D-Link DES-2218 - Manual (Page 56)

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Using the Console Interface
A bridge priority ranges from 0 to 65535, with 0 being the highest
bridge priority.
On the port level, these terms are used:
♦
Root Port. Each bridge has a root port except for the root bridge.
This is the port that has the lowest path cost to the root bridge. In case
there are several such ports, then the one with the lowest port
identifier becomes the root port.
♦
Port Identifier This is a combination of the port priority (a parameter
that can be set) and the physical port number (a unique, unchangeable
number assigned by the bridge).
♦
Designated Port This is the port on each designated bridge that is
attached to the LAN segment for which the bridge is the designated
bridge.
♦
Port Priority This parameter can be set. The smaller the number
you set, the higher the port priority is. The higher the port priority,
the higher the chance the port has for becoming the root port. Port
priority ranges from 0 to 255, with 0 being the highest port priority.
♦
Path Cost Use this parameter to specify preferred paths on the
network. The smaller the path cost, the more chance the port of
becoming the root port. By convention, a 10 Mbps LAN port has a
path cost of 100, while a 100 Mbps port has a path cost of 10.
In the following figure, three DES-2218 switches are used to bridge three
LANs together. Switch 3 is selected as the root bridge because it has the
lowest bridge ID. Switch 3, as the root bridge, also becomes the designated
bridge for LANs 1 and 3. Switch 1 becomes the designated bridge for LAN
2.
Only designated bridges have designated ports, so Switch 2 does not have
any designated ports. Port 2 of Switch 2 becomes the root port because it has
a lower root path cost to the root bridge than Port 1. For Switch 1, Port 1 is
the root port, while Port 2 is the designated port. For Switch 3, both ports are
designated ports.