From: anthony.sequeira@thomson.com
Date: Sat Dec 09 2006 - 11:37:21 ART
Thanks to Ivan for pointing out an issue with the Multicast TTL question
from last week!
Please note: the TTL threshold works OPPOSITE of how you might think!
Only packets that have a TTL GREATER than the TTL configured on the
interface are forwarded. Please examine the question and answer below
carefully for those of you that are interested in this feature:
7-7-5. An administrator in your network has configured the ip multicast
ttl-threshold command on a router and set the TTL Threshold to 12. Which
of the following TTL values of packets will trigger a packet drop?
Choose all that apply.
a. 6
b. 12
c. 13
d. 15
Answer: a, b
Topic Domain: 7d
Issues:
What is the multicast TTL? How does the TTL Threshold work in multicast
environments?
Solutions:
Multicast TTL is a method for scoping multicast traffic. Multicast
packets have a TTL value that potentially controls their propagation. If
a router is configured with a threshold - the packet is not forwarded if
it has a TTL less than or equal to the configured threshold. In other
words, the TTL on the multicast packet must be greater than the
threshold to be forwarded. Another method of scoping multicast traffic
is to use the administrative multicast address range.
More Information:
IP Multicast Troubleshooting Guide
http://www.cisco.com/warp/public/105/mcastguide0.html#ttlthreshold
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