Enabling MSTP
Context
Enabling MSTP on a ring network immediately triggers spanning tree calculation. If basic configurations are not performed on switches and interfaces before MSTP is enabled, network flapping may occur upon changes to parameters such as device priority and interface priority.
Procedure
- Run system-view
The system view is displayed.
STP/RSTP-enabled devices calculate spanning trees by exchanging BPDUs. Therefore, all the ports participating in spanning tree calculation must be enabled to send BPDUs to the CPU for processing. By default, a port is enabled to send BPDUs to the CPU. You can run the bpdu enable command in the interface view to enable a port to send BPDUs to the CPU. The S5720-EI, S5720-HI, S5730-HI, S5731-H, S5731-S, S5731S-H, S5731S-S, S5732-H, S6720-EI, S6720-HI, S6720S-EI, S6730-H, S6730S-H, S6730-S, and S6730S-S do not support the bpdu command.
- Run stp enable
MSTP is enabled on the switch.
By default, the MSTP function is enabled on the device.
If the management network interface for an MSTP-enabled device is a VLANIF interface of a VLAN, run the ethernet-loop-protection ignored-vlan command to specify this VLAN as an ignored VLAN. During MSTP calculation, the interface on which the ignored VLAN is configured remains in forwarding state. Therefore, services are not interrupted.
After MSTP is enabled on a port, edge port detection is started automatically. If the port fails to receive BPDU packets within (2 x Hello Timer + 1) seconds, the port is set to an edge port. Otherwise, the port is set to a non-edge port.
To prevent the CPU from being affected and faults such as network flapping from occurring, the following lists the recommended maximum number of STP-enabled ports in Up state for different models:
S2720-EI, S5720-LI, S5735-L, S5735S-L, S5735S-L-M, S5720S-LI, S5730-SI, S5730S-EI, S6720-LI, S6720S-LI, S6720-SI, S6720S-SI, S5720I-SI, S5720-SI, S5735-S, S5735S-S, S5735-S-I, and S5720S-SI: 128
S5720-EI: 200
S5720-HI, S5730-HI, S5731-H, S5731-S, S5731S-H, S5731S-S, S5732-H, S6720-EI, S6720-HI, S6720S-EI, S6730-H, S6730S-H, S6730-S, and S6730S-S: 256
Follow-up Procedure
If the topology of a spanning tree changes, the forwarding paths to associated VLANs are changed. On the switch, therefore, the ARP entries corresponding to these VLANs need to be updated. MSTP processes ARP entries in either fast or normal mode.
In fast mode, ARP entries to be updated are directly deleted.
In normal mode, ARP entries to be updated are rapidly aged.
The remaining lifetime of ARP entries to be updated is set to 0. The switch rapidly processes these aged entries. If the number of ARP aging probe attempts is not set to 0, ARP implements aging probe for these ARP entries.
To specify which mode is used for STP/RSTP convergence, run the stp converge { fast | normal } command in the system view.
By default, the normal MSTP convergence mode is used.
If fast mode is used, ARP entries are frequently deleted. This causes high CPU usage on the device (reaching 100%) and results in frequent network flapping. Therefore, using normal mode is recommended.