5. Multicast > Overview
107
Tsi578 User Manual
June 6, 2016
Integrated Device Technology
www.idt.com
5.1.6
Multicast Work Queue
The multicast work queue accepts packets from ingress ports and forwards them to the broadcast
buffers according to the multicast group table. The multicast work queue can store a maximum of
2208
1
bytes of packet data or seven maximum sized packets inside its buffer. Once the packet buffer
has stored 1936 bytes of data, it forces the ISF to stop further packet transmission.
For information on ISF arbitration for the multicast work queue, refer to
Multicast Work Queue” on page 99
and
“Output Queuing Model for Multicast” on page 100
.
Once the first 8 bytes of a packet have been received by the multicast work queue, the destination ID
and TT fields of the packet are examined and used by the multicast work queue to determine which
multicast group's vector to process in order to replicate the packets. The multicast group table
computes a multicast mask which indicates which ports the packet copies should be sent to. The
ingress port where the original packet was received is always removed from the multicast mask to form
the multicast vector. If the broadcast buffer for a port has detected a maximum latency violation, a
system designer can optionally automatically remove this port from the multicast mask. For more
information, refer to
“Multicast Maximum Latency Timer” on page 119
.
When the multicast work queue has computed the multicast vector, it arbitrates to transmit packet
copies to the broadcast buffers accordingly. The work queue always operates in a cut-through fashion.
A packet is not dropped if it is STOMPed when it is received in the multicast work queue. The
STOMPed packet is replicated to the broadcast buffers. Similarly, if a packet exceeds the time-to-live
counter value in the ingress queue, it is replicated to the broadcast buffers by the multicast work queue.
For more information on error scenarios, refer to
“Error Management of Multicast Packets” on
5.1.7
Broadcast Buffers
Each egress port has a dedicated broadcast buffer associated with it. The broadcast buffers accept
packet copies from the multicast work queue, and forward the packet copies to the egress port. A
broadcast buffer can accept one maximum sized packet (276 bytes) or up to eight smaller packets.
Eight packets can only be accepted if the sizes of the packets, individually rounded up to the nearest
multiple of eight, sum to less than 280 bytes.
For more information on ISF arbitration for the broadcast buffers, refer to
the Broadcast Buffer” on page 100
“Output Queuing Model for Multicast” on page 100
.
The broadcast buffers wait until a packet has been completely received before starting arbitration with
the egress port.
1. Packets are stored in an 8-byte boundary. Packets with length of non-multiple of 8-bytes is rounded up to the nearest multiple of 8 for
storage in the buffer. The packets are not altered and are transmitted exactly as they are received.
