From: Bob Sinclair (bob@bobsinclair.net)
Date: Sat Nov 03 2007 - 10:34:09 ART
Champika G wrote:
> Dear Group
> When a question says that multicast traffic should not be forwarded if
> it has a ttl of < 13 over a interface,
>
> Then the ip multicast ttl-threshhold must be 12 or 13?
>
>
With ip multicast ttl-threshold 8 I get the following debug using
various starting TTLs:
R4#
*Mar 1 00:42:13.051: IP(0): s=172.16.35.5 (FastEthernet0/0)
d=239.192.1.1 id=140, ttl=7, prot=1, len=114(100), threshold denied
R4#
*Mar 1 00:42:34.015: IP(0): s=172.16.35.5 (FastEthernet0/0)
d=239.192.1.1 (Serial1/0) id=141, ttl=8, prot=1, len=100(100), mforward
That is: With the threshold set to 8, packets with ttl 7 are denied,
but packets with ttl 8 are permitted.
So, it appears that the TTL in the packet must EQUAL or EXCEED the
threshold in order to pass traffic.
This conflicts with the following statement from the configuration guide:
" Only multicast packets with a TTL value greater than the threshold are
forwarded out the interface."
But is in accord with this Tech Note document:
http://www.cisco.com/en/US/tech/tk828/technologies_tech_note09186a0080094b55.shtml#ttlthreshold
So, in your scenario I would set the threshold to 13. Any multicast
packet with a ttl less than 13 would be denied.
HTH,
--Bob Sinclair CCIE 10427 CCSI 30427 www.netmasterclass.net
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