3Com TR - Manual (Page 26)

1-16 C HAPTER 1: O VERVIEW
table, a switch knows which port must be used to
reach each known MAC address.
Every time a switch receives a packet, it examines its
bridging table to determine if the source MAC
address is contained in its bridging table. If it is not, it
creates a new entry. The switch then searches its
bridging table for the address contained in the
packet’s destination address field. The switch then
forwards the packet to the port associated with the
destination MAC address. If the port specified in the
bridging table is the same port on which the packet
was received, the switch discards the packet.
The “learned” entries in the switch’s bridge table are
subject to aging. That means that if the switch does
not receive a packet from each entry before the
entry’s Age Timer expires, the switches bridging table
deletes the entry. This feature allows entries associ-
ated with dormant stations to be removed from the
switch’s bridge table.
Source Route Transparent Bridging
As the name implies, source route transparent (SRT)
bridge performs both source routing and transparent
bridging. If a frame with routing information is
received at the bridge, the bridge performs source
route bridging. Likewise, if a frame without routing
information is received at the switch, the switch per-
forms transparent bridging.
Spanning Tree
Spanning Tree support is provided in the Switch 2000
TR.
■
802.1d Spanning Tree
■ IBM Spanning Tree
802.1d Spanning Tree
Spanning Tree is an industry standard protocol
(802.1d) which prevents redundant paths (loops)
from existing within a network. By ensuring that only
one active data path exists between any two rings,
the Spanning Tree protocol prevents the following:
■ Packet duplications
■ Broadcast storms
■ Packet misordering
In addition, Spanning Tree also provides fault toler-
ance within the network by automatically reconfigur-
ing the active topology if a fault is detected in the
network. This provides contingency paths in the event
that the active path is disabled and guarantees stabil-
ity.
IBM Spanning Tree
IBM Spanning Tree operates the same as 802.1d in
that it resolves network loops. However, IBM Span-
ning Tree uses different addresses and parameters
which do not interoperate with 802.1d Spanning
Tree.
The IBM Spanning Tree algorithm reduces the number
of broadcast frames in source routed networks. A
single path is formed between networks from the
root bridge. IBM Spanning Tree applies only to Span-
ning Tree Explorer frames (STEs). All Route Explorer
frames (AREs), and Specifically Routed Frames (SRFs)