[Xorp-users] (Most) Multicast packets arriving twice

[Dave Price]

Dear All,

	While setting up my XORP router here, we have
made progress, but we now have noticed that many
multicast packets seem to arrive twice.

A first we thought it was all packets, but my opinion
is starting to change.

	If I start a beacon client  (we are using the Java version
chatting to the JANET beacon server at http://ulcc.beacon.ja.net/global/)
then all beacon traffic appears to arrive twice to us from what we can see.
I used tcpdump and snoop on various interfaces to spot the duplicates.

[Aside: actually I think the Java Beacon client has bugs
in some ways as it seems to count things in odd manners, especially
with duplicates but thats another matter]

	However, if I stop the beacon client and allow traffic to die
away and then use sdr to start vic/rat attached to "Places all round the world"
that multicast traffic only appears to me to arrive once.

	Any thoughts??

	Our one thought is that the main link feeding
our site is actually a pair of load balancing links
between riverstone routers.  Could a copy of each packet be arriving
one down each link we thought?   But if that's the case, how
come the "places around the world" traffic arrives once but the
beacon traffic arrives twice?

	Another thought was shared trees v source trees.  Could
we be seeing the traffic arrive once down a RPT shared tree from
the RP and then again on a SPT source tree directly from each source??

	How would I tell from the arriving packets
(I don't I can can I ??) which way they had arrived?

	Could I tell from the "show pim ..." commands whether or not
XORP has tried to send prunes to the RP for the groups/sources
arriving via SPT

or ???  thoughts??

Thanks,

Dave Price

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	| David Price, Computer Science				|
	|							|
	|  Computer Science, University of Wales, Aberystwyth,	|
	|  Penglais Campus, Aberystwyth, Ceredigion, SY23 3DB	|
	|                                                       |
	| Email: dap@aber.ac.uk WWW: http://www.aber.ac.uk/~dap |
	|  Phone: +44 1970 622428   FAX: +44 1970 622455	|
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