3Com TR manual Congestion Control, Forwarding Modes

can use any path in the network. ARE packets are flooded through all bridges onto all rings. This creates multiple copies if redundant paths exist in the net- work.

When multiple requests are received at the destina- tion; each one causes a response. Implementing Spanning Tree ensures that the number of broadcast packets are significantly reduced.

For more information on Spanning Tree, refer to “Spanning Tree Fields in Chapter 4.

Congestion Control

Congestion control prevents loss of packets due to congestion on destination ports. You have a choice among:

■Flow Control—Flow control balances the band- width use of the client ring to that of the server ring. This is especially useful in focused load condi- tions where many ports are sending data to a single port.

Flow control monitors buffer usage and shifts the load onto the end station. This allows the Switch 2000 TR to use the available bandwidth to access the port by balancing traffic on the sending ports.

■Nonblocking—Occasionally packets that are des- tined for a particular port cannot be delivered. This is especially true if a ring is busy or not operating. These packets inadvertently impede the delivery of packets destined for known good or free rings.

Nonblocking provides a means for discarding pack- ets that can not be delivered while ensuring deliv-

Token Ring Switching Concepts 1-17

ery of packets with known good or free destinations.

■Off—In Off mode there is no congestion control. Packet transfers are based on queue availability in the receive buffer. If there is no buffer space on the receiving port, packets are dropped.

Forwarding Modes

Switch 2000 TR provides three forwarding methods.

■Store-and-Forward—Switch 2000 TR can use a conventional store-and-forward method typically found in bridges. In this mode, packets are received and buffered (stored) in their entirety before they are forwarded. This guarantees that errored frames on the source ring are not for- warded to the destination. Although store-and-forward guarantees packet stability, it also involves a transit delay depending on the length of the packet.

■Cut-thru—Cut-thru avoids the transit delay found in standard store-and-forward methods. Although cut-through can be applied only between ports that are operating at the same wire speed or from a high speed port to a low speed port, it does pro- vide better transit rates by forwarding a frame as soon as enough information is available to deter- mine the destination port.

■Dynamic Cut-thru—In Dynamic Cut-thru mode the switch monitors error rates on the source. If a user set threshold is exceeded, the switch port changes to store and forward mode until the error rate reduces. At this point the port reverts to