3Com TR - Manual (Page 27)

Token Ring Switching Concepts 1-17
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-
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