Re: How to get the multicast address in a network


latha wrote:
Hi all,

I have one question.
How to get the multicast group addresses in which our system is also a
member.

On a Linux host (end station) you can use:
$ /sbin/ip maddress list
1: lo
inet 224.0.0.1
2: eth0
link 01:00:5e:00:00:01
inet 224.0.0.1

I have not joined any groups. See here for ip command ref:
http://snafu.freedom.org/linux2.2/docs/ip-cref/ip-cref.html#SECTION000100000000000000000

I have used "host -l mcast.net " for printing the multicast addresses,
but
it was coming like

;; communications error to 172.16.200.3#53: end of file

172.16.200.3 is our Domain name server.

Can anyone help me to get the multicast group addresses.

It is not clear what info you are wanting about multicast. Do you wish
a list of _which_ hosts are joining which multicast groups? Just a
list of which multicast groups have any members?

Multicast group membership is dynamic and has no connection with DNS.

Routers running multicast routing protocols only take note of whether
_any_ hosts are listening to a multicast address. They do not maintain
a list of multicast-to-unicast IP mappings so have no idea _which_
hosts are listening. They do maintain a "list" of which interfaces
have listeners for each active group. Is this what you need?

Switches and multicast, on the other hand, require different treatment
to avoid flooding each port with multicast packets just because one
host has joined a group. They _may_ maintain a list of
multicast-to-port # addresses that can be queried if the switch is
running:

-- CGMP (Cisco Group Management Protocol)
-- IGMP Snooping
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/ipmulti.htm#xtocid15

-- GMRP:GARP (GARP [Generic Attribute Registration Protocol] Multicast
Registration Protocol [GMRP])
http://www.javvin.com/protocolGMRP.html

See here for an inclusive intro:
http://www.intelligraphics.com/articles/ipmulticasting2_article.html

There are also several application layer protocols (especially MADCAP)
with which I have no experience. See the Cisco doc above. You are not
likely running these without knowing it. Then again, you never can be
sure ;-)

So how to get whatever info you are wanting will depend on your setup.
And remember, google is your friend.

cheers,
prg

.

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