From: Donny MATEO (donny.mateo@sg.ca-indosuez.com)
Date: Sun May 11 2003 - 23:19:04 GMT-3
Hi Robert,
when you tried sparse mode, I take it that C-RP is directly connected to your MA ?
Here is a catch. When you configure a router to use pim, the router will automatically join the group 224.0.1.40 (This is where you listen for RP-mapping message). Now when you configure a router to be a MA, it will start listening on 224.0.1.39 for C-RP
announce message. Ok so far so good.
Here is what happened, if your C-RP is directly connected, C-RP will multicast RP-Announce message to the MA (I'm not sure, but perhaps you can check this by observing the show ip mroute 224.1.0.39 and see the output interface).
Since the MA is joining group 224.0.1.39 and is directly connected (so it does not have to send join message upstream the RFP tree etc,) it will receive the data directly.( you can think of this the MA is a host or pc with multicast software listening to
the multicast address of 239.0.1.39) Ok that settle the problem of the MA getting message from C-RP.
Now MA will send the RP-Mapping message to all PIM router. Now since all pim router are directly connected, the same thing would happen. MA will annouce RP-Mapping message with the multicast address of 239.0.1.40 (remember all PIM router will
automatically join this group), and since PIM routers are directly connected, so they will receive the message without having to rely on sparse mode mechanism.
So far so good.
Now why sparse-dense mode then ? Well, this is for the case when C-RP(s) and MA(s) are not directly connected and you have a lot of PIM router that is more than 1 hop away from the MA(s). In short, this is for those cases whereby you have an
intermediate router. The same thing would happened as the above, but with one catch here. The intermediate router wouldn't pass along the message if it's configured with sparse mode. Why ? Because in sparse mode you need explicit join before your upstream
router will deliver the mesage to you, and for this you need an RP, so you know where to send the join message (upstream router on the path to RP). . So this broke the C-RP announce and RP-Mapping announce message mechanism that we have so far neglect.
This is why then cisco came up with a method called the sparse-dense mode. With this method, the router will behave this way. If it has an RP of a group, then it will work on sparse mode. If it does not, it will work as a dense mode. ( Sparse Dense mode
is not a new mode, it's just a mode that enabled the router to use both method of multicast as needed or should I say as you design them to be. If a particular group has an RP (either static or auto-RP), then the router will understand to treat the
traffic as a sparse mode, in the even when there is no RP for a particular group, the router would interpret it as to treat the group using Dense behaviour.)
Now observed. C-RP will announce C-RP message out of all it's interface. And this message would be flooded down the network. If there is an intermediate router, the router will flood all the C-RP message out of it's interface (except the one it receive
the message from). Why ? Because there is no RP for this 223.0.1.39 group so router with sparse-dense mode configured in it's interface will treat it as a Dense multicast. And on a Dense multicast the first step is always to flood the entire downstream
peers with multicast traffic (neighbor would send a join/prune message later on if it doesn't want to receive the traffic). This way MA will eventually got the C-RP announce message.
The same will hold true for the RP-Mapping announcement. The MA will flood the RP-Mapping information in Dense mode through all the PIM router. And since all PIM router will automatically join the 224.0.1.40 group, they will always receive the RP-Mapping
message (they will maintain the Dense mode multicast tree by keeping a keepalive join/prune message to their upstream router which will in turn foward it to the multicast source which is/are the MA(s). This is due to the fact that all PIM router will join
group 224.0.1.40 by default, so as long as they are PIM router, the tree will always exist for the RP-mapping group. This would make sure all router get the RP-mapping message from MA.
So, I hope it's clear now on the statement why sparse-dense mode is required. If other has anything to add (correction or something), I would really would like to hear them.
Donny
CCIE # 11189
"Robert Yee"
<robert@bluespud. To: "Donny MATEO" <donny.mateo@sg.ca-indosuez.com>
com> cc: <ccielab@groupstudy.com>, <nobody@groupstudy.com>
Sent by: Subject: RE: Auto-RP with NBMA
nobody@groupstudy
.com
05/11/2003 07:14
AM
Please respond to
"Robert Yee"
Hi Donny,
Here is the link forthe my second statement regarding Auto-RP and sparse-dense:
http://www.cisco.com/en/US/tech/tk828/tk363/technologies_white_paper09186a00800d6b63.shtml#87563
It's the first sentence of the 3rd paragragh titled 'Sparse-dense mode for Auto-RP'
I've tried configuring it in sparse mode and it works.
Thanks for the help.
Robert
-----Original Message-----
From: Donny MATEO [mailto:donny.mateo@sg.ca-indosuez.com]
Sent: Friday, May 09, 2003 7:25 PM
To: Robert Yee
Cc: ccielab@groupstudy.com; nobody@groupstudy.com
Subject: RE: Auto-RP with NBMA
Hi Robert.
For your confusion number 1 :
=> if you read the document through, you will actually understand that the comand ip pim nbma is used to optimized behaviour of a sparse-mode pim on nbma network. You see if you were using Dense mode, the ip pim nbma command is useless. Why ? Because in
Dense mode there is no such thing as explicit join and such, the data would be flooded down the three in a certain interval.
The command is created to let's say adjust the NBMA network to behave well (in definition of sparse mode). In sparse mode multicast, a node will explicitely notify it's upstream multicast router when it wants to receive a certain traffic. Now the router
behaviour in NBMA network when fowarding multicast would be to duplicate it and send it downstream so it's kindda beats the purpose of having using sparse mode in the first place (by all means if that's the case why bother, just use dense then). SPARSE
MODE MUST BE IN USE because => IP PIM NBMA is designed to work and solve problem related to SPARSE MODE on NBMA network.
As for the second statement, can you provide the link where I can verify this statement. There are some information on auto-rp that is flooded in Dense mode thus requiring allrouter to join this particular group (the group for rp advertisement and group
mapping advertisement, i believe the router join one by default). I need to dig up some info on this before I can answer you, brain is getting rusty again... :0
Only thing that is good and bad about sparse-dense is that if a router don't have a rp-group maping, it will ressume operation using Dense mode.. ;) That's probably the catch to say that if it's only using sparse mode and the rp mapping info and rp
candidate info is announce using multicast mechanism than how the router going to communicate (sparse needs rp, and rp mapping info can only be annouce once MA receive info about candidate rp. But candidate rp does not know how to join to rp announce
group as it does not have rp mapping from MA).. ;)
If I can remember correctly all node will automatically join the dense group used for the rp-mapping announcement, you prolly can read more on Beau Williamson Developing IP Multicast network.
My advice is go ahead and configure auto RP using sparse mode... see how it goes. If memory serves me right, I think it should be fine.
Others may have something to add as well.. ;)
Regards,
Donny
CCIE #11189
"Robert Yee"
<robert@bluespud. To: "Donny MATEO" <donny.mateo@sg.ca-indosuez.com>
com> cc: <ccielab@groupstudy.com>, <nobody@groupstudy.com>
Sent by: Subject: RE: Auto-RP with NBMA
nobody@groupstudy
.com
05/10/2003 12:11
AM
Please respond to
"Robert Yee"
Hi Danny,
Thanks for the reply. I do understand exactly what yo uare talking about. I've read through 4 differnt versions of the link that you have provided previously. All the docs use the identical examples, that is why I asked whether of not Auto-RP in an NBMA
enviroment should be set up for 'sparse-dense' or 'sparse'. From what I have read, Auto-RP and IP PIM NBMA-MODE seem contrary to each other when it comes to the PIM mode:
IP PIM NBMA-MODE:
In order to deal with the problems introduced by NBMA networks, it is necessary to provide the router with information about the underlying L2 topology of the nbma network. Cisco IOS accomplishs this with the following interface command:
ip pim nbma-mode
In order forthis command to run correctly, SPARSE MODE MUST BE IN USE...The reason has to to with the sparse mode join message...
AUTO-RP:
A prerequisite for Auto-RP is that all interfaces must be configured in SPARSE-DENSE mode...
Both descriptions are very clear as to excactly what mode they need to run in, respectively. Yet they both seem contrary to each other. This is where my confusion is.
I'm still stuck with the question. Which mode do I configure if I'm using Auto-RP in a NBMA environment.
Thanks,
Robert
-----Original Message-----
From: Donny MATEO [mailto:donny.mateo@sg.ca-indosuez.com]
Sent: Friday, May 09, 2003 2:29 AM
To: Robert Yee
Cc: ccielab@groupstudy.com; nobody@groupstudy.com
Subject: Re: Auto-RP with NBMA
Hi Robert,
the real problem of this hub and spoke with the relation with RP and MA placing has nothing to do with the configuration of the mode of PIM.
The issue is (if I can remember correctly) that a router pass on Discovery message in a flooding fashion. Meaning it wont' pass it up to the interface it's receiving the information from.
So in your case, the MA must be put in the Hub or somewhere up stream of the hub router. If you places MA in one of the spoke, what will happen is the spoke router will flood the discovery message (the rp mapping) to the hub. And the hub will flood it to
it's other interface(s) except the one it received the message from. In this case the other spoke router will never get the rp discovery message and thus will not have any rp - group mapping information.
About ip pim nmba-mode, actually this is only an optimization method, does not really has anything to do with mode and rp or ma placement. You see to understand the problem you need to understand how router works on nbma network. When one router (say hub)
want's to send multicast traffic to nbma network, it's using a technique called pseudobroadcast (basically it's just the data link layer of a router replicate the packet and send them one by one to the peer(s) in the NBMA network. requiring peer_number x
resources that he need when doing it in a broadcast network (such as ethernet). Now this is not good. Specially in sparse mode. Since in sparse mode node will explicitly tells the upsteam (by sending join/prune message) when it has a downstream multicast,
client. So here the router behaves like dense mode and flood all the traffic downstream => bad for multicast, imagine movie streaming packet with 100 clients...urghh...
so what happen here you use the nbma feature of PIM, with this feature the router would track which peer in the nbma network that actually want's to receive the multicast traffic and only send them to those peer(s).
check this out if you're not clear
http://www.cisco.com/en/US/tech/tk828/tk363/technologies_white_paper09186a00800d6b61.shtml
Donny Mateo,
CCIE #11189
"Robert Yee"
<robert@bluespud. To: <ccielab@groupstudy.com>
com> cc:
Sent by: Subject: Auto-RP with NBMA
nobody@groupstudy
.com
05/09/2003 03:37
PM
Please respond to
"Robert Yee"
Hi All,
I've been reading about configuring Auto-RP and also Multicast over NBMA (in this case, frame-relay). All the literature point to these 2 things:
1. The use Auto-RP, the routers should be configured for 'sparse-dense-mode'
2. To use 'ip pim nbma-mode' accross a frame relay network, the routers should be configured for 'sparse' mode
So what if you want to configure Auto-RP over NBMA? All the stuff I read only points to where to place the MA, but does not say whether to use 'sparse-dense' or 'sparse'?
Can anyone shed some ligh on this?
TIA,
Robert
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