From: Frank B (frank@xxxxxxxxxxxx)
Date: Sun Aug 04 2002 - 15:50:13 GMT-3
First, to verify they are dense mode groups look at the flags when you
do a sh ip mroute. The first is always D for dense. Also look at the
entries for the groups you have senders and receivers for and you'll
notice they probably have an S for sparse. This is also why you use
sparse-dense mode on interfaces when using auto-RP--so you can join
these two dense mode groups.
But, this is an interesting story, it's like the chicken and the egg
problem. Auto-RP was designed to provide routers with the address of
the RP via 2 multicast groups 224.0.1.39 where candidate RPs can
announce their candidacy to the mapping agent(s) AND 224.0.1.40 where
mapping agents can advise multicast routers who they selected as the
RP(s) for particular groups from among all of the candidates.
If these groups were sparse mode groups you would have to join the
shared tree toward the RP to receive traffic from the group (at least to
begin with.) But you don't know the address of the RP yet
right?...that's why you want auto-RP in the first place. So, that's why
these groups MUST be dense. It eliminates the chicken and the egg issue
and makes sure the RP info is flooded about.
I highly recommend getting Beau Williamson's "Developing IP Multicast
Networks-Volume 1" it explains just about everything there is regarding
IP multicast--certainly more than enough for the average network geek.
If possible, go to Networkers and hear him speak on the subject--he's
definitely "da man" when it comes to IP multicast.
Later and aloha, Frank
-----Original Message-----
From: D. J. Jones [mailto:meganac@attbi.com]
Sent: Sunday, August 04, 2002 7:37 AM
To: Frank B
Subject: Re: Multicast question
Frank,
why would you consider 224.0.1.39 and 224.0.1.40 dense-mode groups? If
they are being used in the context of Auto-rp which depends on pim
sparse-mode, why would they not be sparse-mode groups? Thanks..dj
----- Original Message -----
From: "Frank B" <frank@buff-net.com>
To: "'Tony H.'" <aamercado31@yahoo.com>; "'Szeto Jeff'"
<jytszeto@hotmail.com>
Cc: <ccielab@groupstudy.com>
Sent: Saturday, August 03, 2002 8:43 PM
Subject: RE: Multicast question
> Hey Tony and Jeff,
>
> I don't notice anything unusual except that you're using interface e1
> as the rp-address and you don't show the config for e1. That may be an
> issue.
>
> I'm certainly NOT a multicast expert but here's my take...if anyone in
> the group can clarify or correct anything please do. But I believe
> when you configure an interface with ip igmp join-group G you have
> congigred the router to act as a receiver. So the term Jeff used
> "hosting" is, well at least to me, misleading. You're not "hosting"
> the groups you're sort of acting like a receiver for traffic destined
> for the group.
>
> When you issue a ping to a multicast group address you're sending or
> "sourcing" packets from yourself to a mutlticast address--your're now
> a multicast traffic "source"! When you ping from r2 or r3 to
> 224.1.1.1 you should get a response from r1--do it several times, it
> took 5-9 pings for me until it worked. Also, if you issue a show ip
> mroute afterward you'll see a (*,G) entry for 224.1.1.1 and an
> (S,G) entry--maybe 2 entries on r2 one for each interface configured
> for ip pim--with the local router as the source. One will probably
> have a null Outgoing Inteface List (OIL) and the other should have the
> interface through which the rp is reachable...maybe?
>
> Assuming the rp is reachable this set-up will work. I recently tested
> a very similar scenario. But, if the rp-address is not reachable by
> your r3 it can't register with the rp as a source for the group so I
> don't think the ping will work. The first packet(s) are encapsulated
> in a PIM register and then unicast to the rp. If the rp has receivers
> (which it does in your case..e0 of r1) it then joins the shortest-path
> tree and then kills the unicast registers from continuing with a
> register stop.
>
> Oh yea, you probably should make sure both r2 and r3 have the two
> dense-mode groups 224.0.1.39 and 224.0.1.40 in the mroute table. I'm
> thinking you'll have both a (*,G) and a S,G) entry for both groups on
> both routers. If not, even if the rp was reachable, the router won't
> know the ip address of the rp yet...clearly a problem
>
> I rambled a bit too much but I hope this helps. As always, don't take
> any advice from this list as truth without testing it yourself first!
>
> Aloha, Frank
>
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
> Of Tony H.
> Sent: Friday, August 02, 2002 7:29 AM
> To: Szeto Jeff; ccielab@groupstudy.com
> Subject: Re: Multicast question
>
>
> Hi
>
> Design wise R2 should be hosting the 2 groups and you
> will find all things pingable
>
> However with r1 hosting the 2 groups, r2 will prune
> back the multicast route to r3. This is normal
> behavior for Sparse mode as to decrease multicast
> traffic. If you were to add some clients off r3, then
> r2 will go from prune to active (join) and r3 should
> be able to ping r1
>
> --- Szeto Jeff <jytszeto@hotmail.com> wrote:
> > Hi,
> >
> > I am practising multicast and cannot do a simple
> > config.
> >
> >
> >
> --e0--R1--s0-------HDLC------s1-R2-S0------HDLC-----s1-R3
> >
> > R1 ethernet joined two groups:224.1.1.1 and
> > 224.2.1.1
> > I use sparse-dense-mode, auto-rp
> > The problem is R3 cannot ping both multicast groups
> > while
> > R2 does.
> >
> > Please help, the related config is at the end,
> >
> > Thank you.
> >
> > Regards,
> >
> > Jeff
> >
> > R1
> > ip multicast-routing
> >
> > interface Ethernet0
> > ip address 172.16.20.2 255.255.255.0
> > ip pim sparse-dense-mode
> ip igmp join-group 224.1.1.1
> ip igmp join-group 224.2.1.1
> >
> > interface Serial0
> > ip address 172.16.12.2 255.255.255.0
> > ip pim sparse-dense-mode
> > ip pim send-rp-announce Ethernet1 scope 15
> > ip pim send-rp-discovery scope 15
> >
> >
> > R2
> > ip multicast-routing
> > interface Serial0
> > ip address 172.16.14.1 255.255.255.0
> > ip pim sparse-dense-mode
> >
> > interface Serial1
> > ip address 172.16.12.1 255.255.255.0
> > ip pim sparse-dense-mode
> >
> > R3
> >
> > ip multicast-routing
> > interface Serial1
> > ip address 172.16.14.4 255.255.255.0
> > no ip directed-broadcast
> > ip pim sparse-dense-mode
> >
> >
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