hi all ,
im just have a question ,
when i need to put
ip igmp jion group x.x.x.x
as i understand , the reciver send igmp join request to the router that enabled the multicasting
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 ?
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 .
very nice ,
we need to join multicast to interfaces of router that is the destination multicast that the sender is sending ?
the the src multicast of reciver that want to listen to multicast being sent from transmitter ??
or both ??
or any thing else ?
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.
sender and reciver are indifferent subnets,
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
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 ????
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.
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
im talking here only about tx transimitter and rx reciver that only using multicast and not other things
thanks Jon for clarification ,
now assume the sender is sending to 188.8.131.52 , 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 ?
who decide us to use
(s,g) or (*,g)
or , how it implemented in mroute table ?
or the router ?
or the reciver ?
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.
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
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
does that mean it get from revicer ??
plz correct me i may be wrong so much here
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 (
"""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 ?
how transmitter will check for loops ?
It doesn't. The routers do the RPF check.
Can you clarify which bit is not clear ?