless of packet TTL setting.)
Example of Configuring PIM-DM Operation at the VLAN Level. The
network in figure 3-4 uses VLAN 25 for multicast traffic. However, this VLAN
is multinetted and there is only one subnet (10.38.10.x) in VLAN 25 that is
common to all three routing switches. Thus, when configuring VLAN 25 on
these routing switches to perform multicast routing, it is necessary to use
ip
pim-dense < source-ip-address > to designate the common subnet as the source
address for outbound multicast traffic on VLAN 25. (If only identical subnets
were present in the multinetted VLAN 25 configuration on all three devices,
then the
ip pim-dense ip-addr any command would be used instead.) Note that
the other VLANs in the network are not multinetted and therefore do not
require the
ip pim-dense ip-addr < any | source-ip-address > option.
For this example, assume that the VLANs and IP addressing are already
configured on the routing switch.
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