User's Manual

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In a complicated layer-2 network, the administrator may hope a switch in the core layer as the root of the
network, but it cannot manage all switches in the access layer (That's because the switch in the access layer
may belong to other clients.) Thus, the inappropriate configuration of other switches may cause the core switch
cannot become the root.
To avoid the root role is occupied by switches outside the management area, you can configure Root Guard
function on the boundary switch. If an interface configured Root Guard receives information that a higher BPDU
is chosen as Port Port, Root Guard will automatically set the port as the blocking state and resumes it as the
designated port.
In PVST and MSTP mode, Root Guard can work independently in each STP. In MSTP mode, if a boundary
interface in CIST is blocked because of Root Guard, the interface will be blocked in all MSTI. The boundary
interfaces are those connected to the LAN host, STP switch, RSTP switch or MSTP switch outside the region.
In interface configuration mode, run command spanning-tree guard root to enable Root Guard characteristic.
Note:
Root Guard characteristic acts differently somehow in SSTP/PVST and RSTP/MSTP. In
SSTP/PVST mode, Root port is always blocked by Root Guard. In RSTP/MSTP mode, Root
port won’t be blocked until receiving higher level BPDU. A port which formerly plays the Root
role will not be blocked.
37.1.7 Loop Guard
The Loop Guard attribute can protect a port after it changes from a root port or an alternate port to a designated
port. This function can prevent a port from generating a loop when the port cannot receive BPDU continuously.
You can enable this feature by using the spanning-tree loopguard default global configuration command. After
enabled the command, a Root port or Alternate port wil change to designated port and set as the block state.
If the port receives high priority BODU in a while, it will resumes from Loop Guard automatically.
In PVST and MSTP mode, Loop Guard can work independently in each STP. In MSTP mode, if a boundary
interface in CIST is blocked because of Root Guard, the interface will be blocked in all MSTI.
Note:
Root Guard characteristic acts differently somehow in SSTP/PVST and RSTP/MSTP. In SSTP/PVST mode,
designated port is always blocked by Loop Guard. In RSTP/MSTP mode, the port will be blocked when it
changes to designated port if it cannot receive BPDU. An interface receiving low priority BPDU and is of the
designated role will not be blocked by Loopt Guard.
37.2 Configuring STP Optional Characteristic
37.2.1 STP Optional Characteristic Configuration Task
 Configuring Port Fast
 Configuring BPDU Guard
 Configuring BPDU Filter
 Configuring Uplink Fast