3com TR ユーザーズマニュアル

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.

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 support is provided in the Switch 2000 
TR. 

■

802.1d Spanning Tree 

■

IBM 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 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)