From: cebuano (cebu2ccie@cox.net)
Date: Sun Mar 16 2003 - 01:05:19 GMT-3
I'm in the same boat. Basically this is one of the many technologies
better understood if presented in a hands-on type of example, instead of
just being described. Similar to how Caslow does in his book. Now if
only there was a practical workbook on the 3550 so we could test out the
features using TESTED and WORKING samples.
Sorry Jim, I'll have to re-read this chapter for the nth time before I
can answer your questions.
Elmer
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
ccie2be
Sent: Saturday, March 15, 2003 8:21 PM
To: Group Study
Subject: 3550 - MVR
Hi all,
I just finished reading for the nth time, the section on MVR (Multicast
Vlan
Registration) in Chapter 18 of the 3550 Configuration Guide. I don't
know if
it's just me or if that section just isn't all that clear but I really
don't
get it.
I have a bunch of questions on it but my first question is this:
How important a topic is this? If I can't get this right will I lose
all the
points for Multicast?
1) Regarding MVR itself, it seems that this feature creates a
separate vlan
for multicast traffic and that access ports connected to hosts that want
multicast traffic "belong" to both their normal vlan and the multicast
vlan.
But, how can that be if access ports can only belong to 1 vlan? (Ports
that
carry the traffic of multiple vlans are trunks.)
2) Consider the situation where you have a multicast router connected
via a
trunk to a Cat switch that doesn't have this feature. On the Cat switch
there
are several vlans defined: vlan 5, 6, and 7 with hosts attached to each
vlan.
Let's also say, CGMP is running between the router and the switch.
Here's my
understanding (taken from the Beau Williamson book) of what happens when
a
host wants to join a multicast group.
i) host sends an igmp report to the router (using the mac addr
equivalent
of the ip mcast group)
ii) switch floods mcast frame to all ports regardless of vlan
iii) the router adds that m-group to its outgoing interface list if
it's
not already there.
iv) router, using cgmp, informs switch to add the m-group to it's
cam
table
v) the switch, using info from the cgmp message adds the host's port
# to
the m-group entry
As a result, the cam table will now have 2 mac addresses associated with
the
port:
a) the mac addr of the host hanging off the port used for regular
unicast
traffic and
b) the mac addr of the ip multicast group so that mcast traffic goes to
that
host
What's interesting in this process, is that all this activity seems to
be
completely independent of vlans. If I'm understanding this correctly,
unknown
multicast traffic is flooded to all ports - not just the ports in the
same
vlan unlike broadcast traffic which only goes to those ports in the same
vlan.
So, here's what I can't figure out: How does the above process change
with
MVR? And, what's the benefit of MVR over the above process?
I sure hope someone out there in group study can help me with this. I'd
really appreciate that.
Jim
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