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why TTL value 1 in IGMP protocol and what are the different values possible in TTL section for Multicasting.

What I got from the google is that if TTL value is 1 then it won't leave the LAN, but if it won't leave the LAN how the msg will be broadcasted ?

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2 Answers 2

up vote 9 down vote accepted

The reason that it doesn't have to leave the LAN is that IGMP (or MLD if you use IPv6) only talks to the multicast router on the LAN. The multicast router will take care of the routing.

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If the TTL is 1, no router will forward it. That's the whole point. – Ricky Beam Jun 27 '14 at 18:20
The IGMP message isn't forwarded. It is meant to talk to the multicast router on the local subnet. It is never forwarded. The multicast router will probably use PIM or a similar protocol to talk to upstream multicast routers to get the data to flow to where it is needed. – Sander Steffann Jun 27 '14 at 18:23
Which protocol is commonly used by multicast router for routing, like PIM or DVMRP or MOSPF and based on what they are used ? Thank You – dillip_beta Jun 30 '14 at 3:44
I most commonly see PIM-SM and PIM-SSM. The first choice is between a dense (send multicast packets down a link unless requested not to) or sparse (don't send multicast packets unless asked to) protocol. After that it is mostly a matter of taste, local policy or special requirements. – Sander Steffann Jun 30 '14 at 8:18

TTL == 1 means stay on this network; it will not be forwarded by any router. This is true of multicast and unicast. The purpose for doing this with multicast is to limit how far the message can spread. For example, (All Systems on this Subnet) is required to have a TTL of 1 to keep it "on this subnet". (NTP) can have as high a TTL as you like to reach as far across your network as you need.

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He is asking about the IGMP messages, your answer seems to be about the actual multicast packets. – Sander Steffann Jun 27 '14 at 18:24
what are the different values possible in TTL section for Multicasting. sigh – Ricky Beam Jun 27 '14 at 19:28
TTL always means the same. It needs to be high enough so that it can reach the destination you are talking to. If that destination is on your local LAN then TTL=1 is enough. – Sander Steffann Jun 28 '14 at 19:20
Ty Ricky, in the case of NTP what should be the TTL value ? will it be 31 or 255. Ty – dillip_beta Jun 30 '14 at 3:53
here, I'm seeing "64" and I did nothing special to openntpd. – Ricky Beam Jun 30 '14 at 4:51

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