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Configuring Spanning Tree Parameters
Restricting Port Roles (Root Guard)
All ports are automatically eligible for root port selection. A port in a CIST/MSTI instance or per-VLAN
instance can be prevented from becoming the root port by restricting the role of the port (also referred to
as enabling root guard). This is done using the
vlan restricted-role
example:
-> spantree cist port 1/2 restricted-role enable
-> spantree cist linkagg 10 restricted-role enable
-> spantree vlan 100 port 8/1 restricted-role enable
-> spantree vlan 20 linkagg 1 restricted-role enable
Note that the above commands also provide optional syntax; restricted-role or root-guard. For example,
the following two commands perform the same function:
-> spantree vlan port 2/1 restricted-role enable
-> spantree vlan port 2/1 root-guard enable
When root guard is enabled for a port, it cannot become the root port, even if it is the most likely
candidate for becoming the root port. However, this same port is designated as the alternate port when the
root port is selected.
Enabling the restricted role status is used by network administrators to prevent bridges external to the core
region of the network from influencing the Spanning Tree topology. However, note that enabling the
restricted role status for a port may impact connectivity within the network.
Restricting TCN Propagation
All ports automatically propagate Topology Change Notifications (TCN) or Topology Changes (TC) to
other ports. To restrict a port from propagating topology changes and notifications, use the
restricted-tcn
command or the
VLAN or flat) is active for the switch. For example:
-> spantree cist port 2/2 restricted-tcn enable
-> spantree cist linkagg 5 restricted-tcn enable
-> spantree vlan 10 port 1/5 restricted-tcn enable
-> spantree vlan 20 linkagg 1 restricted-tcn enable
Enabling the restricted TCN status is used by network administrators to prevent bridges external to the
core region of the network from causing unnecessary MAC address flushing in that region. However, note
that enabling the restricted TCN status for a port may impact Spanning Tree connectivity.
Limiting BPDU Transmission
The number of BPDUs to be transmitted per port per second can be limited using the
txholdcount
command for a CIST instance or the
instance. Both of these commands apply to all ports and link aggregates and are supported when the
switch is running in either the per-VLAN mode or the flat mode. For example:
-> spantree cist txholdcount 5
-> spantree vlan 10 txholdcount 5
OmniSwitch AOS Release 7 Network Configuration Guide
command regardless of which mode (per-VLAN or flat) is active for the switch. For
spantree vlan restricted-tcn
spantree cist restricted-role
command regardless of which mode (per-
spantree vlan txholdcount
March 2011
Configuring STP Port Parameters
command or the
spantree
spantree cist
spantree cist
command for a per-VLAN
page 5-43
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