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IP Multicast fundamentals with lab –
PIM, IGMP, Dense and Sparse mode and Multicast network components

We usually think about multicast in the context of streaming video, audio, gaming, live transmissions, but also routing protocols use multicast to send updates. Moreover, if you are going to deal with Data Center Interconnect,  you gonna come across multicast trafic. But, before we go over the lab let’s get to know what is the multicast transmissions, what elements consists of and how is being used.


IP Multicast enables us transmission “one to any”. The “Any” means the hosts (multicast users) that are the members of specified multicast group. Multicast user uses particular ip address to signify that he want to joint to the group of multicast traffic. 


Multicast Addresing

IP Multicast uses Class D with a range of addresses –

Within above scope we distinguish below reserved addresses

For local multicast, there is a link locally scope of multicast addresses – Also within this scope there are a couple of addresses that we should be aware of: – multicast to all hosts – multicast to all multicast routers – multicast to all OSPF routers (updates sends by Designated Router) – multicast to all OSPF Designated Routers ( LSA sends to Designated Router)

The TTL for these addresses is 1, thats mean they will never be send outside the link (broadcast domain)

Transient addresses utilized by applications
– global range –
– 224.2.x.x Multicast babckbone in the Internet

Generally all addresses from the range may be used locally and never should  leave the LAN, but we also distinguish among them:
– addresses for site local scope
– addresses for organization local scope

IP Multicast network components
Multicast source – server that is sending out multicast traffic

Multicast routers – routers that understand multicast traffic and are able to route its further

Multicast clients – clients of the multicast traffic

Multicast routing protocolPIM Protocol Independent Multicast, protocol uses by routers on the path of multicast traffic

Group management protocol – uses by clients to indicate that want receive multicast traffic, the protocol is IGMP Internet Group mnagement Protocol

Distribution tree – the topology with routers that participate in relying the multicast traffic to particular network segments with clients

PIM doesn’t rely on any underlaying routing protocol!

Multicast configuration

PIM Dense Mode
Push technology – assumption is that clients are densely populated and each segment of the network has the clients that want to join to the multicast group. Network segments that are not interested in of Multicast traffic just reject its. This is why this technology is called “Flood and Prune”. This technology is unefficient but is very simple to configure. Will work out in small environment

The configuration boils down to running 2 commands on each router in the multicast topology:
globally: ip multicast-routing
and on each interface: ip pim dense-mode

if the client wants to join to the group :
on the interface : ip igmp join-group [Mcast IP address]

PIM Sparse Mode
Pull technology, assumption is that clients are sparsely populated, also there is  a router (or routers) in topology that is RP (Randezvous Point). The server is sending a multicast traffic to this RP, and then RP is sending further to the Clients. Sparse Mode technology is much more efficient than Dense Mode, the network is not being flooded out with unnecessary multicast traffic.

The configuration with statically assigned IP address of RP on the RP and the other routers is the same :

ip multicast-routing
ip pim rp-address [RP address]
on the interface:
ip pim sparse-mode

The Client:
ip igmp joinn-group [Mcast IP address]

AUTO RP – CISCO proprietary

A method for dynamic RP address assigment that relies upon Multicast groups for its operation (information about RP is spreading with use of multicast, Cisco call it “Chicken and egg” problem, cause we need RP in order to get working multicast environment, but in order assign address to RP we need multicast environment).

When we run show ip mroute we gonna see address or these multicast addresses are being used by Auto RP in its operation

Sparse Dense Mode technology for Auto RP

Initially sparse-dense mode technology was invented to overcome “chicken and egg” issue. All routers for Auto RP groups and, that don’t have RP run plain Dense mode to spread information about RP. Everything else is being run in Sparse mode. But it caused a problem. If we lose the RP, then we have a “Dense flood” in entire multicast topology. To overcome this issue, Cisco has invented the command “ip pim autorp listener” and Sparse-Dense technology stopped to be needed anymore.

Besides plain ip multicast aware routers in multicast topology There are 2 kinds of AUTO RP routers that have to be deployed in order to obtain fully working AUTO RP topology.

Candidate RP – the router that wants to be RP
ip pim send-rp-announce [interface or ip address] scope [TTL]

Mapping Agent – device that spread information about Candidate RP
ip pim send-rp-discovery [interface] scope [TTL]

We may have these 2 roles : Candidate RP and Mapping Agent on the same machine. Also on each router we have to run command “ip pim autorp listener” !

Let’s lab it out !

The topology that we will be working with is simple. There are a couple of a routers in the multicast topology, also there is a server with running video stream and two Clients that are willing to watch this video. I used GNS3 as usuall, server and 2 hosts are just Windows 7 machines with running “VLC media player”. Of course the server is streaming, the hosts are plugging to the server via multicast group. We have also 2 routers required by Auto RP Mapping Agent and Candidate RP. I configured loopback 0 with address on the Candidate RP that will be used as an Candidate RP IP address for the other routers. I aslo implemented EIGRP routing protocol across entire topology just for the sake of reachability particular routers. Interface Lo0 is also in EIGRP AS.

How to setup VLC Server and Client ?

Menu Media, choose Stream. Click Add and choose the file that you want to stream. Click Stream at the bottom, then Next. Form drop-down list choose RTP / MPEG Transport Stream and click Add. Next, input multicast group address, that video will be accessible at and stream name, leave port 5004 as is.Next window, deactivate “Activate Transcoding”. In the last window to the outpu line add “ttl=255” because default value is 0 and click Stream. We are done!

Menu Media, choose Open Network Stream and input rtp://, this is equivalent of command “ip igmp joinn-group” .Click Play

Configuration of Multicast Group routers

On ALL routers globally
ip multicast-routing
on the interfaces
ip pim sparse-mode

Additionally on Candidate RP (R3)
ip pim send-rp-announce scope 255

and on Mapping Agent  (R2)
ip pim send-rp-discovery e1/0 scope 255

also on all Routers we have to run command :

ip pim autorp listener



First of all from now on we should be able to watch the video on the Clients machine, but also we have a few useful commands:

show ip pim interface 

show ip pim neighbors – let’s see the neighborhood from the perspective of Randezvous Point


show mroute – let’s check the multicast routing information base (MRIB) , I marked entry (*, asteriks stands for ANY. For any IP address to RP is, also we see routing for ANY to Auto RP addresses


show ip pim rp mapping – let’s use this command on R4 router that border with the Client1. We may conclude that for ANY multicast group ( Randesvouz Point is, the source of info is Mapping Agent  (R2) with interface, of course via Auto RP


and at the end let’s capture the traffic on the link between the Client 1 and R4. As we see the Client 1 with an Ip address reports its membership to Multicast Group with IP address with using Group Management Protocol IGMP v2


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