[j-nsp] Odd drop behavior on low-rate multicast streams

[John Neiberger]

We have a simple setup like this:

[receiver] ---- [ Cisco-rtr-A] ---- [ Cisco rtr-B] ----- [MX960] -----
[Cisco rtr-C] ---- [source]

The receiver has joined a specific S,G, but this is very low rate,
perhaps just a couple of multicast packets per week. It's a multicast
messaging setup related to emergency alerts. What's weird is that if
you look at the multicast routing tables on the Cisco routers, the S,G
will be there. However, the multicast route is not present on the
Juniper router. It does show up under "show pim join", but there is no
route. The odd behavior we're seeing is that the first message from
source to receiver is always dropped. If we configure the source to
send multiple times, the first one fails but all subsequent messages
succeed. Our thought is that the multicast route is timing out on the
MX 960, so the first packet gets dropped, although it apparently
causes the route to be setup again since the rest of the packets
succeed.

Any idea if we're on the right track? It seems like we're just running
into some sort of timeout issue on this router, but we're not sure
yet.

Thanks,
John