Multicast Ports?

Answered Question
Oct 1st, 2007
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We use multicast heavily throughout our Datacenter for heartbeat purposes for servers and other cisco proprietary uses HSRP etc...My question is, in multicast is having different ports make a difference if using the same multicast address with other services. Or should we use a unique multicast address for each service that does not conflict? I always thought that ports for multicast were irrelavant and only unique addresses were needed? Sorry if this seems to basic of a question. Thx!

Correct Answer by Kevin Dorrell about 9 years 7 months ago

Not really ... just like unicast, it is the combination of the address and the port that matters. But you would do well to use different multicast IP addresses for different application because switches will distribute multicast packets according to the IP address (regardless of port).


So if you have two applications that use the same IP address but different ports, a machine that is interested in either application will have to listen to both sets of traffic and filter out the port it is not interested in. If they are using different IP addresses, the switch will do that for them.


(Actually, its a bit more complicated because the switch distributes according to groups of 32 addresses, so there may be some overlap even if the addresses are different ... if the addresses fall in the same MAC group.)


Kevin Dorrell

Luxembourg


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ankbhasi Mon, 10/01/2007 - 08:30
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Hi Friend,


It will be good to use different address if assigning a multicast address for some application is in your control. I do not think port number is relevant for multicast traffic as it is mostly UDP based traffic.


Regards,


Ankur

jcmattos1 Mon, 10/01/2007 - 09:49
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So in your expert opinion will we expect problems if there is an IP address conflict even if UDP ports used are different?

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Kevin Dorrell Mon, 10/01/2007 - 11:53
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Not really ... just like unicast, it is the combination of the address and the port that matters. But you would do well to use different multicast IP addresses for different application because switches will distribute multicast packets according to the IP address (regardless of port).


So if you have two applications that use the same IP address but different ports, a machine that is interested in either application will have to listen to both sets of traffic and filter out the port it is not interested in. If they are using different IP addresses, the switch will do that for them.


(Actually, its a bit more complicated because the switch distributes according to groups of 32 addresses, so there may be some overlap even if the addresses are different ... if the addresses fall in the same MAC group.)


Kevin Dorrell

Luxembourg


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