Table 5-1 Stp Path Costs; Table 5-2 Stp Port States - ZyXEL Communications ZyWALL 5 User Manual

ZyWALL 5 Internet Security Appliance
5.2.2 STP Terminology
The root bridge is the base of the spanning tree; it is the bridge with the lowest identifier value (MAC
address).
Path cost is the cost of transmitting a frame from the root bridge to that port. It is assigned according
to the speed of the link to which a port is attached. The slower the media, the higher the cost - see the
next table.
LINK SPEED
Path Cost 4Mbps
Path Cost 10Mbps
Path Cost 16Mbps
Path Cost 100Mbps
Path Cost 1Gbps
Path Cost 10Gbps
On each bridge, the root port is the port through which this bridge communicates with the root. It is
the port on this switch with the lowest path cost to the root (the root path cost). If there is no root port,
then this bridge has been accepted as the root bridge of the spanning tree network.
For each LAN segment, a designated bridge is selected. This bridge has the lowest cost to the root
among the bridges connected to the LAN.
5.2.3 How STP Works
After a bridge determines the lowest cost-spanning tree with STP, it enables the root port and the ports
that are the designated ports for connected LANs, and disables all other ports that participate in STP.
Network packets are therefore only forwarded between enabled ports, eliminating any possible
network loops.
STP-aware bridges exchange Bridge Protocol Data Units (BPDUs) periodically. When the bridged
LAN topology changes, a new spanning tree is constructed.
Once a stable network topology has been established, all bridges listen for Hello BPDUs (Bridge
Protocol Data Units) transmitted from the root bridge. If a bridge does not get a Hello BPDU after a
predefined interval (Max Age), the bridge assumes that the link to the root bridge is down. This bridge
then initiates negotiations with other bridges to reconfigure the network to re-establish a valid network
topology.
5.2.4 STP Port States
STP assigns five port states (see next table) to eliminate packet looping. A bridge port is not allowed
to go directly from blocking state to forwarding state so as to eliminate transient loops.
PORT STATE
Disabled STP is disabled (default).
Blocking Only configuration and management BPDUs are received and processed.
Listening All BPDUs are received and processed.
5-2

Table 5-1 STP Path Costs

RECOMMENDED VALUE
250
100
62
19
4
2

Table 5-2 STP Port States

RECOMMENDED RANGE
100 to 1000
50 to 600
40 to 400
10 to 60
3 to 10
1 to 5
DESCRIPTION
ALLOWED RANGE
1 to 65535
1 to 65535
1 to 65535
1 to 65535
1 to 65535
1 to 65535
Bridge Screen