
Configuring IP Multicasting Services
4-14 114064 Rev. B
We recommend the following values:
Specifying a Threshold
Threshold values control the scope of datagram delivery. The threshold is the
minimum IP time-to-live (TTL) required for a multicast datagram to be forwarded
out a given interface. DVMRP compares the TTL value of each multicast
datagram to be forwarded with the threshold configured for that interface. If the IP
TTL is less than the threshold value you specify, the datagram is dropped by the
router.
For mrouted compatibility, multicast datagrams originated by the router have a
TTL of 1. These datagrams are not compared against the TTL. Unicast datagrams
originated by the router (for example, for route reports issued via a tunnel) have a
TTL of 255. A datagram that is to be forwarded through a tunnel is first compared
against the threshold and, if accepted, is then encapsulated in an IP datagram with
a TTL of 64.
By default, DVMRP uses a TTL value of 1. You can use Site Manager to specify a
TTL value from 1 to 254.
T
able 4-3 lists the originating TTL values recommended for certain types of
multicast applications and the threshold values recommended for routers to permit
the forwarding of packets from these applications. These values are recommended
for topologies that will connect to the MBone.
Hop Metric
LAN, or tunnel across a single LAN 1
Multihop tunnel 2 or 3
Serial link, or tunnel across a serial link 1
Backup tunnel Primary tunnel metric + 1
Site Manager: Threshold parameter: page A-8
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