Configuration Guide User guide

1462 FastIron Configuration Guide
53-1002494-02
IGMP snooping overview
An IGMP device's responsibility is to broadcast general queries periodically, and to send group
queries when receiving a leave message, to confirm that none of the clients on the port still want
specific traffic before removing the traffic from the port. IGMP V2 lets clients specify what group
(destination address) will receive the traffic but not to specify the source of the traffic. IGMP V3 is
for source-specific multicast traffic, adding the capability for clients to INCLUDE or EXCLUDE
specific traffic sources. An IGMP V3 device port state could be INCLUDE or EXCLUDE, and there are
different types of group records for client reports.
The interfaces respond to general or group queries by sending a membership report that contains
one or more of the following records associated with a specific group:
• Current-state record that indicates from which sources the interface wants to receive and not
receive traffic. This record contains the source address of interfaces and whether or not traffic
will be included (IS_IN) or not excluded (IS_EX) from this source.
• Filter-mode-change record. If the interface state changes from IS_IN to IS_EX, a TO_EX record
is included in the membership report. Likewise, if the interface state changes from IS_EX to
IS_IN, a TO_IN record appears in the membership report.
• An IGMP V2 leave report is equivalent to a TO_IN (empty) record in IGMP V3. This record means
that no traffic from this group will be received regardless of the source.
• An IGMP V2 group report is equivalent to an IS_EX (empty) record in IGMP V3. This record
means that all traffic from this group will be received regardless of source.
• Source-list-change record. If the interface wants to add or remove traffic sources from its
membership report, the report can contain an ALLOW record, which includes a list of new
sources from which the interface wishes to receive traffic. It can also contain a BLOCK record,
which lists the current traffic sources from which the interface wants to stop receiving traffic.
IGMP protocols provide a method for clients and a device to exchange messages, and let the device
build a database indicating which port wants what traffic. The protocols do not specify forwarding
methods. They require IGMP snooping or multicast protocols such as PIM or DVMRP to handle
packet forwarding. PIM or DVMRP can route multicast packets within and outside a VLAN, while
IGMP snooping can switch packets only within a VLAN.
If a VLAN is not IGMP snooping-enabled, it floods multicast data and control packets to the entire
VLAN in hardware. When snooping is enabled, IGMP packets are trapped to the CPU. Data packets
are mirrored to the CPU in addition to being VLAN flooded. The CPU then installs hardware
resources, so that subsequent data packets can be switched to desired ports in hardware without
going to the CPU. If there is no client report or port to queriers for a data stream, the hardware
resource drops it.
MAC-based implementation on
FastIron X Series devices
On both switch and router software images, IGMP snooping is MAC-based. This differs from IGMP
snooping on the BigIron router images, which match on both IP source and group (S,G) entries
programmed in the Layer 4 CAM. In contrast, the FastIron X Series images match on Layer 2
destination MAC address entries (*,G).
When Layer 2 CAM is used, traffic is switched solely based on the destination MAC address.
Consequently, traffic of the same group coming to the same port, regardless of its source, is
switched in the same way. In addition, the lowest 23 bits of the group address are mapped to a
MAC address. In this way, multiple groups (for example, 224.1.1.1 and 225.1.1.1) have the same