Multicast mac address steve deering

Stephen Deering is a former Fellow at Cisco Systems, where he worked on the development technologies, including multicast routing, mobile internetworking, scalable addressing, and support for multimedia applications over the Internet.
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Get Mac Address From IP Address - All Devices On Network

It really make sense to use multicast to talk to relevant peer rather than using unicast or even broadcast which rather inefficient. Using multicast will make the communication exclusive and avoid sending the packet to non-relevant end.


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Multicast is used commonly in an area where traffic is exclusive to its member and unidirectional. For example video stream, using multicast will only require the source to generate 1 stream and send it to the multicast address and the client will just need to join the multicast address to view the video content.

No multiple streams required for many clients. Similar approach is used for TV broadcast and stock ticker application. The traffic flows from Sender S or we usually call it Server and it is unidirectional with the exception PIM-Bidir , Client will never send multicast packet to the Group G as the client will only join the Group and listen.

For example, a Sender is sending a packet to a multicast group The first 25 bits are always Look at the break down below. It appears that both This is something to keep in mind when designing the multicast network or creating multicast application. This site uses Akismet to reduce spam. Learn how your comment data is processed. Search for: Search. This method ensures that all group members on the subnet, and any routers listening for members of the group, hear the report.

The NICs of hosts that are not members of the group reject the reports based on their Layer 2 address. If all group members on a subnet respond to a query, bandwidth is unnecessarily wasted.


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After all, the router needs to know only that there is at least one member of the group on the subnet; it does not need to know exactly how many there are, or who they are. Another problem with all group members responding to a query is the possibility of collisions if all members respond at once. Backing off and retransmitting consumes more network and host resources. If many group members are on the subnet, there is an increased probability that multiple collisions will occur before everyone sends his report.

Sending reports to the group address eliminates multiple reports on a subnet.

Stephen Deering

When a query is received, each group member starts a timer based on a random value. The member does not send a report until the timer expires. Because the timers are random, it is much more likely that one member's timer will expire before the other timers. This member sends a report, and because the report is sent to the group address, all other members hear it.

These other members, hearing the report, cancel their timers and do not send a report of their own. As a result, only one report is generally sent on the subnet. One report per subnet is all the router needs. The possibility was raised in the preceding section that multiple routers might be attached to a subnet, all of which need to know whether group members are present. Figure shows an example. Two routers are attached to the subnet, both of which receive the same multicast stream from the same source over different routes. If one router or route fails, the group members can continue to receive their multicast session from the other router.

US6331983B1 - Multicast switching - Google Patents

Under normal circumstances, however, it is inefficient for both routers to forward the same data stream onto the subnet. The routers are aware of each other because of their routing protocols.

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So one way to ensure that only one router forwards the session onto the subnet is to add a designated router, or querier, function to the multicast routing protocol. The querier is responsible for forwarding the multicast stream. The other router or routers only listen, and they begin forwarding the stream only if the querier fails.

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The problem with allowing the routing protocol to elect a querier is that multiple IP multicast routing protocols are available. If the two routers in Figure are running incompatible protocols, their respective querier election processes will not detect each other; each will decide that it is the querier, and both will forward the data stream.

The local group management protocol, however, is independent of the routing protocols. The routers have to run this common protocol to query group members, so it makes sense to give the querier function to the group management protocol.

This guarantees that the routers are speaking a common language on the subnet and can agree on which is responsible for forwarding the session. Regardless of which of the several routing protocols is used in a multicast internetwork, IGMP is always the "language" spoken between hosts and routers.

All hosts that want to join multicast groups, and all routers with interfaces on subnets containing multicast hosts, must implement IGMP. It is a control protocol like ICMP, sharing some functional similarities. Like ICMP, it is responsible for managing higher-level data exchanges. For this reason, the default can be changed with the ip igmp version command. However, version 3 is briefly discussed in this section with the expectation that Cisco IOS Software may support it in the near future. Membership Report messages are sent to indicate that a host wants to join a group.

The messages are sent when a host first joins a group, and sometimes in response to a Membership Query from a local router. When a host first learns of a group and wants to join, it does not wait for the local router to send a query. If the host had to wait for a query, it might never get the opportunity to join. Instead, when the host first joins a group, it sends an unsolicited Membership Report for the group.

Multicast sessions are identified in the routers by a source, group pair of addresses, where source is the address of the session's originator and group is the Class D group address. If the local multicast router does not already have knowledge of the multicast session the host wants to join, it sends a request upstream toward the source. The data stream is received, and the router begins forwarding the stream onto the subnet of the host that requested membership. The destination address of the Membership Report message's IP header is the group address, and the message itself also contains the group address.

To ensure that the local router receives the unsolicited Membership Report, the host sends one or two duplicate reports separated by a short interval. RFC recommends an interval of 10 seconds. The local router periodically polls the subnet with queries. Each query contains a value called the Max Response Time, which is normally 10 seconds specified in units of tenths of a second.


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When a host receives a query, it sets a delay timer to a random value between 0 and the Max Response Time. If the timer expires, the host responds to the query with one Membership Report for each group to which it belongs.

Bite the bytes: Multicast in Ethernet LANs

All multicast-enabled devices are members of the "all systems on this subnet" group, represented by the group address Because this is a default, hosts do not send Membership Reports for this group. Because the destination of the Membership Report is the group address, other group members that might be on the subnet hear the report in addition to the router. If the host receives a Membership Report for a group before its delay timer expires, it does not send a Membership Report for that group. In this way, the router is informed of the presence of at least one group member on the subnet, without all members flooding the subnet with reports.

When a host leaves a group, it notifies the local router with a Leave Group message. The message contains the address of the group being left, but unlike Membership Report messages, the Leave Group message is addressed to the "all routers on this subnet" address of This is because only the multicast routers on the subnet need to know that the host is leaving; other group members do not. RFC recommends that a Leave Group message be sent only if the leaving member was the last host to send a Membership Report in response to a query.

As the next section explains, the local router always responds to a Leave Group message by querying for remaining group members. If group members other than the "last responder " leave quietly , the router continues forwarding the session and does not send a query.