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Dell Configuration Guide for the S4048–ON System 9.14.2.6

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Prevent Network Disruptions with BPDU Guard

Configure the Portfast (and Edgeport, in the case of RSTP, PVST+, and MSTP) feature on ports that connect to end stations. End stations do not generate BPDUs, so ports configured with Portfast/ Edgport (edgeports) do not expect to receive BDPUs.

If an edgeport does receive a BPDU, it likely means that it is connected to another part of the network, which can negatively affect the STP topology. The BPDU Guard feature blocks an edgeport after receiving a BPDU to prevent network disruptions, and Dell EMC Networking OS displays the following message. 
3w3d0h: %RPM0-P:RP2 %SPANMGR-5-BPDU_GUARD_RX_ERROR: Received Spanning Tree BPDU on
BPDU guard port. Disable TenGigabitEthernet 3/4.

Enable BPDU Guard using the bpduguard option when enabling PortFast or EdgePort. The bpduguard shutdown-on-violation option causes the interface hardware to be shut down when it receives a BPDU. Otherwise, although the interface is placed in an Error Disabled state when receiving the BPDU, the physical interface remains up and spanning-tree will only drop packets after a BPDU violation.

The following example shows a scenario in which an edgeport might unintentionally receive a BPDU. The port on the Dell EMC Networking system is configured with Portfast. If the switch is connected to the hub, the BPDUs that the switch generates might trigger an undesirable topology change. If you enable BPDU Guard, when the edge port receives the BPDU, the BPDU is dropped, the port is blocked, and a console message is generated.

NOTE Unless you enable the shutdown-on-violation option, spanning-tree only drops packets after a BPDU violation; the physical interface remains up.
Dell EMC Networking OS Behavior: Regarding bpduguard shutdown-on-violation behavior:
  • If the interface to be shut down is a port channel, all the member ports are disabled in the hardware.
  • When you add a physical port to a port channel already in the Error Disable state, the new member port is also disabled in the hardware.
  • When you remove a physical port from a port channel in the Error Disable state, the Error Disabled state is cleared on this physical port (the physical port is enabled in the hardware).
  • You can clear the Error Disabled state with any of the following methods:
    • Perform a shutdown command on the interface.
    • Disable the shutdown-on-violation command on the interface (the no spanning-tree stp-id portfast [bpduguard | [shutdown-on-violation]] command).
    • Disable spanning tree on the interface (the no spanning-tree command in INTERFACE mode).
    • Disabling global spanning tree (the no spanning-tree in CONFIGURATION mode).
Figure 1. Enabling BPDU Guard
Illustration of enabling BPDU guard.

Dell EMC Networking OS Behavior

BPDU guard:
  • is used on edgeports and blocks all traffic on edgeport if it receives a BPDU.
  • drops the BPDU after it reaches the RP and generates a console message.

Example of Blocked BPDUs

DellEMC(conf-if-te-1/7)#do show spanning-tree rstp brief
Executing IEEE compatible Spanning Tree Protocol
Root ID Priority 32768, Address 0001.e805.fb07
Root Bridge hello time 2, max age 20, forward delay 15
Bridge ID Priority 32768, Address 0001.e85d.0e90
Configured hello time 2, max age 20, forward delay 15

Interface                           Designated
Name   PortID  Prio Cost  Sts Cost  Bridge ID            PortID
---------- -------- ---- ------- --- ------- --------------------
Te 1/6 128.263 128  20000 FWD 20000 32768 0001.e805.fb07 128.653
Te 1/7 128.264 128  20000 EDS 20000 32768 0001.e85d.0e90 128.264

Interface
Name   Role   PortID  Prio Cost  Sts Cost  Link-type Edge
---------- ------ -------- ---- ------- --- ----------------
Te 1/6 Root   128.263 128  20000 FWD 20000 P2P       No
Te 1/7 ErrDis 128.264 128  20000 EDS 20000 P2P       No
DellEMC(conf-if-te-1/7)#do show ip interface brief tengigabitEthernet 1/7
Interface           IP-Address OK Method  Status Protocol
TenGigabitEthernet 1/7 unassigned YES Manual up     up

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