Enterasys Networks Security Router X-PeditionTM - Manual (Page 182)

Describing the XSR’s IP Multicast Features
7-6 Configuring PIM-SM and IGMP
Receiving a Query
When a LAN contains multiple multicast routers, IGMPv3 chooses a single querier per subnet
using the same querier election mechanism as IGMPv2, namely by IP address. When a router
receives a query with a lower IP address, it sets the Other-Querier-Present timer to Other Querier
Present Interval and stops sending queries on the network if it was the previously elected querier.
After its Other-Querier Present timer expires, it will begin sending General Queries. If the
previous querier stops sending queries, other multicast routers will wait a certain period, which
can be configured by the
ip igmp querier-timeout command, then take over as the querier.
Receiving a Report
Three types of group records comprise a Report message: Filter-Mode-Change, Source-List-Change,
and Current-State. When multicast routers receive the Report message, they update the IGMP state
table.
Source-Specific Forwarding Rules
When a multicast router receives a datagram from a source destined to a particular group, a
decision must be made whether to forward the datagram onto an attached network or not. The
multicast routing protocol makes this decision using IGMPv3 information to ensure that all
sources/groups desired on a sub-network are forwarded to that sub-network. IGMPv3 data does
not override multicast routing data; for example, if the IGMPv3 filter-mode group for G is
EXCLUDE, a router may still forward packets for excluded sources to a transit subnet.
Interoperating with Older IGMP Versions
IGMPv3 hosts and routers can interoperate with hosts and routers that have not yet been
upgraded to IGMPv3. This compatibility is maintained by hosts and routers taking appropriate
actions depending on the versions of IGMP operating on hosts and routers within a network.
Query Version Distinctions
The IGMP version of a Membership Query message is determined as follows:
• IGMPv1 Query: length = 8 octets and Max Resp Code field is zero
• IGMPv2 Query: length = 8 octets and Max Resp Code field is non-zero
• IGMPv3 Query: length >= 12 octets
Query messages that do not match any of the above conditions (e.g., a Query of length 10 octets)
will be silently ignored.
Behavior of Group Members Among Older Version Queriers
To be compatible with older version routers, IGMPv3 hosts operate in version 1 and version 2
compatibility modes. IGMPv3 hosts will keep state per local interface regarding the compatibility
mode of each attached network. A host's compatibility mode is determined by the Host
Compatibility Mode variable which can be in one of three states: IGMPv1, IGMPv2 or IGMPv3. This
variable is stored per interface and is dependent on the version of General Queries received on
that interface as well as the Older Version Querier Present timers set for the interface.