IBM 8271 Nways Ethernet LAN Switch, User Manual

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6-4 C
HAPTER
6: S
PANNING
T
REE
P
ROTOCOL
How STP Works
STP Initialization Initially, the STP system requires the following before it can configure
the network:
■
Communication between all the bridges. This communication is
carried out using Bridge Protocol Data Units (BPDUs), which are
transmitted in packets with a known multicast address.
■
One bridge to start as a master or Root Bridge, a central point from
which the network is configured.
The Root Bridge is selected on the basis of it having the lowest Bridge
Identifier value. This is a combination of the unique MAC address of
the bridge and a priority component defined for the bridge.
The Root Bridge generates BPDUs on all ports at a regular interval
known as the Hello Time. All other bridges in the network have a Root
Port. This is the port nearest to the Root Bridge, and it is used for
receiving the BPDUs initiated by the Root Bridge.
STP Stabilization Once the network has stabilized, two rules apply to the network:
1 Each network segment has one Designated Bridge Port. All traffic
destined to pass in the direction of or through the Root Bridge flows
through this port. The Designated Bridge Port is the port which has the
lowest Root Path Cost for the segment.
The Root Path Cost consists of the path cost of the Root Port of the
bridge, plus the path costs across all the Root Ports back to the Root
Bridge. Table 6-1 shows the default path costs for the stack.
Table 6-1
Default path costs
Port Type Duplex Cost
100BASE-TX Full 150
Half 300
10BASE-T Full 650
Half 700
F24user.bk Page 4 Wednesday, July 15, 1998 4:27 PM