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CheetahSwitch Workgroup-4508
Forwarding Traffic with Unknown VLAN Tags
Up to 2048 VLANs are supported by the IEEE 802.1Q protocol, but this switch only
supports 16 VLANs. Therefore, if this switch is attached to any device that
forwards frames with unknown VLAN tags, or to endstations which issue VLAN
registration requests for unknown VLANs, this traffic will be dropped.
Class-of-Service (CoS) Support
The CheetahSwitch Workgroup-4508 provides two transmit queues on each port,
with a weighted round-robin scheme. This function can be used to provide
independent priorities for various types of data such as real-time video or voice,
and best-effort data.
Priority assignment to a packet in the CheetahSwitch is accomplished through
explicit assignment by end stations which have applications that require a higher
priority than best-effort. This switch utilizes the IEEE 802.1p and 802.1Q tag
structure to decide priority assignments for the received packets.
IGMP Snooping and IP Multicast Filtering
The Internet Group Management Protocol (IGMP) runs between hosts and their
immediately neighboring multicast router/switch. The protocol’s mechanisms allow
a host to inform its local router/switch that it wants to receive transmissions
addressed to a specific multicast group.
A router, or multicast-enabled switch, can periodically ask their hosts is they want
to receive multicast traffic. If there is more than one router/switch on the LAN
performing IP multicasting, one of these devices is elected “querier” and assumes
the responsibility of querying the LAN for group members.
Based on the group membership information learned from IGMP, a router/switch
can determine which (if any) multicast traffic needs to be forwarded to each of its
ports. At Layer-3, multicast routers use this information, along with a multicast
routing protocol, to support IP multicasting across the Internet.
IGMP provides the final step in an IP multicast packet delivery service since it is
only concerned with forwarding multicast traffic from the local router/switch to
group members on directly attached subnetwork or LAN segment.
This switch supports IP Multicast Filtering by:
• Passively snooping on the IGMP Query and IGMP Report packets transferred
between IP multicast routers and IP multicast host groups to learn IP Multicast
group members, and
• Actively sending IGMP Query messages to solicit IP Multicast group members
(see page 2-23 or 3-25).