Setting the multicast datagram time-to-live (router hop-count) threshold for
the VLAN
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.
The VLAN connected to the multicast source does not receive state refresh packets and thus is not state-refresh
capable. Downstream VLANs in the switches covered in this guide are state-refresh capable. 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 routing switch requires knowledge of the TTL setting of incoming multicast packets:
• A value that is too high can allow multicast 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 multicast traffic regardless of packet TTL setting
Example of configuring PIM-DM operation at the VLAN level
The network in the following figure 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, the
ip pim-
dense ip-addr any
command would be used instead.) 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.
Chapter 3 PIM-DM (Dense Mode)
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