Configuring IP Multicast Protocols
December 2005 © Foundry Networks, Inc. 19 - 13
Failover Time in a Multi-Path Topology
When a port in a multi-path topology fails, and the failed port is the input port of the downstream router, a new path
is re-established within a few seconds, depending on the routing protocol being used.
No configuration is required for this feature.
Modifying the TTL
The TTL defines the minimum value required in a packet for it to be forwarded out of the interface.
For example, if the TTL for an interface is set at 10, it means that only those packets with a TTL value of 10 or
more will be forwarded. Likewise, if an interface is configured with a TTL Threshold value of 1, all packets
received on that interface will be forwarded. Possible TTL values are 1 to 31. The default TTL value is 1.
Configuration Notes
• If the TTL for an interface is greater than 1, PIM packets received on the interface are always forwarded in
software because each packet’s TTL must be examined. Therefore, Foundry does not recommend modifying
the TTL under normal operating conditions.
• Multicast packets with a TTL value of 1 are switched within the same VLAN. These packets cannot be routed
between different VLANs.
Configuration Syntax
To configure a TTL of 24, enter the following:
FastIron SuperX Router(config-if-3/24)# ip pim ttl 24
Syntax: ip pim ttl <1-31>
Dropping PIM Traffic in Hardware
Unwanted PIM Dense or PIM Sparse multicast traffic can be dropped in hardware on Layer 3 Switches. This
feature does not apply to DVMRP traffic. Refer to “Dropping PIM Traffic in Hardware” on page 19-31.
PIM Sparse
Foundry devices support Protocol Independent Multicast (PIM) Sparse version 2. PIM Sparse provides
multicasting that is especially suitable for widely distributed multicast environments. The Foundry implementation
is based on RFC 2362.
In a PIM Sparse network, a PIM Sparse router that is connected to a host that wants to receive information for a
multicast group must explicitly send a join request on behalf of the receiver (host).
PIM Sparse routers are organized into domains. A PIM Sparse domain is a contiguous set of routers that all
implement PIM and are configured to operate within a common boundary. Figure 19.3 shows a simple example of
a PIM Sparse domain. This example shows three Layer 3 Switches configured as PIM Sparse routers. The
configuration is described in detail following the figure.
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