The preceding two sections describe operations
whereby the IGMP hosts and routers all run the same version of IGMP.
What if you have a mixture of versions in your network?
The first possible combination of versions
might involve hosts with a mix of versions, as in Figure
,
but with a Version 1 router. All IGMP operations in this scenario
are driven by the version that the query router uses. When there is
such a combination, the hosts must use Version 1 messages. The
router does not understand Version 2 membership reports, nor does it
understand Version 2 leave messages.
A second case exists when the router supports
IGMP Version 2, but hosts use IGMP Version 1. Although the router
understands more message types than the hosts, it ultimately uses
only Version 1 messages. When the Version 2 router receives the
Version 1 membership report, it remembers that they are present and
uses only Version 1 membership queries. Version 1 queries use the
group address 0.0.0.0 and do not generate group-specific queries. If
it generated group-specific queries, the Version 1 hosts would not
recognize the message and would not know how to respond.
What if there are both Version 1 and Version 2
hosts on the segment with a Version 2 router, as in Figure
?
As in the previous case, the router must remember that there are
Version 1 hosts and must, therefore, issue Version 1 membership
queries. Additionally, if any of the Version 2 hosts send a leave
message, the router must ignore the leave notification. It ignores
the message because it must still issue general queries in case a
Version 1 member is still active on the segment.
If two routers attach to the segment where one
supports Version 1 and the other Version 2, the Version 2 router
must be administratively configured as a Version 1 router. The
Version 1 router has no way to detect the presence of the Version 2
router. Because the two versions use different methods of selecting
the query router, they might not reliably agree on the query router.