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how does router receive multicast traffic ?

pfrancis3
Beginner
Beginner

Hello, if you have a streaming server sending out multicast traffic to a pre-defined multicast address, do you need to configure the nearest facing router to receive this traffic, or does it just come in to the router interface by default ?

I am asking this because if you don't have to configure it to receive the multicast traffic, would the incoming streaming data be a big load on the router ?

Thank you kindly.

5 Replies 5

Philip D'Ath
Advisor
Advisor

The router only needs to be interested in the traffic if it needs to forward it.  Does the router need to forward the traffic in this case?

If you need the router to forward it then you need to enable multicast routing and using a protocol like PIM.

Chapter: Configuring IP Multicast Routing

thanks, my question really is 'how does the multimedia server multicast flow get initiated ?

i.e. is it always pushing out multicast data in to it's facing router, or is the flow from the multimedia server initated by the router or host IGMP report request ?

Thanks kindly.

Well it depends on weather you have a multicast aware switch or not.

If you have a dumber switch, multicast traffic gets broadcast out every port,

If you have a multicast aware switch IGMP is used.  The server sends out an IGMP message creating a multicast group.  Clients who are interested send join requests.  The switch then forwards the multicast packets from the server to every client that has joined the group.

When the router appears in the picture it talks PIM to other routers.  If it needs the multicast stream to send to another router it will send its own IGMP join request to get the traffic.

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

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In no event shall Author be liable for any damages wha2tsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

If you have a multicast aware switch IGMP is used.  The server sends out an IGMP message creating a multicast group.  Clients who are interested send join requests.  The switch then forwards the multicast packets from the server to every client that has joined the group.

Unsure that's exactly correct.

I believe, as least for ASM multicast, hosts just transmit multicast "in the blind".

Clients, independently of any multicast sources, do send IGMP join requests.  This is for the benefit of multicast routers, so such routers "know" a host, or hosts, want a particular multicast stream.

Multicast router repeatedly send out IGMP queries.  This so they can continue to confirm V1 IGMP hosts still want a multicast stream.

IGMP snooping switches, "watch" for the IGMP report and block (to the host) non-desired multicast from ports that don't want it.  I believe the also "watch" for the IGMP query to recognize the multicast router's port (so as to not block it).

(NB: interestingly, I recall, with hubs, multicast clients will suppress their IGMP report when they see another host request the same stream.  But with IGMP snooping switches, the switch suppresses IGMP reports getting to other hosts, so they don't suppress their IGMP reports, so it continues to know which ports want the stream.  However, it must insure at least one IGMP port gets to the multicast router port so the router doesn't cut off a routed multicast stream.  [If there's not multicast router, IGMP snooping switches generally can be configured to also be the IGMP querier.])

Joseph W. Doherty
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Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages wha2tsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

do you need to configure the nearest facing router to receive this traffic, or does it just come in to the router interface by default ?

If the multicast source is in the same LAN segment as the router interface, it should arrive by default.

I am asking this because if you don't have to configure it to receive the multicast traffic, would the incoming streaming data be a big load on the router ?

That's a "it depends" kind of answer. If the router doesn't want the multicast flow, the router's NIC should behave much like a host NIC that doesn't want the flow, which is, if the L2 multicast MAC isn't desired, the frame will be dropped by the NIC.  (Of course, the inbound frame still uses bandwidth to the NIC.)  If the L2 multicast MAC overlaps with a L3 multicast L2 MAC that's desired, the frame is passed up the stack to L3, where the L3 will drop the multicast packet.  The latter, for routers without ASIC support, should add to the CPU load, but the router should do this rather efficiently.  I.e. additional load should be similar to the router receiving unicast traffic it forwards to a null interface.

[edit: the router, might pass all received L2 multicast up the stack, i.e. the NIC doesn't filter it out, so that the router is aware of the multicast stream.  Which it does [NIC filter or not] may depend on whether it's enabled for multicast routing.]

To your later post's question:

i.e. is it always pushing out multicast data in to it's facing router, or is the flow from the multimedia server initated by the router or host IGMP report request ?

I believe the router always gets the flow, so that it "knows" of it, and can also forward it, immediately, if configured for multicast routing.

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