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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 24 Configuring IP Multicast Routing
Configuring Optional Multicast Routing Features
To return to the default setting, use the no ip sdr cache-timeout global configuration command. To
delete the entire cache, use the clear ip sdr privileged EXEC command.
To display the session directory cache, use the show ip sdr privileged EXEC command.
Configuring the TTL Threshold
Each time an IP multicast packet is forwarded by the multilayer switch, the time-to-live (TTL) value in
the IP header is decremented by one. If the packet TTL decrements to zero, the switch drops the packet.
TTL thresholds can be applied to individual interfaces of the multilayer switch to prevent multicast
packets with a TTL less than the TTL threshold from being forwarded out the interface. TTL thresholds
provide a simple method to prevent the forwarding of multicast traffic beyond the boundary of a site or
region, based on the TTL field in a multicast packet. This is known as TTL scoping.
Figure 24-10
shows a multicast packet arriving on Gigabit Ethernet interface 0/2 with a TTL value of 24.
Assuming that the RPF check succeeds and that Gigabit Ethernet interfaces 0/1, 0/3, and 0/4 are all in
the outgoing interface list, the packet would normally be forwarded out these interfaces. Because some
TTL thresholds have been applied to these interfaces, the multilayer switch makes sure that the packet
TTL value, which is decremented by 1 to 23, is greater than or equal to the interface TTL threshold
before forwarding the packet out the interface. In this example, the packet is forwarded out interfaces
0/1 and 0/4, but not interface 0/3.
Figure 24-10 TTL Thresholds
Figure 24-11
shows an example of TTL threshold boundaries being used to limit the forwarding of
multicast traffic. Company XYZ has set a TTL threshold of 100 on all routed interfaces at the perimeter
of its network. Multicast applications that constrain traffic to within the company’s network need to send
Step 3
end
Return to privileged EXEC mode.
Step 4
show running-config
Verify your entries.
Step 5
copy running-config startup-config
(Optional) Save your entries in the configuration file.
Command
Purpose
Si
Multicast datagram
with TTL = 24
Output Interface List
Gigabit Ethernet 0/1: TTL threshold = 16
Gigabit Ethernet 0/1
TTL = 16
Gigabit Ethernet 0/3: TTL threshold = 64
Gigabit Ethernet 0/4
TTL = 0
Gigabit Ethernet 0/4: TTL threshold = 0
Gigabit Ethernet 0/3
TTL = 64
Packet not
forwarded
Gigabit Ethernet 0/2
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