LevelOne GEL-5261, User Manual

Page: 193 / 570

Chapter 7
| Spanning Tree Algorithm
Configuring Interface Settings for STA
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Table 12: Default STA Path Costs
(Continued)
Port Type Short Path Cost Long Path Cost
(IEEE 802.1D-1998) (IEEE 802.1D-2004)
Gigabit Ethernet 10,000 10,000
10G Ethernet 1,000 1,000
Administrative path cost cannot be used to directly determine the root port on a
switch. Connections to other devices use IEEE 802.1Q-2005 to determine the root
port as in the following example.
Figure 108: Determining the Root Port
For BPDU messages received by i1 on SW3, the path cost is 0.
For BPDU messages received by i2 on SW3, the path cost is that of i1 on SW2.
The root path cost for i1 on SW3 used to compete for the role of root port is
0 + path cost of i1 on SW3; 0 since i1 is directly connected to the root bridge.
If the path cost of i1 on SW2 is never configured/changed, it is 10000.
Then the root path cost for i2 on SW3 used to compete for the role of root port
is 10000 + path cost of i2 on SW3.
The path cost of i1 on SW3 is also 10000 if not configured/changed.
Then even if the path cost of i2 on SW3 is configured/changed to 0, these ports
will still have the same root path cost, and it will be impossible for i2 to become
the root port just by changing its path cost on SW3.
For RSTP mode, the root port can be determined simply by adjusting the path
cost of i1 on SW2. However, for MSTP mode, it is impossible to achieve this only
by changing the path cost because external path cost is not added in the same
region, and the regional root for i1 is SW1, but for i2 is SW2.
◆
Admin Link Type – The link type attached to this interface.
■
Point-to-Point – A connection to exactly one other bridge.
■
Shared – A connection to two or more bridges.
■
Auto – The switch automatically determines if the interface is attached to a
point-to-point link or to shared media. (This is the default setting.)
◆ Root Guard – STA allows a bridge with a lower bridge identifier (or same
identifier and lower MAC address) to take over as the root bridge at any time.
Root Guard can be used to ensure that the root bridge is not formed at a
suboptimal location. Root Guard should be enabled on any designated port
connected to low-speed bridges which could potentially overload a slower link
by taking over as the root port and forming a new spanning tree topology. It
could also be used to form a border around part of the network where the root
bridge is allowed. (Default: Disabled)