C
HAPTER
4
| Configuring the Switch
Configuring the Spanning Tree Algorithm
– 145 –
◆ Priority – Defines the priority used for this port in the Spanning Tree
Algorithm. If the path cost for all ports on a switch are the same, the
port with the highest priority (i.e., lowest value) will be configured as
an active link in the Spanning Tree. This makes a port with higher
priority less likely to be blocked if the Spanning Tree Algorithm is
detecting network loops. Where more than one port is assigned the
highest priority, the port with lowest numeric identifier will be enabled.
(Range: 0-240, in steps of 16; Default: 128)
◆ Admin Edge (Fast Forwarding) – You can enable this option if an
interface is attached to a LAN segment that is at the end of a bridged
LAN or to an end node. Since end nodes cannot cause forwarding loops,
they can pass directly through to the spanning tree forwarding state.
Specifying edge ports provides quicker convergence for devices such as
workstations or servers, retains the current forwarding database to
reduce the amount of frame flooding required to rebuild address tables
during reconfiguration events, does not cause the spanning tree to
initiate reconfiguration when the interface changes state, and also
overcomes other STA-related timeout problems. However, remember
that this feature should only be enabled for ports connected to an end-
node device. (Default: Edge)
◆ Auto Edge – Controls whether automatic edge detection is enabled on
a bridge port. When enabled, the bridge can determine that a port is at
the edge of the network if no BPDU's are received on the port.
(Default: Enabled)
◆ Restricted Role – If enabled, this causes the port not to be selected
as Root Port for the CIST or any MSTI, even if it has the best spanning
tree priority. Such a port will be selected as an Alternate Port after the
Root Port has been selected. If set, this can cause a lack of spanning
tree connectivity. It can be set by a network administrator to prevent
bridges external to a core region of the network influencing the
spanning tree active topology, possibly because those bridges are not
under the full control of the administrator. This feature is also know as
Root Guard.
◆ Restricted TCN – If enabled, this causes the port not to propagate
received topology change notifications and topology changes to other
ports. TCN messages can cause temporary loss of connectivity after
changes in a spanning tree’s active topology as a result of persistent
Table 11: Default STA Path Costs
Port Type Link Type IEEE 802.1w-2001
Ethernet Half Duplex
Full Duplex
Trunk
2,000,000
1,000,000
500,000
Fast Ethernet Half Duplex
Full Duplex
Trunk
200,000
100,000
50,000
Gigabit Ethernet Full Duplex
Trunk
10,000
5,000
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