Basic question on multicast:igmp ssm-map static vs igmp access-group?

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Jul 3rd, 2009
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Hi, can you help me with this:


I read the documentation but this is not clear. What would be the impact if instead of 'igmp ssm-map static' instead of igmp access-group command ? Can you tell me the reason this router may be configured with ip igmp ssm-map static instead of ip igmp access-group?


(...)

ip multicast-routing distributed

ip multicast multipath

ip multicast rpf interval 1

ip multicast rpf backoff 120 120

ip igmp ssm-map enable

no ip igmp ssm-map query dns

ip igmp ssm-map static mylist 11.3.15.3

(...)

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Giuseppe Larosa Fri, 07/03/2009 - 22:57
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Hello Marlon,

SSM stands for source specific multicast.


This means IGMPv3: in SSM a join specifies both the group address G and the source address (actually include or exclude directives)


the ip igmp ssm-map static commands build some IGMPv3 static joins as a reaction to receiving IGMPv2 joins from real users the list is processed if a match is found an IGMPv3 join is created using source 11.3.15.3 and the group address.


see


http://www.cisco.com/en/US/docs/ios/ipmulti/command/reference/imc_03.html#wp1011414


ip igmp access-group has another purpose:


http://www.cisco.com/en/US/docs/ios/ipmulti/command/reference/imc_02.html#wp1011497


by using extended IP access-lists it specifies to what (S,G) pairs users in the lan can make joins.


This is a filter on IGMPv3 joins the other one is a converter the joins statically on the routers and can accomodate hosts that are not IGMPv3 capable.


Be aware that if this behaves like

ip igmp join-group means packets of the (S,G) groups are process switched (it shouldn't be so but it is better to think of)


Hope to help

Giuseppe


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