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[All About Switches] Designated Interface Is Blocked After Receiving Low-Priority Packets

Latest reply: Apr 11, 2018 14:19:46 1406 1 1 0

Involved Models and Versions

l   Fixed switches of V100R005C00 and later versions

l   Modular switches of V100R003 and later versions

Problem Description

Figure 1-1 Designated interface is blocked after receiving low-priority packets



In Figure 1-1, Switch1 and Switch2 are connected through GE0/0/1 interfaces. Switch1 has a higher priority than Switch2, so Switch1 is selected as the root bridge.

Normally, GE0/0/1 on Switch1 is the designated port in Forwarding state and GE0/0/1 on Switch2 is the root port in Forwarding state.

When a fault occurs, GE0/0/1 on Switch1 becomes the designated interface in Discarding state. Or, the interface switches between Learning and Discarding states.


Generally, MSTP BPDUs may be transmitted unidirectionally, and Switch2 does not process high-priority packets sent by Switch1. Check whether the link has poor quality. If the link quality is normal, run the debugging stp interface interface-type interface-number packet all command to check whether Switch2 sends low-priority packets. For fixed switches with earlier versions of V100R005 and modular switches with earlier versions of V100R003, BPDU transparent transmission is disabled on an interface by default. As a result, the device discards received MSTP BPDUs.

Root Cause

Normally, Switch1 considers that Switch1 is the root bridge. GE0/0/1 on Switch2 is the root port, so it does not send MSTP BPDUs to Switch1.

When Switch2 does not receive or process MSTP BPDUs sent by Switch1, Switch2 considers itself as the root bridge and periodically sends low-priority MSTP BPDUs to Switch1.

For fixed switches with earlier versions of V100R005 and modular switches with earlier versions of V100R003, when the designated port receives MSTP BPDUs with a lower priority than the root bridge, the switch considers an exception occurs. Multiple spanning trees exist on the network and may cause loops, so the port is blocked.

After 15s, the designated port transitions to the Learning state for 15 s. If the port continuously receives low-priority packets, it enters the Discarding state. This process is repeated.


If unidirectional forwarding is caused by low link quality, replace the link.

If the link quality is normal, configure bpdu enable on GE0/0/1 of Switch2.


For fixed switches in V100R005 and later versions and modular switches in V100R003 and later versions, BPDU transparent transmission is enabled on an interface by default.

Pay attention to this problem during version upgrade, and check whether the blocking status is changed before and after the upgrade.

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MVE Created Apr 11, 2018 14:19:46 Helpful(0) Helpful(0)

useful document, thanks
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