All things related to multicast

[John Kristoff <>]

On Mon, 7 Mar 2005 16:53:22 -0500 (EST)
Caren Litvanyi 
<>
 wrote:

> Other potential problems I have heard involve HSRP or VRRP
> interaction with PIM.  I do not have the gear to build this
> scenario in a lab.  Can anyone here relate specific experience
> with HSRP breaking multicast?  Or how they got multicast working
> with HSRP?  Or can report "it just works"?

I've had some experience with HSRP.  In fact, there was a thread that
touched one aspect of something I ran into.  That thread starts here:

  <https://mail.internet2.edu/wws/arc/wg-multicast/2004-11/msg00028.html>

In a test, I upgraded a router that was the HSRP DR for a stub subnet.
However, before doing so I set 'ip pim dr-priority 10' on the other.
This forced the standby router to become DR, which was useful in our
scenario for a client on the stub network that was receiving a TV feed.
The app appeared to receive it's multicast traffic and continue without
any noticeable interruption.

I've not explored HSRP and multicast extensively, but it appears to
work fine.  I don't believe there is anything unique we've had to do
to make it work.  The only original thing I believe we've been doing
is to poke a hole in the stub HSRP-enabled subnets anti-spoofing ACL
or uRPF check.  That is, allow any source UDP traffic to 224/4 since
multicast packets will come into the subnet via one HSRP router and
hit the router interface of the other.  If you deny those packets it
shouldn't hurt, but if you're doing any logging or flow collection
you may want to just let the router get them to keep the noise level
down.

> I would be very grateful to people who can relate scenarios 
> where some local/LAN network configuration or design "breaks"
> otherwise properly configured multicast, and how you discovered
> and fixed the problem.  So this is sort of a re-issue of Alan's
> request for LAN issues.  Particularly if you were able to work
> around them with some configuration or topology change.  I have
> not run a very large campus LAN since about 1999, so I don't have
> recent experience to help people in this area, and am somewhat
> at a loss.

I'm pretty sure it's been mentioned either here or the multicast
training team before, but there is the spanning tree topology change
issue we ran into on our LANs.  This is when Windows hosts enabled for
bridging sends TCNs, which in a nutshell cause our switches to remove
multicast filtering state and flood all multicast for short while.

John