IGMPv1 and IGMPv2 Interoperability:
à IGMPv2 is designed to be backward compatible with IGMPv1.
à To achieve this, RFC has defined some special interoperability rules
- IGMPv2 Hosts with IGMPv1 Routers:
à When IGMPv1 routers see an IGMPv2 report, it will treat it as an invalid IGMP message type and ignore it.
à To maintain proper group membership, the IGMPv2 host must therefore send IGMPv1 reports only when IGMPv1 router is active as the IGMP Querier.
à The IGMPv2 hosts can differentiate between IGMPv1 and IGMPv2 Queries. It can be done by examining the octet corresponding to the Maximum Response Time field.
In IGMPv1 Queries, this field is zero whereas IGMPv2 Queries contains a non-zero Maximum Response Time value. Therefore, when an IGMPv1 Query is heard, the host will mark the interface as an IGMPv1 interface and stop sending IGMPv2 messages.
à To maintain this interface state, the IGMPv2 hosts will start a 400-second, IGMPv1 router countdown timer whenever an IGMPv1 Query is received on the interface. This timer is reset when another IGMPv1 Query is received. When this countdown timer expires, the interface is once again marked as an IGMPv2 interface to allow IGMPv2 messages to be sent again.
à Using this approach, IGMPv2 host must be prepared to allow Membership Reports to be suppressed by either IGMPv1 or IGMPv2 Membership Report from other hosts on the network.
- IGMPv1 Hosts with IGMPv2 Routers:
à IGMPv1 hosts will respond in the usual way when there is IGMPv1 or IGMPv2 Queries because they are in essence the same format.
à The difference is only in the second octet of the IGMPv2 message, which is ignored by IGMPv1 hosts.
à If an IGMPv2 router is the IGMP Querier and there are IGMPv1 host and IGMPv2 host for a group, both IGMPv1 and IGMPv2 Reports are received in response to a General Query. Because this IGMPv2 Reports do not trigger Report Suppression in IGMPv1 hosts. In addition, IGMPv1 hosts do not understand IGMPv2 Reports, so ignore them.
à If IGMPv1 host is the last member of the group, it will not respond to the Group-Specific Query which is sent for received Leave group message (as IGMPv1 host treat it as an invalid IGMPv1 message type) and the IGMPv2 router will timeout this group wrongly.
- Mixed IGMPv1 and IGMPv2 Routers:
à There are different implementations for IGMPv1 and IGMPv2 mixed environment.
à there is no single method that is completely reliable.
Startup Query Interval The startup query interval is the amount of time in seconds between successive General Query messages sent by a querier during startup. You can also click the scroll arrows to select a new setting. The default startup query interval is one-fourth of the value for the query interval.
Startup Query Count The startup query count is the number of general query messages sent at startup. You can also click the scroll arrows to select a new setting. The default startup query count is 2.
Last Member Query Count The last member query count is the number of Group-Specific Query messages sent before the router assumes that there are no members of the host group being queried on this interface. You can also click the scroll arrows to select a new setting. The default last member query count is 2.
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