Multicast Routing----Part 2

Hello People!!!!

This article is a continuation of my previous multicast Routing --part 1.

As I was talking about IGMP messages or we can also call it an IGMP report which the client sends to the router to connect to a multicast group.

Let's say we have PC in our lab and they want to receive multicast traffic and they are going to talk to the next-hop router to say hey I want to receive the multicast traffic for this particular multicast group (I am taking here 239.1.1.1 as the multicast group).

The way the PC request for multicast traffic is through a protocol called IGMP and it stands for the Internet group of the management protocol.

Different Version of IGMP:

IGMP Version 1

IGMP Version 2

IGMP Version 3

IGMP Version 1

With IGMP version 1 the receiver sends the packet to the router saying that I want to join the multicast group 239.1.1.1. The router sends the traffic destined for that group out of interface out of which that receiver is connected.

It's not necessary that the next-hop router would be there to receive the traffic for that multicat group. If the router is not here to receive the traffic for that group then it will graft request to the multicast distribution tree to talk (that we will talk later)to another router to get the traffic for that group. In IGMP Version 1 there is TTL of 60 sec and every 60 sec the router would be asking the PC hey do you still want to receive the traffic for the multicast group 293.1.1.1 and it will continue. What if the PC does not want to receive the traffic after some time? But still, in this case the router would be contacting the PC every 60 sec for the update.

IGMP Version 2

With the IGMP version 2 also the PC contacts the router to get the traffic for a particular multicast group. If they no longer want the traffic then they will proactively contact the router and will say, hey I do not want to receive the traffic for this group. I am leaving. That is not possible with the IGMP version 1. That's the biggest difference between IGMP version 1 and IGMP version 2.

IGMP v3

In the real world, we will most of the time see IGMP v2. With the IGMP version 3, the PC will send IGMP version 3 message only to notify the router that they not only want to receive the traffic destined for this multicast group but also want to receive traffic if they are coming from a specific source. Remember we spoke about the Source-Specific multicast address range in our previous article. So we use that Class D address range here in IGMP version 3.

Here you got a couple of video servers that send the same video stream, in this case, with IGMP version 3 which also defines the specific source multicast the PC can say that they want to join group 239.1.1.1 and include a source of 10.3.1.1.1. Just from the video server 2.

Now here is one more concept that is very important and its called IGMP snooping.

Routers use PIM (Protocol Independent Multicast which we will discuss later in the next article) to figure out where to forward multicast traffic but what about Ethernet switches?

Layer two switches are simple devices. They learn source MAC addresses and insert these in their MAC address tables. When a frame arrives, they check for the destination MAC address, perform a lookup in the MAC address table and then forward the frame. This works very well for unicast traffic but it’s a problem for multicast traffic.

IGMP Snooping

As the name implies, this is done by listening to IGMP traffic between the router and the hosts. So the main job of the IGMP snooping is to determine to which port we have the receiver connected. When the router receives the multicast traffic it forwards that to the switch and switch knows exactly that the multicast traffic is coming for this group. It also knows the list of the receiver who has requested for the traffic from that multicast group, so it will forward the traffic to those ports where the receivers are connected.

How is switch going to implement this?

So the switch is monitoring the IGMP messages which are getting exchanged between the receivers and the routers.

Once IGMP snooping is enabled, the switch will detect multicast enabled routers and it does so by listening to the following messages:

• IGMP General Query (0100.5e00.0001)
• OSPF (0100.5e00.0005 and 0100.5e00.0006)
• PIM version 1 and HSRP (0100.5e00.0006)
• PIM version 2 (0100.5e00.000d)
• DVMRP (0100.5e00.0004)

Advantages of IGMP Snooping

A Layer 2 switch supporting IGMP Snooping can passively snoop on IGMP Query, Report, and Leave (IGMP version 2) packets transferred between IP Multicast routers/switches and IP Multicast hosts to determine the IP Multicast group membership. IGMP snooping checks IGMP packets passing through the network picks out the group registration and configures Multicasting accordingly.

Without IGMP Querying/Snooping, Multicast traffic is treated in the same manner as a Broadcast transmission, which forwards packets to all ports irrespective of the fact that a particular receiver does not want to receive t the traffic destined for that multicast address. We can see the same from the below diagram where without IGMP snooping the traffic is forwarded to all the ports on the switch and with IGMP snooping switch has forward only to the intended receivers.

With IGMP Querying/Snooping, Multicast traffic is only forwarded to ports that are members of that multicast group. IGMP Snooping generates no additional network traffic, which significantly reduces the Multicast traffic passing through your switch.

We will talk about PIM, Multicast Sparse, and Dense mode in the next article.