[c-nsp] CGMP question

[Nick Hilliard]

On 27/06/2012 20:25, Bernie wrote:
> Hello! I'm new to working with CGMP and PIM in general and have a
> conundrum-
> 
> I've come into a new network where multicast is configured. There is no
> experienced network engineer on site with historical info on this setup.
> 
> I believe CGMP is misconfigured because taking my personal laptop and
> connecting it to a user port shows roughly 75% of the packets received
> (600/800) in 30 seconds are CGMP Join messages (Wireshark pcap).

CGMP is misconfigured because it exists in the configuration.  You don't
want to run CGMP on a modern multicast network.  If possible you want to
run igmpv3, if not then igmpv2.

> The core 6500 switch is configured as follows-
> 
>   ip pim rp-address x.x.x.x 85
>   ip pim send-rp-announce LoopbackXX scope 16 group-list 85 interval 30
> 
> interface LoopbackXX
>   description PIM RP loopback IP
>   ip address x.x.x.x y.y.y.y
>   ip pim sparse-dense mode
> 
> Interface Vlan55
>   description User Vlan
>   ip pim sparse-dense-mode
>   ip cgmp
>   no ip mroute-cache
> 
> From the core it's layer 2 out to edge switches (3750 stacks) on the user
> vlan. I don't see any explicit configs on the 3750 stacks so I'm thinking
> that's where my problem is, but I don't know how to proceed!

You've obviously inherited a legacy multicast configuration.  This is going
to be a complete pain for a variety of reasons, including older software
versions on the c6500 have hilarious multicast bugs (that's "hilarious" as
in "omfg"), and because it appears to be a legacy configuration.  If you
are planning to change anything, you're going to have piles of fun because
you'll need to ensure compatibility with all the existing multicast
configuration.  As things have changed a _lot_ since the days when CGMP was
the only real option in town, it's likely that this is going to be
troublesome, depending on how many networking devices you have.

If you're new to multicast, you will need to spend some time immersing
yourself in order to understand it.  It's completely different to regular
unicast ip transmission.  Be careful about changing stuff until you're
happy you have a good idea of what's going on.  Wireshark is good.  Get
some good quality multicast clients and set up monitoring stuff so that you
can determine when stuff has been knocked offline.

Also bear in mind that lots of networking devices have varying levels of
multicast support.  For example, 3750 stacks will sort-of support igmpv3,
but it's really a front-end wrapper on an igmpv2 engine.

You will probably also want to implement pim and igmp snooping to stop
multicast frames from being flooded across all your vlan ports.

Nick