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Community Member

need help in MULTICASTING

hi all ,

im just have a question ,

when i need to put

ip igmp jion group x.x.x.x

command ?

as i  understand , the reciver send igmp join request to the router that enabled the multicasting

so ,

if the reciver can tell the router that he want to recive on the multicast x.x.x.x

why i need to let router join that address that the reciver need to recive ?

14 REPLIES
Community Member

need help in MULTICASTING

Thats becasue the source could be anywhere in the network, and to be able to send the multicast from source to the destination you have to enable multicast routing on all routers in the network.  The receiver does not know who the source is and has to rely on the router to use the multicast routing information to process the join request and get the multicast from the source .

HTH

Regards

Umesh

Community Member

need help in MULTICASTING

hi ,

very nice ,

so ,

we need to join multicast to  interfaces of router that is the destination multicast that the sender is sending ?

or

the the src multicast of reciver that want to listen to multicast being sent from transmitter ??

or both ??

or any thing else ?

regards

Hall of Fame Super Blue

need help in MULTICASTING

If the sender is on a different subnet than the receiver then the router needs to be running PIM on both interfaces to be able to route the traffic from source to receiver.

IGMP is needed for end devices to register their interest in a multicast stream.  The router itself does not use IGMP to receive the multicast stream. It uses PIM to talk to other routers and request the stream.

Note PIM uses sparse and dense mode so the way the router actually gets the stream is slightly different but it does not use IGMP for this.

Please ask further if needed.

Jon

Community Member

need help in MULTICASTING

hi ,

sender and reciver are indifferent subnets,

agian

i agree with u to enable

multicast routing & pim on interfaces

but im asking about :

where i need to join groups of multicast to interfaces ?

as i understood , the transmitter is just reansmitting so will not recive

but the reciver should tell the router that he need to recive on x.x.x. as an example

now

my question agian :

what cases i need to tell routers interface that u have to join a specific group multucast ???a

as i understood , the reciver will say to ther router

hey " i need to recive on x.x.x.x"address , then by pim , that address reach all routers in the network and the transmitter can send the stream to reciver.

inmy less understanding , i think we dont need to use "ip igmp join group X.x.x.x"

but still believe that we need it

my question is

when we will need it ?

is there cases that forve us to let interface of router join a specific multicast address ????

regards

Hall of Fame Super Blue

need help in MULTICASTING

I think i understand.

You do not need to use "ip igmp join ..." on a router interface for it to receive the multicast stream. The only time i have actually used that on a router interface was when i was testing and didn't have end hosts to work with.

Usually it is the end host eg. PC etc. that makes the IGMP join request.

You do not need to enable this on router interfaces.

Jon

Community Member

need help in MULTICASTING

hi  jon ,

thanks alot ,

another question :

as the tx and rx are using multicasting ,

do they need to be having normal ip address ??

can they only have multicast address ?

is it mandatory to have both ip & multicast address

again ,

im talking here only about tx transimitter and rx reciver that only using multicast and not other things

regards

Hall of Fame Super Blue

need help in MULTICASTING

Yes, they need an IP address.

A multicast group address is still an IP address, it just uses a different range.

Jon

Community Member

need help in MULTICASTING

hi  ,

thanks Jon for clarification ,

now assume the sender is sending to 224.50.60.70 , the question now is ,

will the src ip of the transmitter will be a normal ip ?? or will be multicast ip ??

also , i read that multicasting routing table is based on src ip , while routinmg table is based on destination ip ,

can u clarify me this issue ?

regards

Community Member

need help in MULTICASTING

also ,

who decide us to  use

(s,g) or (*,g)

or , how it implemented in mroute table ?

the transmitter?

or the router ?

or the reciver ?

Hall of Fame Super Blue

Re: need help in MULTICASTING

To answer both questions -

the source IP of the multicast IP stream will be the source IP of the sender.

Multicast routing is about routing packets away from the source as opposed to routing towards the destination. When you use PIM that uses the IP routing table but it does not do a lookup on the destination IP, it does a look up on the source.  This is called a Reverse Path Forwarding (RPF) check and is the way multicast routers with PIM avoid loops.

Simply put the router looks at the route for the source IP network in the IP routing table. If the multicast stream was received on the same interface that the next hop for the route points out of then it passed the RPF check and can be forwarded. If the stream is received on an interface that is not the same interface that the route itn the IP routing table points to then the stream is dropped because the router cannot ensure there is no loop.

The group entries in the route table are created by the router. The actual entries you see are used differently depending on whether it is PIM sparse or PIM dense mode. When they are created is also dependant on when the router receives the stream and when a host or a downstream router requests the stream.

Jon

Community Member

need help in MULTICASTING

hi ,

nice explamation,

agiain

Q1-

does that mean that the sender dont need multicast address to be assigned with ?

as i understood it has a unicast addresss and send on multicast destination , so i think it dont need multicast addrresss beside the unicast address. ??

correct me if i was wrong

Q2-

as i understood , the reciver tell router that he want to listen to 224.x.y.z as an example

so , assumign igmpv2 is running , the router will have a route as

(*,224,x,y,z)

does that mean it get from revicer ??

plz correct me i may be wrong so much here

regards

Hall of Fame Super Blue

need help in MULTICASTING

No the sender has to use a multicast address it's just that the source unicast IP is used to check for loops.

if there was no multicast address then how would any clients register their interest for the group ie. the group is the multicast IP address. The difference between unicast and mutlicast is that the destination IP in a mulicast packet is the multicast group address but it still has to have one.

In terms of the mroute entries like i say when the entries are created depends on what is happening. The router uses these entries to keep track of which interface the multicast stream is received on ie. the RPF check and also which interface(s) it needs to forward the stream out of.

So if the router receives an IGMP request from a host on a specific interface (think subnet) and there are no other hosts (or routers) that have previously requested the stream then that interface will be added to the specific mroute entry so the router knows it has to to forward it out that interface.

There are differences, as i say, between the (*, G) entries and the (, G) entries and how they are used when running in PIM sparse or dense mode.

Jon

Community Member

need help in MULTICASTING

"""No the sender has to use a multicast address it's just that the source unicast IP is used to check for loops."""

acutally no understanding this

how transmitter will check for loops ?

imn understanding the RFP in routers ,

but sorry i didnt understand the sentense above

can u explain it more ?

regards

Hall of Fame Super Blue

Re: need help in MULTICASTING

how transmitter will check for loops ?

It doesn't. The routers do the RPF check.

Can you clarify which bit is not clear ?

Jon

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