Paradyne BitStorm 2400, Руководство пользователя

Страница: 22 / 90

1. BitStorm 2400 Overview
1-12
December 2002 2400-A2-GB20-10
In an effort to reduce the number of broadcast messages being generated in
response to the Query or Group-Specific Query message, all hosts in the
broadcast domain will see the reports for other hosts in that group. Hosts that see
another’s report will simply reset their timer on that group and will not generate a
duplicate of the Report Message.
IGMP-enabled devices (routers or snooping switches) will learn which interfaces
have at least one interested host for each group. They will not be able to determine
if more than one host is interested in the group on a single interface.
For typical situations, IGMP report packets should be suppressed so that they are
not forwarded between the subscriber ports on StormPort CPE modems. If IGMP
reports are flooded to the other ports, the modem will not get an accurate view of
the ports that have interested hosts, i.e., the hosts on EtherLoop ports 2–6 will see
a report from a host on EtherLoop 1. Hosts on ports 2–6 therefore would not
generate reports for the groups. If port-to-port communication has been enabled
on the modem, then IGMP Report packets should be allowed to be forwarded.
When a Membership Report is received, the router (or IGMP snooping switch)
does the following:
!
Adds the group being reported to the list of multicast group memberships on
the network on which it received the report.
!
Set/Resets the interval timer for that group membership
!
Repeats Membership Query at regular intervals
Leave Group
When a host leaves a multicast group, it does one of the following:
!
Sends a Leave Group Message to the routers (224.0.0.2) IP group address.
This option is only available to IGMP V2 hosts.
!
Quietly leaves a multicast group by no longer generating IGMP Reports for
that group in response to Queries.
IGMP Operation
IGMP V1 Operation
In IGMP V1, routers (or IGMP Proxy function) will periodically (typically every
10 seconds) issue IGMP General Query messages. Every multicast-enabled end
station will receive the Query Message and prepare to generate IGMP report
messages for each of the multicast groups it wishes to participate in.
The IGMP router will keep track of the Report Messages received for each group.
If the IGMP router completes three query/report cycles and does not detect a
report for a specific group, it will assume that there are no interested end stations
and will stop forwarding the multicast messages for this group on this interface. If
this was the last or only interface interested in a specific group, the router will
signal the other multicast routers to prune itself from the multicast forwarding tree.