Network Topology
AP1--SW1==SW2--AP2 staticlink-aggregation between SW1 andSW2
Problem Description
when AP1 send a arp broadcast, it will receive the arp back; when AP2 send a arp broadcast, it will not receive the arp back.
There seems to be a one direction loops.
Process Analysis
check the stp status in the sw1 and sw2, we found there are disputed record.
dis stp abnormal
-port ---[Bridge-Aggregation1]---
MST ID BlockReason Time
0 Disputed 01:28:23 12/29/2023
0 Disputed 01:28:22 12/29/2023
0 Disputed 01:28:20 12/29/2023
%Dec 29 01:26:11:014 2023 H3C STP/6/STP_DISPUTE_RESTORATION: Instance 0's port Bridge-Aggregation1 exited the dispute state.
%Dec 29 01:26:12:810 2023 H3C STP/4/STP_DISPUTE: Instance 0's port Bridge-Aggregation1 received an inferior BPDU from a designated port which is in forwarding or learning state. The designated bridge ID contained in the BPDU is 32768.a274-58da-0200, and the designated port ID contained in the BPDU is 128.2.
%Dec 29 01:26:12:812 2023 H3C STP/6/STP_DISPUTE_RESTORATION: Instance 0's port Bridge-Aggregation1 exited the dispute state.
%Dec 29 01:26:14:607 2023 H3C STP/4/STP_DISPUTE: Instance 0's port Bridge-Aggregation1 received an inferior BPDU from a designated port which is in forwarding or learning state. The designated bridge ID contained in the BPDU is 32768.a274-58da-0200, and the designated port ID contained in the BPDU is 128.2.
%Dec 29 01:26:14:609 2023 H3C STP/6/STP_DISPUTE_RESTORATION: Instance 0's port Bridge-Aggregation1 exited the dispute state.
%Dec 29 01:26:16:401 2023 H3C STP/4/STP_DISPUTE: Instance 0's port Bridge-Aggregation1 received an inferior BPDU from a designated port which is in forwarding or learning state. The designated bridge ID contained in the BPDU is 32768.a274-58da-0200, and the designated port ID contained in the BPDU is 128.2.
%Dec 29 01:26:16:403 2023 H3C STP/6/STP_DISPUTE_RESTORATION: Instance 0's port Bridge-Aggregation1 exited the dispute state.
%Dec 29 01:26:18:184 2023 H3C STP/4/STP_DISPUTE: Instance 0's port Bridge-Aggregation1 received an inferior BPDU from a designated port which is in forwarding or learning state. The designated bridge ID contained in the BPDU is 32768.a274-58da-0200, and the designated port ID contained in the BPDU is 128.2.
%Dec 29 01:26:18:186 2023 H3C STP/6/STP_DISPUTE_RESTORATION: Instance 0's port Bridge-Aggregation1 exited the dispute state.
%Dec 29 01:26:19:980 2023 H3C STP/4/STP_DISPUTE: Instance 0's port Bridge-Aggregation1 received an inferior BPDU from a designated port which is in forwarding or learning state. The designated bridge ID contained in the BPDU is 32768.a274-58da-0200, and the designated port ID contained in the BPDU is 128.2.
%Dec 29 01:26:19:982 2023 H3C STP/6/STP_DISPUTE_RESTORATION: Instance 0's port Bridge-Aggregation1 exited the dispute state.
%Dec 29 01:26:21:778 2023 H3C STP/4/STP_DISPUTE: Instance 0's port Bridge-Aggregation1 received an inferior BPDU from a designated port which is in forwarding or learning state. The designated bridge ID contained in the BPDU is 32768.a274-58da-0200, and the designated port ID contained in the BPDU is 128.2.
%Dec 29 01:26:21:780 2023 H3C STP/6/STP_DISPUTE_RESTORATION: Instance 0's port Bridge-Aggregation1 exited the dispute state.
%Dec 29 01:26:23:570 2023 H3C STP/4/STP_DISPUTE: Instance 0's port Bridge-Aggregation1 received an inferior BPDU from a designated port which is in forwarding or learning state. The designated bridge ID contained in the BPDU is 32768.a274-58da-0200, and the designated port ID contained in the BPDU is 128.2.
The disputed mechanism occurs when a high-priority device continuously receives low-priority BPDU messages. The device will dispute this interface, and the interface will temporarily be unable to forward messages. For a low-priority device, the reason for sending low-priority BPDU messages is not receiving high-priority messages from this interface. We have observed very short periods of disputes, but they quickly recover, indicating that the messages can be forwarded normally.
The detailed analysis of this process:
1. The aggregated BAGG10 is the root port, continuously sending BPDU (Bridge Protocol Data Unit) messages to XGE1/0/28 (with F=1, L=1, TC=0 in the Flag field).
3. When the aggregation receives BPDU messages from XGE1/0/27 (with F=1, L=1, TC=0 in the Flag field), it considers the priority of the message lower than its own and not a normal root port negotiation BPDU message (F=1, L=1, TC=1). This triggers the dispute mechanism, stopping the forwarding of traffic. It then sends BPDU messages outward (with F=0, L=0, TC=0 in the Flag field).
4. When XGE 1/0/28 receives BPDU messages (with F=0, L=0, TC=0 in the Flag field), it immediately responds with the normal BPDU message (with F=1, L=1, TC=1 in the Flag field), resolving the dispute. Normal traffic forwarding resumes.
This cycle of events repeats continuously. The time from 3 to 4 is short, and the time from 4 to 3 is long, allowing traffic to be forwarded.
This issue essentially causes a loop in the broadcast traffic from SW1 to SW2, resulting in the AP receiving its own ARP packets. There are no issues with the broadcast traffic from SW2 to SW1.
Solution
This issue essentially causes a loop in the broadcast traffic from SW1 to SW2, resulting in the AP receiving its own ARP packets. There are no issues with the broadcast traffic from SW2 to SW1.
root cause: sw2 static link-aggregation configuration missing.
Solution: configure static link-aggregation for XGE1/0/27 and 1/0/28